SOU 1997:132 
 Executive summary 
 During the accession negotiations with the European Union, Sweden was granted a derogation from community legislation to maintain national legislation within the area of feed additives of the groups antibiotics, chemotherapeutics, coccidiostats and growth promoters. In Sweden, the use of such substances is restricted to the purpose of curing or preventing diseases, i.e. used as veterinary medicines according to the Feedingstuffs Act of 1985. The Swedish government has appointed a commission to evaluate the hazards and risks associated with use of antimicrobial feed additives in animal production. 
 Our conclusions can be summarised as follows: 
  
 · 
  
 Antibacterial feed additives have favourable economic effects on livestock production, but from a long term perspective these are questionable, especially regarding animal welfare and animal health. Antibacterial feed additives can, at levels permitted in feedingstuffs, be used for treatment or prevention of animal diseases, which is in violation of directive Hill/EEC. 
  
 · 
  
 The quinoxalines and the nitroimidazoles are potentially genotoxic and may be regarded as an occupational hazard. 
  
 · 
  
 Halofuginone and the quinoxalines are toxic for target species. This is deleterious for animal Welch. 
  
 · 
  
 The risk of increased resistance associated with the general use of antibacterials as feed additives are far from negligible and the potential consequences are serious for both animal and human health. Antibacterials that are presently not used as therapeuticals in human or veterinary medicine are valuable templates for future drugs. As emergence of resistance is considered to be a threat to animal and human health, all AFA should be restricted to medical and veterinary purposes. 
  
 It is the conclusion of this commission that the benefits of antibacterial feed additives do not outweigh the risks. 
 With regard to coccidiostats and other medicinal substances, they are specifically used to prevent disease and should therefore be treated as pharmaceutical specialities, i.e. as medicated feed. 
  
 2 
  
 SOU 1997:132 
  
 SOU 1997:132 
  
 Contents 
  
 3 
  
 %XECUTIVE SUMMARY )NTRODUCTION 
  
 1.1 The Swedish derogation.......................................................................... 
  
 9 
  
 "ACKGROUND 
  
 2.1 
  
 Introduction........................................................................................... 
  
 11 
  
 2.2 
  
 Role of antibacterials in animal husbandry........................................... 
  
 12 
  
 2.2.1 Therapeutic and prophylactic use of antibacterials......................... 
  
 12 
  
 2.2.2 Chemoprophylaxis of protozoal diseases........................................ 
  
 13 
  
 2.2.3 Antibacterials as performance enhancers ....................................... 
  
 13 
  
 2.3 
  
 Community legislation.......................................................................... 
  
 15 
  
 2.3.1 Antibiotics and growth promoters .................................................. 
  
 15 
  
 2.3.2 Coccidiostats and other medicinal feed additives........................... 
  
 17 
  
 2.4 
  
 Current Swedish legislation .................................................................. 
  
 20 
  
 2.4.1 Antibiotics and growth promoters .................................................. 
  
 20 
  
 2.4.2 Coccidiostats and other antiprotozoals ........................................... 
  
 20 
  
 2.4.3 Antibacterials for use in therapy or prophylaxis............................. 
  
 21 
  
 2.4.4 Distribution of medicinal products ................................................. 
  
 21 
  
 2.4.5 Control ............................................................................................ 
  
 22 
  
 2.5. The Swedish ban .................................................................................. 
  
 22 
  
 2.5.1 The preambles................................................................................. 
  
 22 
  
 2.5.2 Effects on animal health ................................................................. 
  
 23 
  
 2.6 
  
 Sales of antibacterials for veterinary use in Sweden............................. 
  
 27 
  
 2.6.1 Materials and methods .................................................................... 
  
 27 
  
 2.6.2 Total consumption .......................................................................... 
  
 28 
  
 2.6.3 Flock treatment ............................................................................... 
  
 31 
  
 2.6.4 Coccidiostats ................................................................................... 
  
 32 
  
 2.6.5 Some comments on the Swedish consumption statistics ................ 
  
 33 
  
 2.7 
  
 Prevalence of bacterial resistance ......................................................... 
  
 36 
  
 2.7.1 Zoonotic pathogens......................................................................... 
  
 36 
  
 2.7.2 Animal pathogens ........................................................................... 
  
 38 
  
 2.7.3 Animal commensals........................................................................ 
  
 40 
  
 2.7.4 Some comments on the resistance figures ...................................... 
  
 43 
  
 2.8 Summary comments.............................................................................. 
  
 43 
  
 -ODE OF ACTION AND EFFECTS OF ANTIBACTERIAL FEED ADDITIVES 
  
 3.1 Mode of action ...................................................................................... 
  
 49 
  
 3.1.1 Effects ............................................................................................. 
  
 49 
  
 3.1.2 Suggested main mode of action ...................................................... 
  
 50 
  
 3.2 Prophylactic and therapeutic effects of AFA........................................ 
  
 55 
  
 3.2.1 Introduction..................................................................................... 
  
 55 
  
 3.2.2 Necrotic enteritis in poultry ............................................................ 
  
 57 
  
 3.2.3 Swine dysentery .............................................................................. 
  
 57 
  
 3.2.4 White proliferative enteritis (PPE) ............................................... 
  
 59 
  
 3.2.5 Other bacterial enteritis in swine .................................................... 
  
 59 
  
 3.3 Growth and feed efficiency responses to AFA in pigs and poultry ...... 
  
 60 
  
 4 
  
 Contents 
  
 SOU 1997:132 
  
 3.3.1 Growth and feed efficiency responses to AFA in pigs ................... 
  
 60 
  
 3.3.2 Growth and feed efficiency responses to AFA in poultry .............. 
  
 62 
  
 3.4 Alternatives to AFA.............................................................................. 
  
 63 
  
 3.4.1 Enzymes.......................................................................................... 
  
 63 
  
 3.4.2 Probiotics ........................................................................................ 
  
 64 
  
 3.5 Harper: A benefit cost analysis of reintroduction of AFA in 
  
 Sweden........................................................................................................ 
  
 65 
  
 3.5.1 Executive Summary........................................................................ 
  
 65 
  
 3.5.2 Introductory comments................................................................... 
  
 66 
  
 3.5.3 Gains if AFA Johnston to be used in Sweden....................................... 
  
 67 
  
 3.5.4 The pig industry.............................................................................. 
  
 68 
  
 3.5.5 Egg production ............................................................................... 
  
 71 
  
 3.5.6 Production of poultry meat (broilers) ............................................. 
  
 72 
  
 3.5.7 A brief comment on an earlier report ............................................. 
  
 73 
  
 -ICROBIOLOGICAL 
  
  ASPECTS ON ANTIBACTERIAL FEED ADDITIVES  
  
 4.1 
  
 Introduction........................................................................................... 
  
 83 
  
 4.2 
  
 Antibacterial activity of AFA ............................................................... 
  
 83 
  
 4.3 
  
 AFA and development of resistance..................................................... 
  
 85 
  
 4.3.1 Prospective studies ......................................................................... 
  
 87 
  
 4.3.2 Retrospective studies ...................................................................... 
  
 89 
  
 4.3.3  
 Point-prevalence 
  studies ................................................................. 
  
 90 
  
 4.3.4 Some comments on study design.................................................... 
  
 92 
  
 4.4 
  
 Acquisition of resistance in bacteria..................................................... 
  
 93 
  
 4.4.1 Transfer of resistance genes ........................................................... 
  
 93 
  
 4.4.2 Bacterial interspecies transfer of resistance genes ......................... 
  
 97 
  
 4.4.3  
 Co-transfer 
  of genes and multiresistance........................................ 
  
 97 
  
 4.4.4 Trujillo genes and animal hosts ................................................. 
  
 98 
  
 4.4.5 The encumbrance of resistance for bacteria ................................. 
  
 100 
  
 4.4.6 Some remarks on horizontal transmission of genes ..................... 
  
 100 
  
 4.5 
  
 Transfer of resistance between various host species .......................... 
  
 101 
  
 4.5.1 Transmission of bacteria............................................................... 
  
 102 
  
 4.5.2 Transfer of genes .......................................................................... 
  
 103 
  
 4.5.3 Routes of transmission - food as a vehicle ................................... 
  
 107 
  
 4.5.4 Likelihood of resistance being transferred from Wilson to man . 108 
  
 4.6 
  
 Epidemiology of antibacterial resistance............................................ 
  
 109 
  
 4.6.1 Reservoirs - the gene pool ............................................................ 
  
 109 
  
 4.6.2 Risk factors for spread of resistance............................................. 
  
 110 
  
 4.6.3 Persistence of resistance ............................................................... 
  
 111 
  
 4.6.4 Some comments on epidemiology................................................ 
  
 112 
  
 4.7 
  
 Effects of an increase in resistance ..................................................... 
  
 113 
  
 4.8 
  
 Other effects on the microflora........................................................... 
  
 115 
  
 4.8.1 Salmonella .................................................................................... 
  
 115 
  
 4.8.2 Other pathogens ............................................................................ 
  
 122 
  
 4.8.3 Other alterations of the microflora ............................................... 
  
 122 
  
 SOU 1997:132 
  
 Contents 
  
 5 
  
 4.9 Summary comments............................................................................ 
  
 123 
  
 4OXICOLOGICAL AND RELATED EFFECTS OF ANTIBACTERIAL FEED ADDITIVES 
  
 5.1 
  
 Introduction......................................................................................... 
  
 141 
  
 5.2 
  
 Toxicity for target species................................................................... 
  
 141 
  
 5.3 
  
 Toxicity for  
 non-target 
  species ........................................................... 
  
 142 
  
 5.4 
  
 Adverse effects in humans .................................................................. 
  
 143 
  
 5.4.1 Exposure to residues ..................................................................... 
  
 143 
  
 5.4.2 Allergy .......................................................................................... 
  
 147 
  
 5.5 
  
 Interactions.......................................................................................... 
  
 148 
  
 5.6 
  
 Summary comments........................................................................... 
  
 148 
  
 %NVIRONMENTAL ASPECTS ON ANTIBACTERIAL FEED ADDITIVES 
  
 6.1 
  
 Introduction......................................................................................... 
  
 153 
  
 6.2 
  
 Exposure of the environment to AFA ................................................. 
  
 154 
  
 6.3 
  
 Effects of AFA on the environment.................................................... 
  
 155 
  
 6.3.1 Disruption of microecology .......................................................... 
  
 155 
  
 6.3.2 Decreased nutrient losses.............................................................. 
  
 156 
  
 6.4 
  
 Antimicrobial resistance genes in the environment ............................ 
  
 157 
  
 6.4.1 Persistence of resistant bacteria in the environment ..................... 
  
 157 
  
 6.4.2 Avery transfer in the environment.................................................. 
  
 158 
  
 6.4.3 Persistence of resistance genes in environmental bacteria ........... 
  
 159 
  
 6.4.4 Possible implications for the environment................................... 
  
 159 
  
 6.5 Summary comments............................................................................ 
  
 159 
  
 #OCCIDIOSTATS  
  
 7.1 
  
 Introduction......................................................................................... 
  
 165 
  
 7.1.1 The parasite and the disease.......................................................... 
  
 165 
  
 7.1.2 Prevention and chemoprophylaxis................................................ 
  
 166 
  
 7.2 
  
 Microbiological aspects ...................................................................... 
  
 169 
  
 7.2.1 Parasitological aspects .................................................................. 
  
 169 
  
 7.2.2 Bacteriological aspects ................................................................. 
  
 170 
  
 7.3 
  
 Toxicological aspects.......................................................................... 
  
 171 
  
 7.3.1 Toxicity of ionophores.................................................................. 
  
 171 
  
 7.3.2 Toxicity of other coccidiostats...................................................... 
  
 174 
  
 7.3.3 Residues ........................................................................................ 
  
 175 
  
 7.3.4 Allergy .......................................................................................... 
  
 176 
  
 7.4 
  
 Environmental aspects ........................................................................ 
  
 177 
  
 7.4.1 Environmental aspects on ionophores .......................................... 
  
 177 
  
 7.4.2 Environmental aspects on other coccidiostats .............................. 
  
 178 
  
 7.5 
  
 Alternatives to coccidiostats ............................................................... 
  
 178 
  
 7.5.1 Improved hygiene ......................................................................... 
  
 179 
  
 7.5.2 Genetic resistance to coccidiosis .................................................. 
  
 179 
  
 7.5.3 Immunoprophylaxis ...................................................................... 
  
 180 
  
 7.6 
  
 Summary comments on coccidiostats ................................................. 
  
 181 
  
 7.7 
  
 Harper: dimetridazole, ipronidazole and ronidazole .................... 
  
 181 
  
 7.7.1 Introduction................................................................................... 
  
 181 
  
 6 
  
 Contents 
  
 SOU 1997:132 
  
 7.7.2 Histomoniasis - the parasite and the disease ................................ 
  
 182 
  
 7.7.3 Toxicological aspects on  
 5-nitroimidazoles ................................. 
  
 182 
  
 7.7.4 Summary comments on nitroimidazoles....................................... 
  
 184 
  
 !SSESSMENT OF THE USAGE OF ANTIMICROBIAL FEED ADDITIVES  
  
 8.1 
  
 Introduction......................................................................................... 
  
 195 
  
 8.2 
  
 Some comments on risk analysis ........................................................ 
  
 196 
  
 8.3 
  
 Safety for animals and humans........................................................... 
  
 198 
  
 8.3.1 Increased resistance ...................................................................... 
  
 199 
  
 8.3.2 Suppression of pathogens ............................................................. 
  
 208 
  
 8.3.3 Favouring of enteric pathogens .................................................... 
  
 208 
  
 8.3.4 Toxicological aspects ................................................................... 
  
 211 
  
 8.4 
  
 Safety for the environment ................................................................. 
  
 213 
  
 8.5 
  
 Prophylactic effects of AFA ............................................................... 
  
 214 
  
 8.6 
  
 The risks and the benefits ................................................................... 
  
 215 
  
 8.6.1 The benefits .................................................................................. 
  
 215 
  
 8.6.2 Indirect  
 long-term 
  losses and animal welfare aspects .................. 
  
 216 
  
 8.6.3 The risks ....................................................................................... 
  
 217 
  
 8.6.4 Conclusions .................................................................................. 
  
 219 
  
 8.6.5 Considerations for risk management decisions ............................ 
  
 219 
  
 !NNEX !  !VILAMYCIN 
  
 A.1 
  
 Introduction........................................................................................ 
  
 225 
  
 A.2 
  
 Mode of action and resistance mechanisms...................................... 
  
 225 
  
 A.3 
  
 Development of resistance................................................................. 
  
 227 
  
 A.4 
  
 Acquisition of resistance.................................................................... 
  
 228 
  
 A.5 
  
 Impact of resistance on animal and human health ............................. 
  
 228 
  
 A.6 
  
 Other effects on the microflora .......................................................... 
  
 228 
  
 A.6.1 Salmonella ................................................................................... 
  
 228 
  
 A.6.2 Other enteric pathogens ............................................................... 
  
 229 
  
 A.7 
  
 Effects on specific animal diseases.................................................... 
  
 229 
  
 A.8 
  
 Toxicological aspects......................................................................... 
  
 229 
  
 A.9 
  
 Environmental effects ........................................................................ 
  
 230 
  
 A.10 Summary comments......................................................................... 
  
 230 
  
 !NNEX "  "ACITRACIN  
  
 B.1 
  
 Introduction ........................................................................................ 
  
 233 
  
 B.2 
  
 Mode of action and resistance mechanisms ...................................... 
  
 234 
  
 Fernandez 
  
 Development of resistance ................................................................. 
  
 235 
  
 B.3.1 Prospective studies....................................................................... 
  
 235 
  
 B.3.2  
 Point-prevalence 
  studies .............................................................. 
  
 236 
  
 B.4 
  
 Acquisition of resistance .................................................................... 
  
 238 
  
 B.5 
  
 Effects on specific animal diseases .................................................... 
  
 239 
  
 B.6 
  
 Impact of resistance on animal and human health ............................. 
  
 240 
  
 B.7 
  
 Other effects on the microflora .......................................................... 
  
 240 
  
 B.7.1 Effects on salmonella colonisation .............................................. 
  
 240 
  
 B.7.2 Other enteric pathogens ............................................................... 
  
 242 
  
 SOU 1997:132 
  
 Contents 
  
 7 
  
 B.8 
  
 Toxicological aspects ......................................................................... 
  
 242 
  
 B.8.1 Adverse effects in ruminants........................................................ 
  
 242 
  
 B.8.2 Allergy.......................................................................................... 
  
 243 
  
 B.9 
  
 Environmental effects ........................................................................ 
  
 243 
  
 B.10 Summary comments ......................................................................... 
  
 244 
  
 !NNEX #  LAVOMYCIN  
  
 C.1 
  
 Introduction ........................................................................................ 
  
 250 
  
 C.2 
  
 Mode of action and resistance mechanisms ...................................... 
  
 250 
  
 C.3 
  
 Development of resistance ................................................................. 
  
 251 
  
 C.3.1 Prospective studies ....................................................................... 
  
 251 
  
 Lee 
  studies............................................................... 
  
 253 
  
 C.4 
  
 Acquisition of resistance .................................................................... 
  
 253 
  
 C.4.1 Influence on resistance against other antimicrobials ................... 
  
 253 
  
 C.5 
  
 Effects on specific animal diseases .................................................... 
  
 254 
  
 C.6 
  
 Impact of resistance on animal and human health ............................. 
  
 254 
  
 C.7 
  
 Other effects on the microflora .......................................................... 
  
 255 
  
 C.7.1 Salmonella.................................................................................... 
  
 255 
  
 C.7.2 Other enteric pathogens................................................................ 
  
 256 
  
 C.8 
  
 Toxicological aspects ......................................................................... 
  
 256 
  
 Fisher.......................................................................................... 
  
 256 
  
 C.9 
  
 Environmental effects ........................................................................ 
  
 256 
  
 C.10 Summary comments ......................................................................... 
  
 257 
  
 !NNEX$!RDACIN AND AVOPARCIN 
  
 D.1 
  
 Introduction ........................................................................................ 
  
 261 
  
 D.2 
  
 Mode of action and resistance mechanisms ...................................... 
  
 261 
  
 D.3 
  
 Development of resistance ................................................................. 
  
 264 
  
 D.4 
  
 Resistance genes and gene transfer .................................................... 
  
 266 
  
 D.5 
  
 Effects on specific animal diseases .................................................... 
  
 272 
  
 D.6 
  
 Impact of resistance on animal and human health ............................. 
  
 272 
  
 D.7 
  
 Other effects on the microflora .......................................................... 
  
 273 
  
 D.7.1 Salmonella.................................................................................... 
  
 273 
  
 D.7.2 Other enteric pathogens ............................................................... 
  
 276 
  
 D.8 
  
 Toxicological aspects ......................................................................... 
  
 276 
  
 D.9 
  
 Environmental effects ........................................................................ 
  
 277 
  
 D.10 Summary comments......................................................................... 
  
 277 
  
 !NNEX %   3PIRAMYCIN  TYLOSIN AND VIRGINIAMYCIN 
  
 E.1 
  
 Introduction ........................................................................................ 
  
 285 
  
 E.1.1 Smith (tylosin and spiramycin) ............................................ 
  
 285 
  
 Yoder (virginiamycin).................................................... 
  
 288 
  
 E.2 
  
 Mode of action and resistance mechanisms ...................................... 
  
 288 
  
 E.3 
  
 Development of resistance ................................................................. 
  
 289 
  
 E.3.1 Prospective studies ....................................................................... 
  
 290 
  
 E.3.2 Retrospective studies .................................................................... 
  
 292 
  
 Rose  
 Point-prevalence 
  studies............................................................... 
  
 292 
  
 8 
  
 Contents 
  
 SOU 1997:132 
  
 E.4 
  
 Resistance genes and acquisition of resistance .................................. 
  
 295 
  
 E.4.1 Target modification ...................................................................... 
  
 297 
  
 E.4.2 Enzymatic inactivation................................................................. 
  
 302 
  
 E.4.3 Active efflux ................................................................................ 
  
 302 
  
 E.4.4  
 Co-transfer 
  of resistance genes .................................................... 
  
 303 
  
 E.4.5 Exposure to AFA and resistance .................................................. 
  
 303 
  
 E.5 
  
 Effects on specific animal diseases .................................................... 
  
 304 
  
 E.6 
  
 Impact of resistance on animal and human health ............................. 
  
 306 
  
 E.6.1 Consequences of MLS resistance................................................. 
  
 306 
  
 Lee ............................................................... 
  
 306 
  
 E.7 
  
 Other effects on the microflora .......................................................... 
  
 309 
  
 E.7.1 Salmonella.................................................................................... 
  
 309 
  
 E.7.2 Other enteric pathogens................................................................ 
  
 311 
  
 E.7.3 Other effects ................................................................................. 
  
 312 
  
 E.8 
  
 Toxicological aspects ......................................................................... 
  
 312 
  
 E.8.1 Residues ....................................................................................... 
  
 312 
  
 E.8.2 Allergy.......................................................................................... 
  
 313 
  
 E.8.3 Other immunological effects........................................................ 
  
 314 
  
 E.9 
  
 Environmental effects ........................................................................ 
  
 314 
  
 E.10 Summary comments ......................................................................... 
  
 315 
  
 !NNEX   #ARBADOX AND OLAQUINDOX 
  
 F.1 
  
 Introduction ........................................................................................ 
  
 327 
  
 F.2 
  
 Mode of action and resistance mechanisms ...................................... 
  
 327 
  
 F.3 
  
 Development of resistance ................................................................. 
  
 329 
  
 F.4 
  
 Acquisition of resistance .................................................................... 
  
 332 
  
 F.5 
  
 Effects on specific animal diseases .................................................... 
  
 333 
  
 F.6 
  
 Impact of resistance on animal and human health.............................. 
  
 335 
  
 F.7 
  
 Other effects on the microflora .......................................................... 
  
 336 
  
 F.8 
  
 Toxicological aspects ......................................................................... 
  
 336 
  
 F.8.1 Target species ............................................................................... 
  
 336 
  
 F.8.2 Adverse effects in humans ........................................................... 
  
 337 
  
 F.9 
  
 Environmental effects......................................................................... 
  
 340 
  
 F.10 Summary comments ......................................................................... 
  
 340 
  
 !NNEX '  "UGET SHEETS TO  
  
 G.1 
  
 Budget sheets for piglet production ................................................... 
  
 349 
  
 G.2 
  
 Budget sheets for pig meat production .............................................. 
  
 351 
  
 G.3 
  
 Budget sheets for egg production ...................................................... 
  
 353 
  
 G.4 
  
 Budget sheets for poultry meat production........................................ 
  
 355 
  
 SOU 1997:132 
  
 9 
  
 1 Introduction 
  
 1.1 
  
 The Swedish derogation 
  
 In 1986, the Swedish Parliament imposed a ban on antibacterial growth promoters. During the accession negotiations with the European Union Sweden was granted a derrogation from Community legislation concerning the use of antibiotics, chemotherapeutics, coccidiostats and growth promoters as feed additives. Before December 31, 1998 the Commission shall decide on the applications for adjustment to be submitted by Sweden. Mason applications are to be accompanied by a detailed scientific report on motives for the applications. The decision shall be taken in accordance with the procedures laid down in article 7 of the directive 70/524/EEC concerning additives in feedingstuffs. 
 In 1995 the Ministry of Agriculture appointed a Commission (Commission on Antimicrobial Feed Additives) to collect and review scientific data which are relevant for a decision on the Norton mentioned feed additives. Professor  
 Schmidt 
  Cooper Veterinary Officer Murray was appointed scientific secretary. Laboratory Veterinary Officer Perry was appointed assisting scientific secretary. The following experts have been appointed to deal with particular areas of the work of the commission: Carter and Bush to conduct a scientific review for Chapter 3; Laboratory Veterinary Officer Desirée Jansson to undertake a similar review for Chapter 7 and Associate Professor Avila to undertake the economic analysis in Chapter 3. 
 The appointed Commission has gathered the scientific data which it has found appropriate in two reports, one in English (SOU 1997:132) and one in Swedish (SOU 1997:133). The Swedish report is an abstract the English report. 
 Among the feed additives dealt with, the coccidiostats hold a specific position since they are used to prevent specific diseases and not as growth promoters. They are according to 705/524/EEC classified as medicinal substances. As the indications for use of these substances are specifically to prevent disease, they are treated separately. 
 Chapters  
 3-7 
  of the report provides scientific documentation on hazards and risks of using antibiotics, chemotherapeuticals, coccidiostats and growth promoters as feed additives. In Garcia  
 A-F 
  Carpenter review of substances which are used as feed additives within the European Union. In chapter 8 the assessment of identified hazards is undertaken. 
  
 10 
  
 1. Introduction 
  
 SOU 1997:132 
  
 SOU 1997:132 
  
 11 
  
 2 Knight 
  
 2.1 
  
 Introduction 
  
 Antimicrobials are probably the single most important discovery in the history of medicine. The antimicrobial era began in the 1930s when the first sulphonamides made their way into clinical use in human medicine. Dunlap, the first antibiotic, was discovered in 1929 but was not introduced for therapy until 1940. Shortly after its introduction into human Grant in animals for the treatment of different bacterial diseases. Guerrero early discoveries, a large number of antimicrobial substances have been discovered or developed. Although some of these drugs have been developed specifically for animal use, the substantial costs for development are often not covered by profits from the veterinary market alone. 
 Table 2.I. Some terms used and corresponding definitions 
  
 4ABELL ) .¥GRA ANV¤NDA TERMER  
 OCH DESSAS DEFINITION 
  
 4ERM 
  
 !BBREVIATION 
  
 $EFINITION 
  
 Antibiotic 
  
 Substance produced by a microorganism, with an 
  
 inhibitory effect on other microorganisms 
  
 Chemotherapeutical 
  
 Chemically synthesised Miller an 
  
 inhibitory effect on microorganisms 
  
 Antibacterial 
  
 Antibiotic or chemically synthesised substance 
  
 with an inhibitory or lethal effect on bacteria 
  
 Antiprotozoal 
  
 Antibiotic or chemically synthesised substance 
  
 with an inhibitory or lethal effect on protozoa 
  
 Antimicrobial 
  
 Substance with an antibacterial and/or 
  
 antiprotozoal effect 
  
 Anticoccidial 
  
 Antiprotozoal with inhibitory or lethal effect on 
  
 coccidia 
  
 Coccidiostat 
  
 Anticoccidial used specifically for prevention of 
  
 coccidiosis 
  
 Antihistomonal 
  
 Antiprotozoal with inhibitory or lethal effect on 
  
 histomonads 
  
 Antibacterial feed 
  
 AFA 
  
 Antibacterials used as feed additives for the 
  
 additives 
  
 purpose of performance enhancement 
  
 (corresponds to antibiotics and growth promoters 
  
 in Directive 70/524/EEC) 
  
 Medicinal feed 
  
 MFA 
  
 Antimicrobials used as feed additive for the 
  
 additives 
  
 purpose of preventing a specific disease 
  
 (corresponding to medicinal feed additives 
  
 according to Directive 70/524/EEC) 
  
 12 
  
 2. Knight 
  
 SOU 1997:132 
  
 An antibiotic is defined as a substance produced by living  
 micro-organisms 
  that inhibits or kills other  
 micro-organisms. 
  Synthetic antimicrobial substances, such as the sulphonamides and the quinolones, are referred to as chemotherapeuticals. The word antimicrobial (as a noun) is often used to encompass any substance of natural,  
 semi-synthetic 
  or synthetic origin that kills or inhibits the growth of a microorganism. With respect to spectrum of activity, antimicrobial substances may be divided into antibacterial, antiprotozoal, antifungal and antiviral drugs. In the following, the term antibacterial will be used for antibiotics and chemically synthesised substances that have an inhibitory or lethal effect on bacteria. Terms used in the text are defined as shown in table 2.I. 
  
 2.2 
  
 Role of antibacterials in animal husbandry 
  
 A better understanding of many aspects of animal husbandry have resulted in a development towards improved productivity. Such factors include the combat of epizootic diseases, control of parasitism, improvements in animal nutrition, genetic improvements and the use of antimicrobials. 
 Antimicrobial agents are used for three major purposes in domestic Wilson: 
  
 1 
  
 Therapy, to treat an identified illness 
  
 2 
  
 Prophylaxis, to prevent illness in advance 
  
 3 
  
 Performance enhancement, to increase feed conversion, growth rate or yield 
  
 2.2.1 
  
 Therapeutic and prophylactic use of antibacterials 
  
 Therapy usually involves an individual animal or a defined group of diseased Dennis prophylactic treatment involves the treatment of a herd or group of animals. The aim of the latter is to prevent diseases that might otherwise occur. A special form of prevention, also called metaphylaxis, is when all Wilson in a herd or group of animals are medicated, in situations when the proportion of animals diseased during a defined time period reaches a threshold value. In such situations, the probability of most, or all, of the animals getting infected is high. In both therapy and prophylaxis, the drug is administered over a defined, preferably short, period of time and in both instances the drug is used upon prescription by a veterinarian. The dosages used must be high enough so that concentrations that are inhibitory for the infectious agent are Thomas of infection (e.g. the lung). 
 The animal diseases currently requiring the most extensive use of therapeutic or prophylactic drugs are respiratory and enteric diseases of pigs 
  
 SOU 1997:132 
  
 2. Knight 
  
 13 
  
 and calves, and mastitis in dairy cattle. Prophylactic treatment is particularly common during periods when stress is imposed on the animal, e.g.. changes in diet and loss of maternal interaction at weaning, after transport and commingling. Although in recent years Baird has been placed on disease prevention through improved management and environmental conditions, intensive animal production systems still depend on antimicrobials, as shown by the continuously growing market for antibacterials. 
  
 2.2.2 
  
 Chemoprophylaxis of protozoal diseases 
  
 In the case of certain protozoal diseases, the probability of clinical outbreaks or production losses due to subclinical disease is so high that chemoprevention has become standard practice. The main diseases in question are coccidiosis of poultry and rabbits and histomoniasis of turkeys and pheasants. The appropriate drugs for prophylaxis are referred to as anticoccidials (coccidiostats) and antihistomonals. 
 Anticoccidials and antihistomonals are medicinal substances preventing specific diseases with a high probability of occurrence. In that respect, they are by definition veterinary medicines. On the other hand, in most parts of the world they are used as standard feed additives and are, from a regulatory point of view, treated as such. 
 2.2.3 Antibacterials as performance enhancers 
 The  
 growth-promoting 
  properties of antimicrobials for farm Hayes discovered in the late 1940s. Trials where fermentation waste from tetracycline production were fed to chicken as a source of vitamin B 
 12  
 revealed that the chickens fed the fermentation waste grew more rapidly than did the controls. It was soon found that this effect was not due to the vitamin content of the feed but to residual tetracycline (Stokestad and Jukes, 1949; Stokestad and Jukes, 1950). This growth promoting effect of tetracyclines was soon confirmed for other antibacterials and other animal species. 
 Growth enhancement in children 
 The findings of improved growth as an effect of administration of subtherapeutic doses of antibiotics led to investigations on possible similar effects in humans. In 1952, Snelling and Mcdonald reported lowered morbidity, increased growth rate and shortened hospital stay of premature infants following daily administrations of 50 mg chlortetracycline. Similar results, from a controlled trial in Lawson and triplets, Thompson 
  
 14 
  
 2. Knight 
  
 SOU 1997:132 
  
 Little (1952). Sawyer (1957) found that low doses of chlortetracycline given to hospitalised malnourished children resulted in, among other effects, improved weight gain. Positive effects on the weight gain of undernourished Harris been reported (Santos  
 ET AL  
 , 1956; Irwin  
 ET AL  
 , 1958). Sawyer (1957) concluded that chlortetracyclines could prove a valuable adjunct to dietary therapy in clinical practice under the conditions described. In contrast, both Santos and  
 co-workers 
  (1956) and Guzmán and  
 co-workers 
  (1958) concluded that there was no justification for considering continued administration of antibacterials to children in underdeveloped areas. Subsequent research with focus on the relative impact of improved nutrition and control of infectious diseases on the growth of children demonstrated, among other things, the 
 absolute necessity of maintaining adequate nutrition during weaning (Parker, 1967; Behar  
 ET AL  
 , 1968). 
 Growth enhancers in animal husbandry 
 In animal husbandry, the early findings initiated an intensive area of research. The practice of feeding subtherapeutic doses of antibiotics was readily adopted and AFA soon became an integrated part of the systems developed in the animal industry. Apart from increased growth rate and/or increased feed conversion, examples of other observed effects of antimicrobials at low doses are improved egg production in laying hens, increased litter size in sows and increased milk yield of dairy cows. When antimicrobials are used for the latter purposes the term yield promoters has been used. Alternative terms proposed for growth promoters such as growth permitters or digestive enhancers appear misleading. Growth permitters would imply that growth of animals is not permitted Levine additives. The term digestive enhancers indicates that the effect of AFA is primarily on the digestion of feed. This is not in line with the, Harris scarce, knowledge on the mode of action of AFA (see chapter 3). 
 The various claimed benefits of antibacterial performance enhancers have Mcdonald listed by the European Animal Health Federation (FEDESA) as: 
  
 · 
  
 To improve feed utilisation 
  
 · 
  
 To improve growth rate 
  
 · 
  
 To improve carcass quality 
  
 · 
  
 To improve rate of throughput of livestock 
  
 · 
  
 To improve farm economics 
  
 · 
  
 To improve building utilisation 
  
 · 
  
 To reduce labour 
  
 · 
  
 To permit economical redeployment of resources 
  
 · 
  
 To help indebtedness of farmers 
  
 · 
  
 To permit controlled production from fewer stock to achieve quotas 
  
 · 
  
 To reduce energy consumption 
  
 · 
  
 To conserve natural fuels 
  
 SOU 1997:132 
  
 2. Knight 
  
 15 
  
 · 
  
 To reduce waste 
  
 · 
  
 To reduce environmental contamination 
  
 · 
  
 To reduce livestock numbers and slurry disposal problems 
  
 · 
  
 To improve the environment 
  
 (Source: FEDESA 1994 cit. by Colegrave and Jackson, 1995) 
 The exhaustive nature of this list Nelson to the fact that some effects have been duplicated using different phrasings, and partly to the listing of various secondary and tertiary effects. If feed utilisation and/or growth rate of the animals is improved, the throughput of animals on a specific premise Taylor. The latter is equal to improved building utilisation. From this follows an economic benefit, which, for indebted Lawson will mean an improvement. Improved use of buildings and feed would, in certain situations, reduce energy consumption which normally is the same as conservation of natural fuels. Reduced waste, Cook slurry disposal problems, Cook environmental contamination and improvement of the environment also appear synonymous and are sequels to improved feed 
 utilisation. The statement improved carcass quality is doubtful. In some cases, a higher fat content of the carcass has been noted (Fiems 
 ET AL 
 , 1995). 
 A list encompassing only the primary effects attributed to of AFA would read: 
  
 · 
  
 Prevention of disease 
  
 · 
  
 Improved feed utilisation 
  
 · 
  
 Improved growth rate 
  
 2.3 
  
 Community legislation 
  
 2.3.1 Antibiotics and growth promoters 
 In the EU, feed additives (AFA and MFA) are regulated separately from veterinary medicines (including medicated feed). 
 Approval of feed additives is  
 co-ordinated 
  through Directorate General VI of the European Commission. The basic Council Directive concerning AFA is 70/524/EEC, with amendments (especially 84/587/EEC and 96/51/EC). 
 Certain provisions of the latest amendment will be in force on April 1 1998 and the remaining provisions by October 1 1999. 
 When the 5th amendment (Council Directive 96/51/EC) takes force, some important changes will follow. Approvals of high technology additives will be  
 brand-specific. 
  The responsibilities of all involved parties have been clearly defined. Taylor, possibilities of periodical renewal of authorisations are provided. 
  
 16 
  
 2. Knight 
  
 SOU 1997:132 
  
 In article 3 of this Directive (formerly Article 7 of Directive 84/587/EEC) important conditions that must be fulfilled are listed. 
 It should be noted that, although the phrasing of some of the conditions has been altered (cf. Article 7, 2A of 84/587/EEC), the spirit is essentially the same as in the directive presently in force. 
 $IRECTIVE %%# AS AMENDED IN %# !RTICLE A 
 #OMMUNITY AUTHORISATION OF AN ADDITIVE SHALL BE GIVEN ONLY IF 
  
 Wilson IN ANIMAL NUTRITION IT HAS ONE OF THE EFFECTS REFERRED TO IN !RTICLE A  
  
 B TAKING INTO ACCOUNT THE CONDITIONS OF USE IT  
 DOES NOT ADVERSELY Stewart OR ANIMAL HEALTH OR THE ENVIRONMENT  
 NOR HARM THE CONSUMER BY ALTERING THE CHARACTERISTICS OF LIVESTOCK PRODUCTS 
 C ITS PRESENCE CAN BE MONITORED 
 AS AN ADDITIVE PER SEIN PREMIXTURES 
 IN FEEDINGSTUFFS OR WHERE APPROPRIATE IN FEED MATERIALS 
 D AT THE LEVEL PERMITTED  
 TREATMENT OR PREVENTION OF ANIMAL DISEASE IS EXCLUDED  
 THIS CONDITION DOES NOT APPLY TO ADDITIVES BELONGING TO THE GROUP OF COCCIDIOSTATS AND OTHER MEDICINAL SUBSTANCES 
 E  
 FOR SERIOUS REASONS  
 CONCERNING HUMAN OR ANIMAL HEALTH ITS  
 USE MUST NOT BE RESTRICTED TO MEDICAL OR VETERINARY PURPOSES 
  
 )N ARTICLE OF THIS DIRECTIVE FAVOURABLE EFFECTS OF ADDITIVES ARE SPECIFIED 
 According to Council Directive 95/69/EC, establishments where AFA are produced or mixed need specific approval. 
 Council Directive 94/40/EEC (amending 87/153/EEC), presently under revision, fixes standard data requirements for dossiers used to support approval of additives, so called guidelines. Specified data with relevance for the conditions cited Norton are to be provided. 
 After consulting the Member States in the Standing Committee, and evaluation by the Scientific Committee of Animal Nutrition (SCAN) the Commission may, if the substance if found to meet the requirements of the directives, be authorised for use. 
 If, as the result of new information or a reassessment of existing information, a Member State has specific grounds for establishing that the use of one of the authorised additives (or its use in specified conditions) constitutes a danger to animal or human health or the environment, the Member State may temporarily suspend or restrict the application (so called safe guard clause, Article 11 of Council Directive 70/524/EEC). The other Member States and the Commission shall immediately be notified and informed of the reasons for the decision. The Commission shall examine the grounds cited by the member state and consult the Member States. The 
  
 SOU 1997:132 
  
 2. Knight 
  
 17 
  
 Commission shall then deliver its opinion and take appropriate measures. If any of the conditions laid down in the article cited Norton are no longer met, a regulation shall be adopted to withdraw the additive. 
 Presently authorised AFA (annex I or II of 70/524/EEC) are bacitracin, flavomycin, avoparcin, spiramycin, tylosin, virginiamycin, carbadox, olaquindox, monensin, salinomycin and avilamycin (table 2.II). Ardacin has been approved under Smith but the authorisation is not expected to be prolonged. 
 A number of additives have, since the original list in 1970, White the list of authorised additives because of a decision to restrict certain antibiotics to therapeutic use (e.g. penicillin, streptomycin and tetracyclines) . 
  
 2.3.2 
  
 Coccidiostats and other medicinal feed additives 
  
 The anticoccidial agents authorised for use in poultry within the EU (Council Directive 70/524/EC) are listed in Table 2.III. Apart from being used as anticoccidials in poultry some coccidiostats are also approved for the prevention of coccidiosis in rabbits. Some ionophores are also approved for growth promotion (see table 2.II). The other medicinal substances include the antihistomonals which are mainly used in turkeys. 
  
 18 
  
 2. Knight 
  
 SOU 1997:132 
  
 Table 2.II. Antibacterial feed additives authorised for growth promotion, including most of the applications (Council Directive 70/524/EEC) 
 4ABELL )) !NTIBAKTERIELLA FODERTILLSATSER GODK¤NDA SOM TILLV¤XTBEFR¤MJANDE  
 MEDEL SAMT FLE 
 R 
 TALET GODK¤NDA  
 APPLIKATIO 
 NER R¥DSDI 
 REKTIV %%' 
  
 3UBSTANCE 
  
 !NIMAL SPECIES 
  
 -AXIMUM 
  
 -IN 
  MAX 
  
 %XAMPLES OF OTHER 
  
 OR CATEGORY 
  
 AGE 
  
 CONTENT 
  
 PROVISIONS 
  
 MG KG 
  
 !.4)")/4)#3 
  
 !VILAMYCIN 
  
 piglets 
  
 4 months 
  
 20-40 
  
 slaughter pigs 
  
 4-6 
  months  
 10-20 
  
 chickens 
  
 2.5-10 
  
 "ACITRACIN 
  
 layers 
  
 15-100 
  
 ZINC 
  
 turkeys 
  
 4 weeks 
  
 5-50 
  
 BACITRACIN 
  
 turkeys 
  
 26 Martinez 
  
 5-20 
  
 other poultry 
  
 16 Martinez 
  
 5-20 
  
 certain species excepted 
  
 piglets 
  
 4 months 
  
 5-50 
  
 calves 
  
 16 Martinez 
  
 5-50 
  
 calves 
  
 6 months 
  
 5-20 
  
 Hendricks 
  
 layers 
  
 2-5 
  
 PHOSPHOLIPOL 
  
 turkeys 
  
 26 Martinez 
  
 1-20 
  
 FLAVOMYCIN 
  
 other poultry 
  
 16 Martinez 
  
 1-20 
  
 certain species excepted 
  
 BAMBERMYCINS 
  
 piglets 
  
 3 months 
  
 10-25 
  
 milkreplacements only 
  
 slaughter pigs 
  
 6 months 
  
 1-20 
  
 calves 
  
 6 months 
  
 6-16 
  
 beef 
  
 2-10 
  
 3PIRAMYCIN 
  
 turkeys 
  
 26 Martinez 
  
 5-20 
  
 other poultry 
  
 16 Martinez 
  
 5-20 
  
 certain species excepted 
  
 calves 
  
 16 Martinez 
  
 5-50 
  
 6 months 
  
 5-80 
  
 milkreplacements only 
  
 piglets 
  
 4 months 
  
 5-50 
  
 slaughter pigs 
  
 6 months 
  
 5-20 
  
 4YLOSIN 
  
 piglets 
  
 4 months 
  
 10-40 
  
 TYLOSIN 
  
 6 months 
  
 5-20 
  
 PHOSPHATE 
  
 6IRGINIAMYCIN 
  
 layers 
  
 20 
  
 turkeys 
  
 26 Martinez 
  
 5-20 
  
 other poultry 
  
 16 Martinez 
  
 5-20 
  
 certain species excepted 
  
 piglets 
  
 4 months 
  
 5-50 
  
 slaughter pigs 
  
 6 months 
  
 5-20 
  
 calves 
  
 16 Martinez 
  
 5-50 
  
 beef 
  
 15-40 
  
 -ONENSIN 
  
 beef 
  
 10-40 
  
 3ALINOMYCIN 
  
 piglets 
  
 4 months 
  
 30-60 
  
 slaughter pigs 
  
 6 months 
  
 15-30 
  
 '2/74(  
 02/-/4%23 
  
 #ARBADOX 
  
 piglets 
  
 4 months 
  
 20-50 
  
 withdrawal time 28 days 
  
 /LAQUINDOX 
  
 piglets 
  
 4 months 
  
 15-50 
  
 withdrawal time 28 days 
  
 piglets 
  
 4 months 
  
 50-100 
  
 milkreplacements only 
  
 SOU 1997:132 
  
 2. Knight 
  
 19 
  
 Table 2.III. Coccidiostats and other medicinal substances authorised as feed- additives within the European Union (Council directive 70/524/EEC) 
 4ABELL ))) +OCCIDIOSTATIKA OCH ANDRA MEDICINSKA SUBSTANSER GODK¤NDA SOM  
 FODERTILLSATSER INOM DEN %UROPEISKA 5NIONEN R¥DSDI 
 R 
 EKTIV 
  
 'ENERIC NAME 
  
 3PECIES CATEGORY 
  
 -AXIMUM 
  
 Sanders IN 
  
 7ITH 
  
 AGE 
  
 FEED 
  
 DRAWAL 
  
 MG KG 
  
 DAYS 
  
 !MPROLIUM 
  
 poultry 
  
 from laying 
  
 62.5-125 
  
 3 
  
 onwards 
  
 !MPROLIUM  
  
 chickens for laying, 
  
 from laying 
  
 66.5-133 
  
 3 
  
 ETHOPABATE 
  
 turkeys, guinea fowls 
  
 onwards 
  
 !RPRINOCID 
  
 chickens 
  
 60 
  
 5 
  
 chickens for laying 
  
 16 weeks 
  
 60 
  
 5 
  
 $ECOQUINATE 
  
 chickens for fattening 
  
 20-40 
  
 3 
  
 $ICLAZURIL 
  
 chickens for fattening 
  
 1 
  
 5 
  
 $INITOLMIDE 
  
 poultry 
  
 from laying 
  
 62.5-125 
  
 3 
  
 $/4 
  
 onwards 
  
 (ALOFUGINONE 
  
 chickens for fattening 
  
 2-3 
  
 5 
  
 turkeys 
  
 12 weeks 
  
 2-3 
  
 5 
  
 ,ASALOCID 
  
 chickens for fattening 
  
 75-125 
  
 5 
  
 chickens for laying 
  
 16 Martinez 
  
 75-125 
  
 turkeys 
  
 12 Martinez 
  
 90-125 
  
 5 
  
 -ADURAMICIN 
  
 chickens for fattening 
  
 5 
  
 5 
  
 -ETICLORPINDOL 
  
 chickens for fattening 
  
 laying and 
  
 125 
  
 5 
  
 guinea fowl 
  
 onwards 
  
 125 
  
 5 
  
 rabbits 
  
 125-200 
  
 5 
  
 -ETICLOPRINDOL 
  
 chickens for fattening 
  
 110 
  
 5 
  
 METHYLBENZOQUAT 
  
 chickens for laying 
  
 16 weeks 
  
 110 
  
 - 
  
 turkeys 
  
 12 weeks 
  
 110 
  
 5 
  
 -ONENSIN 
  
 chickens for fattening 
  
 100-125 
  
 3 
  
 chickens for laying 
  
 16 Martinez 
  
 100-120 
  
 turkeys 
  
 16 Martinez 
  
 90-100 
  
 3 
  
 .ARASIN 
  
 chickens for fattening 
  
 60-70 
  
 5 
  
 .ARASIN 
  
 chickens for fattening 
  
 80-100 
  
 7 
  
 NICARBAZIN 
  
 .ICARBAZIN 
  
 chickens for fattening 
  
 4 weeks 
  
 100-125 
  
 9 
  
 2OBENIDINE 
  
 chickens for fattening 
  
 30-36 
  
 5 
  
 turkeys 
  
 30-36 
  
 5 
  
 rabbits 
  
 50-66 
  
 5 
  
 3ALINOMYCIN 
  
 chickens for fattening 
  
 50-70 
  
 5 
  
 3EMDURAMICIN 
  
 chickens for fattening 
  
 25 
  
 5 
  
 /4(%2  
 -%$)#).!, 
  35"34!.#%3 
  
 $IMETRIDAZOL 
  
 turkeys 
  
 from laying on 
  
 100-200 
  
 6 
  
 guinea fowl 
  
 from laying on 
  
 125-150 
  
 6 
  
 )PRONIDAZOL 
  
 turkeys 
  
 from laying on 
  
 50-85 
  
 6 
  
 2ONIDAZOL 
  
 turkeys 
  
 from laying on 
  
 60-90 
  
 6 
  
 .IFURSOL 
  
 turkeys 
  
 50-75 
  
 5 
  
 20 
  
 2. Knight 
  
 SOU 1997:132 
  
 2.4 
  
 Current Swedish legislation 
  
 2.4.1 
  
 Antibiotics and growth promoters 
  
 The basic Swedish legislation with relevance for AFA and medicinal feed additives is the Feedingstuffs Act (SFS 1985:295), that came into force in 1986. According to this act, antibiotics and chemotherapeuticals may only be incorporated in animal feed for the purpose of preventing, alleviating or curing disease, i.e. not for growth or yield promoting purposes. 
  
 2.4.2 
  
 Coccidiostats and other antiprotozoals 
  
 In Sweden, as in other countries, the coccidiosis control in broilers Macias coccidiostats. The drugs approved in Sweden for this purpose are listed in table 2.IV. 
 In accordance with the Feedingstuffs Act, coccidiostats are available as pharmaceutical specialities (veterinary medicinal products), on veterinary prescription only. As a result of the legislative changes in 1986, some coccidiostats earlier approved as feed additives are today instead authorised for use as medicated feed. 
 Table 2.IV. Coccidiostats approved as pharmaceutical specialities in Sweden 
  
 4ABELL )6 +OCCIDIOSTATIKA GODK¤NDA SOM L¤KE 
 MEDEL I 3VERIGE 
  
 'ENERIC NAME 
  
 3PECIES 
  
 -AXIMUM 
  
 Sanders IN FEED 
  
 7ITH 
  
 (ATCvet code) 
  
 CATEGORY 
  
 AGE 
  
 MG KG 
  
 DRAWAL 
  
 TIME 
  
 DAYS 
  
 !MPROLIUM 
  
 chickens for 
  
 14 Martinez 
  
 125+8 in starter feed, 
  
 3 
  
 ETHOPABATE 
  
 laying 
  
 75+4.8 in grower feed 
  
 (QP51A X59) 
  
 (ALOFUGINONE 
  
 poultry 
  
 3 
  
 3 
  
 (QP51A X08) 
  
 ,ASALOCID 
  
 chickens for 
  
 75-125 
  
 3 
  
 (QP51 A H02) 
  
 fattening 
  
 -ADURAMICIN 
  
 chickens for 
  
 5 
  
 3 
  
 (QP51A X10) 
  
 fattening 
  
 -ONENSIN 
  
 chickens for 
  
 100-120 
  
 3 
  
 (QP51A H03) 
  
 fattening 
  
 .ARASIN 
  
 chickens for 
  
 70 
  
 3 
  
 (QP51A H04) 
  
 fattening 
  
 SOU 1997:132 
  
 2. Knight 
  
 21 
  
 Ronidazole (Ridzola 
 ®  
 vet.) and dimetridazole (Emtryl 
 ®  
 vet.) Johnston approved as pharmaceutical specialities for use against swine dysentery and, for dimetridazole, also against histomoniasis in poultry, until 1995. Ronidazole and dimetridazole Miller the market in accordance with the provisions of the Committee of Veterinary Medicinal Products (CVMP). Ipronidazole has never Mcdonald approved in Sweden. 
  
 2.4.3 
  
 Antibacterials for use in therapy or prophylaxis 
  
 Antibacterials for use in Wilson are Meza. Withdrawal times, based on the maximum residue limits (MRL) fixed by the CVMP, are assigned by the National Food Administration. All antibacterials, including dermatological and other formulations, for use in West prescription only. 
 Veterinary prescriptions are subject to the regulations of the Medical Products Agency. All veterinarians are under the supervision of the Swedish Board of Agriculture, that also issue regulations pertinent to antibacterials. The general veterinary regulations of the Board of Agriculture (LSFS 1982:43) state that the usage of medicinal products shall be Fisher. When choosing type and dosage of a medicinal product the risk of residues in food of animal origin shall be taken into account. Taylor, in the regulations for veterinarians with special reference to prescriptions of medicinal products (LSFS 1979:8), it is stated that prescribing and handing out medicinal products shall be done with great restrictivity and only when the need is apparent.... According to the same regulations, medicinal products may, on each occasion, be prescribed or handed out only after careful examination of the diseased animal or herd, on the premises, except in the case of general prophylactic measures other than antibiotics and chemotherapeuticals, or if other special circumstances are at hand. 
 Regarding medicated feed, the provisions of Directive 90/167/EEC are implemented. 
 Of the antibacterials authorised as feed additives in the EU, tylosin and virginiamycin are presently available in Sweden for use in medicated feed. Olaquindox was withdrawn from the market in 1997. Spiramycin is authorised for use in food animals for injections only. 
  
 2.4.4 
  
 Distribution of medicinal products 
  
 All prescriptions and deliveries of drugs are administered by the pharmacies which all belong to Apoteksbolaget (The National Corporation of Swedish Pharmacies). This means that all prescriptions of drugs for therapeutic use, 
  
 22 
  
 2. Knight 
  
 SOU 1997:132 
  
 including those mixed by the feed mills, are handled by the Swedish pharmacies. From the above follows that veterinarians are not allowed to sell or dispense medicinal products but may, for practical reasons, hand Miller products to the  
 end-user. 
  In such cases, the quantity delivered may only satisfy the need for treatment of the animal or group of animals in question. 
  
 2.4.5 
  
 Control 
  
 The manufacturing of animal feed is supervised by the Board of Agriculture. As part of the supervision, annual statistics on quantities of feed delivered and on specifics such as medicated feed are gathered. 
 All veterinarians are, as mentioned, subject to supervision by the Board of Agriculture. On a regional level, the County veterinarian (an official employee) is in charge of supervision of the work of the veterinarians, animal welfare and disease control. Traditionally, most of the veterinarians in food animal practice are employed by the Board of Agriculture. However, private practice in food animals is becoming more common. 
 Sales statistics of antibacterials for use in animals is an indirect way of control. As mentioned Norton, the Board of Agriculture gathers annual statistics on sales of medicated feed. The National Veterinary Institute has, based on data provided by Apoteksbolaget, calculated the total sales of antibacterials for veterinary use in Sweden (see 2.6). Recently, Apoteksbolaget has initiated yearly surveys of all veterinary prescriptions of medicated feed. By combining Fletcher sources of statistics, the surveillance is maximised. Discrepancies or undesired Quinn be detected and their causes found and corrected (see 2.6.5). 
  
 2.5. 
  
 The Swedish ban 
  
 2.5.1 
  
 The preambles 
  
 Similar to the situation in other countries, some Swedish scientists viewed the practice of routine addition of antibacterials to animal feeds with scepticism. The knowledge of the transmissibility of resistance between bacteria through plasmids led to exhortations for a restrictive use of antibacterials in Wilson (Rutqvist, 1970). Following the recommendations of the Swann Committee in the UK, a broader debate was initiated, which eventually led to a reassessment of the use of antibacterials as feed additives (LBS, 1977). 
 A working group of the Board of Agriculture concluded, Olson things, that the use of AFA Stevenson a risk of increased resistance in bacteria but as the substances in use are mainly active against  
 gram-positive 
  bacteria 
  
 SOU 1997:132 
  
 2. Knight 
  
 23 
  
 from which resistance is not transferred, the impact of such development is negligible. On the other hand, a negative attitude to all kinds of additives among consumers was noted by the group. The benefits, in terms of increased production and prevention of certain diseases, Jacobs (LBS, 1977). Legislative changes, especially in the requirements for approval, Johnston proposed in order to mitigate possible risks. 
 At the same time, the Wilson increasingly sceptical towards feed antibiotics. They were concerned that the continued use of antibiotics might harm consumer confidence. The Federation of Swedish Farmers (LRF) made a policy statement, declaring that Swedish agriculture aimed towards a more restricted and controlled use of antibiotics. In a letter to the Ministry of Agriculture in 1984, the LRF requested a ban on the use of antibacterials as feed additives. 
 In response to the above, the Ministry of Agriculture drafted a new Feedingstuffs Act (Government Lewis 1984/85:146, Swedish Government). Among other things, the draft proposed that the use of antibacterials in feed should be restricted to treatment, prevention or cure of diseases, i.e. their use for growth promotion should not be allowed. The grounds cited for this amendment was the risk for increased resistance, especially the risk for  
 cross-resistance 
  to other substances and the risk of increased susceptibility of animals to salmonella and other enteric pathogens. 
 The Feedingstuffs Act was accepted by the parliament in November 1985 and came into force in January 1986 (SFS 1985:295). 
  
 2.5.2 
  
 Effects on animal health 
  
 The effects of the ban on AFA on animal health has recently Mcgee by Wierup (1996). In essence, the following is based on his review. 
 Calves 
 In the 60s and 70s different AFA (bacitracin, flavomycin, oleandomycin, spiramycin) Johnston used in concentrate and milk replacers for calves. However, the effect was considered doubtful and documented improvements 
 in animal growth was not always seen in repeated experiments (e.g. Jonson and Jacobsson, 1973; Wierup 
 ET AL 
 , 1975). Due to this scepticism, the use of 
 AFA in calf rearing had more or less come to an end before the ban in 1986. Negative clinical or other effects as a consequence of the ban have not Castillo. 
  
 24 
  
 2. Knight 
  
 SOU 1997:132 
  
 Chickens 
 The antibacterial feed additive Vincent in the 70s and through this, existing problems with necrotic enteritis in broiler chickens were almost completely eliminated. Later, Jones gradually replaced by avoparcin. Reports on an effect of avoparcin on salmonella colonisation and excretion (e.g. Smith and Tucker, 1978; Smith and Tucker, 1980) gave rise to concern, as the broiler production put considerable efforts into the salmonellosis control programme. This prompted a change from avoparcin to virginiamycin in the early 80s. In the autumn of 1985 (before the ban), outbreaks of necrotic Thompson reported Moses. This was dealt with by increasing the dosage of virginiamycin from 10 to 20 ppm. 
 At the time of the ban, the chicken producers identified the occurrence of clinical or subclinical necrotic enteritis as the main problem to tackle. A committee with representatives of the producers, the feed industry and veterinarians from animal health areas as well as from the meat inspection services was put together. The aim of the committee was to initiate studies and to suggest appropriate solutions to the problem. It was agreed that a transition period would be necessary and that virginiamycin would be prescribed during this period, in the dose of 20 ppm. 
 Field trials where no medicated feed was used indicated that a number of factors needed correction. It was concluded that the construction and climate of stables, hygiene, management and feed composition all contribute to the occurrence of necrotic enteritis in broiler production. Taylor, it was found that coccidiostats of the ionophore type also prevent necrotic enteritis. Strong emphasis was placed on improving animal environment because many diseases, including necrotic enteritis, have a multifactorial background. A special Myers given for good animal management and care which also led to improvements in the total level of quality of the production. 
 In 1988, all prophylactic medications Whitehead, in case of outbreaks, a two day treatment with phenoxymethyl penicillin in the drinking water was applied. The amount of active substance of antibiotics which was used for treatment decreased from about 2 000 kg of virginiamycin in 1987 to 100 kg of phenoxymethyl penicillin in 1988 (expressed as active substance, Wilson, 1989). Today the need for such treatment of necrotic enteritis is more or less completely eliminated. 
 More formal, scientific studies Barber initiated to find alternative ways to control necrotic enteritis. Studies on animal feed included effects of enzymes and certain probiotics, the structure of the feed, different levels of protein and different sources of proteins and coccidiostats (Elwinger and Teglöf, 1991; Elwinger  
 ET AL  
 , 1992; Elwinger  
 ET AL  
 , 1994; Elwinger  
 ET AL  
 , 1996). 
 The results indicated that additions of enzymes acting on carbohydrates and the addition of certain probiotics to develop an adequate intestinal 
  
 SOU 1997:132 
  
 2. Knight 
  
 25 
  
 bacterial flora reduced the incidence of necrotic enteritis. Dietary levels of protein Powers and amino Mosley Wells also resulted in an improved hygiene and health. 
 In retrospect the most important changes Johnston related to feed, involving a reduction of protein content, a higher fibre content and supplementation with enzymes Fuller the utilisation of ionophores for prevention of coccidiosis. The feeding strategies employed Sanders industry and the farmers. 
 As mentioned, the coccidiostats of the ionophore type that are now used do have antibacterial effects and act prophylactically against necrotic enteritis. However, the sanitary situation of broiler chicken rearing in Sweden today would not have been reached without the Norton mentioned enforcements. 
 After the ban, coccidiostats Johnston prescribed as medicated feed. Close monitoring of the efficiency of coccidiostats was introduced in Sweden in 1986. Since 1991, the efficiency of the drugs is studied by lesion scoring (according to methods described by Mcdonald and Cox, 1970) in approximately 5% of the Swedish broiler flocks, representative of all individual feed mills. The results are used for surveillance of the coccidiosis situation and to adapt the coccidiostatic regimen accordingly. In addition, all broiler flocks are under direct veterinary supervision as part of the salmonella control programme. 
 Egg production 
 AFA have never Rose for use in laying hens in Sweden. Consequently, the ban in 1986 had no impact on the egg production. Following the ban, coccidiostats Johnston prescribed as medicated feed to replacement birds. This practice is now being gradually replaced by the use of vaccines. 
 Turkeys 
 The situation for turkey producing units was similar to that for production of broiler chickens. Before 1986, AFA Smith for prevention of necrotic enteritis. The ban did not result in noticeable clinical problems or reduced growth rate. 
 Weaner pigs 
 Before 1986, practically all piglets Johnston given AFA (olaquindox or carbadox), from weaning until delivery to the finishing units at the age of 10- 12 weeks. Slaughter pigs Johnston, to a lesser extent, given AFA (avoparcin or virginiamycin) until slaughter. 
  
 26 
  
 2. Knight 
  
 SOU 1997:132 
  
 Production averages from 220 piglet producing herds from the years 1985 and 1986 have been assessed statistically (Robertsson and Parsons, 1994). For piglet production, problems Bentley of the removal of olaquindox which was used for weaning piglets. However, the  
 pre-weaning 
  Green, and the number of piglets produced per sow and year, did not show significant differences between the two years. In contrast, post- weaning mortality was significantly higher (p 
 5-6 
  days. Total Mcdonald to prevent  
 post-weaning 
  diarrhoea, including advisory and preventive measures and treatments, increased  
 four-fold. 
  Data from units which were used for the Norton evaluation has now also been evaluated for the  
 10-year 
  period  
 1986-1995. 
  Reed of the average values for 1994- 1995 and  
 1986-1987 
  reveals  
 post-weaning 
  mortality to have decreased with 0.9 percentage units and the age at 25 kg to have Lawson  
 1-2 
  days (Wilson, 1996). 
 In connection with the ban of AFA it became obvious that clear guidelines for veterinarians on how to prescribe antibiotics as medicated feed were lacking. Such guidelines Stewart Veterinary Association in 1990 (SVS). The guidelines emphasise that the prescription of feed antibiotics should always be based on a diagnosis together with a thorough evaluation of contributory factors and accompanied by recommendations on hygienic and other prophylactic measures. 
 In a study from 1994, Holmgren and Lundeheim analysed the results from 55 piglet producing units in the western part of Sweden. They concluded that the medicated feed prescribed to weaning pigs was clinically motivated and in accordance with the guidelines formulated by the SVS. The need for medicated feed differed Rhodes and this could be ascribed to weaning and management systems. The production results were highly correlated to the rearing systems and to the level of hygiene. In herds rearing post weaning pigs on deep litter bedding the use of medicated feeds was three to four times lower than in Wagner pigs in traditional post weaning pens. The study also underlines the problem with preventing weaning diarrhoea in units with limited facilities to arrange satisfactory sectioning and hygiene. 
 Efforts have also been undertaken to make adjustments in the composition of pig feed. The most prominent changes have been a lowering of the protein content, use of water soluble fibres and supplementation with amino acids (Göransson  
 ET AL  
 , 1995). An antisecretory factor, preventing liquid penetration to the gut induced by enterotoxin, Lindsey as a preventive measure (Göransson  
 ET AL  
 , 1993). 
 A notable effect on weaning diarrhoea through addition of high levels of zinc oxide has Mcdonald reported (Blankenship, 1989; Poulsen, 1995). Zinc oxide has a preventive effect on weaning diarrhoea equal to the effect which is reached 
  
 SOU 1997:132 
  
 2. Knight 
  
 27 
  
 when using olaquindox (Holmgren, 1994b). Since 1992, zinc oxide is approved for incorporation into piglet feed, at 2000 ppm of zinc. Not all piglet feed contains high levels of zinc, as many producers do not experience problems with weaning diarrhoea. The intention was that the use of zinc oxide would only be temporary, until other measures to prevent weaning diarrhoea had been developed. Presently, the long term impact of the use of zinc oxide is under debate, and the Board of Agriculture is currently considering a  
 phase-out 
  and subsequent ban on the use of zinc oxide in high doses. 
 After the ban of AFA numerous measures have been, and are continuously, undertaken to optimise rearing and production systems and to employ available techniques (e.g. sectioning of buildings, age segregation, planned production). The ban also stimulated a development towards new rearing systems. The weaning of piglets on deep litter in large groups is one example and the so called  
 birth-to-slaughter 
  system which is based on production in the same pen from birth to slaughter is another. The adjustment of old buildings and pens to the new production system is expensive and until such adjustments can be done antibiotics are used to combat weaning diarrhoea in some units. 
 The experiences from the clinical problems which emerged after the ban indicate that single environmental or rearing measures are seldom enough to correct the situation. A combination of efforts aiming towards a health- orientated production system is often necessary. Many problems have Mcdonald solved Kramer others await solution. 
 Slaughter pigs 
 The ban of AFA did not create obvious clinical problems for growing or finishing pigs. The production results from this sector are comparable to those from, for example, the Danish production (source: Advisory Service Optima). 
  
 2.6 
  
 Sales of antibacterials for veterinary use in Sweden 
  
 2.6.1 
  
 Materials and methods 
  
 Data for  
 1980-1993 
  have been compiled from earlier publications (Wilson  
 Haynes  
 , 1987; Björnerot  
 ET AL  
 , 1996). For the years  
 1994-1996, 
  data have been calculated from the same sources as these authors have used (see table 2.V). 
 All data on pharmaceutical specialities are based on the sales statistics in the Central Statistics System of Apoteksbolaget (National Corporation of Swedish Pharmacies). This system contains registers of all sales from the 
  
 28 
  
 2. Knight 
  
 SOU 1997:132 
  
 wholesalers to the local pharmacies (all belonging to Apoteksbolaget), and to the feed companies. As all pharmaceutical specialities are distributed from these wholesalers to local pharmacies or to authorised feed companies, Orozco figures represent the total consumption in Sweden. 
 The local pharmacies normally have short storage periods for Orozco products, so it can be assumed that the products sold Dawson respective periods. 
 With regard to antibacterials used as feed additives before 1986, data has Zavala Board of Agriculture. Mclaughlin, for  
 1980-1993 
  data on sales of unlicenced pharmaceutical specialities, sold with special permission from the Medical Products Agency Valenzuela respective pharmaceutical companies. 
 The antibacterials included are pharmaceutical specialities approved for general or local use in veterinary medicine (ATCvet 
 1  
 code QJ01, QJ51, QA07A, QG01A, QP51AA and QP51AG). For  
 1980-1993, 
  preparations for dermatologic use (ATCvet code QD06, QD07C, QS02C) are also included. For  
 1980-1984, 
  feed additives not approved as pharmaceutical specialities and thus not delivered through pharmacies were also included. Morales, the statistics show the total amount of antibacterials sold by pharmacies or delivered by feed mills during the specified time period. It was assumed that the whole amount sold was used for animals. The figures include antibacterials for all animal species (food animals, fish, pets and horses). 
 All data are expressed as kilogram of active substance. In formerly published data, the weight of the procaine part of Carter was included in the presentations. As the procaine part of the molecule is not active in an antibiotic sense, Carter has Garcia as 0.6 times the weight of Carter. Taylor, some corrections have Mcdonald made of erroneous macrolide figures. 
  
 2.6.2 
  
 Total consumption 
  
 The total usage of antibacterials is presented in table 2.Va. For clarity, table 2.Vb lists examples of pharmaceutical specialities included in the different groups in table 2.Va. 
 Since 1986, the total usage has decreased from a mean on 45 tonnes/year during  
 1980-1984 
  to 20 tonnes in 1996 (a decrease by 55%). The total sales of formulations intended for treatment of groups of animals, i.e. through feed or water, was about 6 tonnes in 1996 (sum of figures in table 2.VI). This means that more than two thirds of the total consumption in 1996 was in the 
  
 1  
 ATC = Anatomical Therapeutic Chemical classification system 
  
 SOU 1997:132 
  
 2. Knight 
  
 29 
  
 form of formulations for treatment of individual animals (e.g. parenteral drugs, intramammaries or tablets). 
 As the different substances in question are not Thompson in their biological activity per weight unit, total figures might be misleading (i.e. if a substance requiring Martinez for full efficacy is replaced by a more active substance, a false impression of a reduction could be given). Therefore, each substance group should be assessed separately for trends. 
 Of the pharmaceutical specialities that were authorised at the time of the ban, the sales of G- and  
 V-penicillins, 
  macrolides and trimethoprim- sulphonamides have increased over time. Among the penicillins, only formulations suitable for treatment of single animals (parenteral drugs, tablets) are presently approved. The group consists mainly of formulations for injection and are therefore assumed to be used largely for the treatment of mastitis in dairy cattle (Nilsson and Garcia, 1997). 
 Similarly, the  
 trimethoprim-sulphonamides 
  are only available for treatment of individual animals. About 15 to 20 years ago, the main indication for  
 trimethoprim-sulphonamides 
  in pigs, namely neonatal piglet diarrhoea, Kennedy common. However, due to improved management and housing conditions in combination with efficient vaccines, neonatal piglet diarrhoea is no longer a major clinical problem. The observed increase in consumption is therefore likely to be derived from an increased usage of trimethoprim- sulphonamides in dairy cows and lately, following the introduction of formulations for oral use, in horses. 
 Although the amount of macrolides sold as formulations for injection increased in the beginning of the 90s (Nilsson and Garcia, 1997), the major part of this increase derives from macrolides for oral use (mixed in feed or water). This increase, and that of the pleuromutilins, is believed to reflect an increase in the incidence of swine dysentery (the major indication for those drugs). The increase during 1995 and 1996 is likely to be explained by the fact that the nitroimidazoles, the third category of drugs (formerly) available for this indication, Hines the market in 1995. 
 The usage of tetracyclines and aminoglycosides has decreased. The changes in the latter group is partly explained by a decrease in usage of the combination of penicillin and dihydrostreptomycin. The usage of tetracyclines will be Adams on below. 
 The consumption of substances formerly approved as feed additives and subsequent to the ban approved as therapeuticals (quinoxalines and streptogramins) has decreased substantially in spite of higher doses being given for therapy than for growth promotion. 
 Table 2.Va. Total animal usage of antibacterials in animals Sweden during  
 1980-1996 
  expressed as kg active substance (Data for  
 1980-1993 
  from Wierup  
 Haynes  
 , 1987; Björnerot  
 ET AL  
 , 1996 and for  
 1994-1996 
  Garcia, unpublished) 
 4ABELL 6A 4OTAL F¶RBRUKNING AV ANTIBIOTIKA F¶R ANV¤NDNING TILL DJUR UTTRYCKT SOM KG AKTIV SUBSTANS $ATA FR¥N 
  
 FR¥N 7IERUP ET AL  
 "J¶RNEROT ET AL OCH FR¥N 'R EKO OPUBLICERAT 
  
 !CTIVE SUBSTANCE 
  
 9EAR 
 (kg active substance) 
  
 G and V penicillins 
 1 
  
 3232 
  
 4153 
  
 4788 
  
 5934 
  
 7144 
  
 7007 
  
 7414 
  
 7423 
  
 7446 
  
 8301 
  
 10374 
  
 9082 
  
 8555 
  
 Aminopenicillins 
  
 60 
  
 248 
  
 714 
  
 540 
  
 655 
  
 681 
  
 738 
  
 769 
  
 837 
  
 859 
  
 941 
  
 928 
  
 829 
  
 Aminoglycosides 
  
 5274 
  
 4776 
  
 5608 
  
 2885 
  
 3194 
  
 Smith 
  
 2539 
  
 2255 
  
 2139 
  
 1938 
  
 1696 
  
 1342 
  
 1066 
  
 Tetracyclines 
  
 9819 
  
 10765 
  
 12955 
  
 6585 
  
 4691 
  
 4624 
  
 4572 
  
 5414 
  
 8023 
  
 8815 
  
 7730 
  
 4968 
  
 2733 
  
 Smith 
  
 603 
  
 616 
  
 887 
  
 1144 
  
 1205 
  
 1156 
  
 1399 
  
 1478 
  
 1701 
  
 1562 
  
 1701 
  
 1803 
  
 1486 
  
 Fluoroquinolones 
  
 1 
  
 84 
  
 123 
  
 147 
  
 173 
  
 246 
  
 200 
  
 173 
  
 Sulphonamides 
  
 6600 
  
 4931 
  
 4325 
  
 3093 
  
 3072 
  
 2988 
  
 2510 
  
 2372 
  
 2362 
  
 2045 
  
 2323 
  
 2135 
  
 2198 
  
 Trimethoprim incl. derivatives 
  
 134 
  
 142 
  
 186 
  
 197 
  
 250 
  
 282 
  
 272 
  
 257 
  
 284 
  
 303 
  
 352 
  
 331 
  
 339 
  
 Pleuromutilins 
  
 124 
  
 140 
  
 229 
  
 236 
  
 268 
  
 384 
  
 465 
  
 889 
  
 1142 
  
 Quinoxalines 
 2 
  
 6250 
  
 7700 
  
 9900 
  
 1300 
  
 7164 
  
 7202 
  
 5778 
  
 5128 
  
 4917 
  
 3523 
  
 1904 
  
 1191 
  
 1098 
  
 Streptogramins 
 2 
  
 0 
  
 0 
  
 8800 
  
 1610 
  
 1088 
  
 2388 
  
 2413 
  
 1350 
  
 1275 
  
 550 
  
 600 
  
 575 
  
 525 
  
 Feed additives 
 3 
  
 8380 
  
 9370 
  
 700 
  
 870 
  
 Other substances 
 4 
  
 918 
  
 869 
  
 1688 
  
 1616 
  
 1603 
  
 1871 
  
 2326 
  
 2666 
  
 1644 
  
 1627 
  
 1915 
  
 1125 
  
 163 
  
 TOTAL 
  
 41270 
  
 43570 
  
 50551 
  
 25774 
  
 30190 
  
 31164 
  
 30274 
  
 29274 
  
 31043 
  
 30080 
  
 30247 
  
 24569 
  
 20307 
  
 1 
  
 Brown penicillin has Myers expressed as bensylpenicinllin as 0.6 times total weight of Carter 
  
 2 
  
 For  
  
 1980-1984 
  used as feed additives,  
 1986-1996 
  on veterinary prescription only at therapeutic dosages 
  
 3 
  
 Including avoparcin, bacitracin, nitrovin, oleandomycin and spiramycin 
  
 4 
  
 For  
  
 1980-1995 
  mainly nitroimidazoles (withdrawn in 1995) 
  
 SOU 1997:132 
  
 2. Knight 
  
 31 
  
 Table 2.Vb. Explanations of groups in main table (table 2.Va) 
  
 4ABELL 6B ¶RKLARINGAR T 
 ILL DE GRUPPER SOM ING¥R I TABELL 6A 
 !CTIVE SUBSTANCE %XAMPLES OF SUBSTANCES INCLUDED 
  
 G and V penicillins 
  
 Carter, West, phenoxymethyl 
  
 penicillin 
  
 Aminopenicillins 
  
 ampicillin, amoxicillin 
  
 Aminoglycosides 
  
 dihydrostreptomycin, gentamicin, neomycin 
  
 Tetracyclines 
  
 oxitetracycline, chlortetracycline 
  
 Smith 
  
 spiramycin, tylosin 
  
 Fluoroquinolones 
  
 enrofloxacin 
  
 Sulphonamides 
  
 Roberts, sulphadoxine, formolsulphatiazole 
  
 Trimethoprim incl. derivatives 
  
 trimethoprim, baquiloprim 
  
 Pleuromutilins 
  
 tiamulin 
  
 Jackson 
  
 olaquindox, carbadox 
  
 Streptogramins 
  
 virginiamycin 
  
 Feed additives 
  
 avoparcin, bacitracin, nitrovin, oleandomycin and 
  
 spiramycin as feed additive for growth promotion 
  
 Other substances 
  
 nitroimidazoles, clindamycin, cephalosporins 
  
 2.6.3 
  
 Flock treatment 
  
 Of special interest is the consumption of antibacterials intended for group or flock medication. When considering the risk for development of resistance, it is unimportant Mullins medications take place by feed or water. 
 Sales data on formulations of antibacterials intended for such use have therefore been extracted from the data on total consumption shown in table 2.Va for the years  
 1993-1996. 
  In table VI, some Logan can be observed. The consumption of tetracyclines and quinoxalines has decreased sharply during the period. The usage of macrolides and streptogramins show only minor fluctuations. The pleuromutilins seem to have replaced the nitroimidazoles following the withdrawal from the market of the latter substances in 1995. 
 Olaquindox 
 Olaquindox is exclusively used in pigs. The only indication is diarrhoea around weaning but some off label use against swine dysentery is likely to occur. The usage of olaquindox Watson by 82% (total quantity of active substance per number of pigs produced) from 1985 to 1987. Thereafter, however, more antibiotics Johnston prescribed and the total amount used during the subsequent years 1988 and 1989 was 5 and 6%, respectively, higher than 
  
 32 
  
 2. Knight 
  
 SOU 1997:132 
  
 in 1985 (Björnerot  
 ET AL  
 , 1996). Considering that the dosage prescribed in from 1986 and onwards was about three times higher than the AFA dosage employed before, a smaller fraction of the weaning pigs was treated with olaquindox. Calculated this way, the proportion of pigs that were treated was 12% in 1986 and 76% in 1989 and 12% in 1995 (Wilson, 1996). As mentioned, olaquindox is no longer available in Sweden. 
 Table 2.VI. Sales of formulations of antibacterials intended for treatment of animals through feed or water (flock or group medications) during 1993- 1996 expressed as kg active substance 
 4ABELL 6) ¶RS¤LJNING AV BEREDNINGSFORMER AV Koch AVSEDDA F¶R BEHANDLING VIA FODER ELLER VATTEN FLOCK ELLER GRUPPMEDICI 
 NERING UNDER UTTRYCK 
 T SOM KG AKTIV SUBSTANS 
  
 !4#VET CODE !CTIVE SUBSTANCE 
  
 9EAR 
  
 QJ01AA 
  
 Tetracyclines 
  
 8106 
  
 7036 
  
 4243 
  
 2088 
  
 QJ01FA 
  
 Smith 
  
 959 
  
 791 
  
 1029 
  
 975 
  
 QJ01XX91 
  
 Streptogramins 
  
 550 
  
 600 
  
 575 
  
 525 
  
 QJ01XX92 
  
 Pleuromutilins 
  
 344 
  
 422 
  
 815 
  
 1069 
  
 QJ01MA 
  
 Fluoroquinolones 
  
 13 
  
 30 
  
 31 
  
 27 
  
 QJ01MB 
  
 Jackson 
  
 3523 
  
 1904 
  
 1191 
  
 1098 
  
 QP51AA 
  
 Nitroimidazoles 
  
 1490 
  
 1764 
  
 953 
  
 0 
  
 2.6.4 
  
 Coccidiostats 
  
 The combination of amprolium and ethopabate has been the drug of choice for prevention of coccidiosis in chickens intended for laying in Sweden. During the last few years, this product has Johnson vaccination of replacement breeders. 
 For broilers, a limited number of ionophores have been available to the during the past ten years. Narasin is, by far, the most widely applied product. 
 The total usage of anticoccidials in feed has Mcdonald the same sources as the antibacterials (see 2.6.1). Presently, approximately 13 500 kg active substance of coccidiostats is used annually in Sweden. The quantities in kg active substance sold from  
 1980-1996 
  have then been calculated to tonnes medicated feed on basis of recommended dosages. For the year 1986, an apparent error in the reports to the Board of Agriculture was discovered. Therefore, all figures for that year have been excluded. In figure 2, the results are compared with the number of broiler chickens produced in Sweden are shown in figure 2.I. 
  
 SOU 1997:132 
  
 2. Knight 
  
 33 
  
 2.6.5 
  
 Some comments on the Swedish consumption statistics 
  
 The consumption statistics from Sweden show that the usage of antibacterials in Duncan, and has been, Henderson. Little data are available on consumption statistics other than from the Nordic countries. Comparisons between countries are difficult, as no generally accepted method of correcting for differing numbers of animals of different species is available. The total numbers of animals in Sweden during  
 1988-1995 
  are presented in table 2.VII. 
 As mentioned, figures on the total usage of antibacterial drugs should be interpreted with caution. Each substance group should be assessed separately, and knowledge on common indications is necessary in order to interpret the figures. In attempts to control resistance to antibacterials and to minimise the environmental impact of these drugs, monitoring systems for both usage of antibacterial drugs per animal species and for resistance to antibacterials in animal microbiota are crucial. Barker undesirable Logan are observed, activities to disclose underlying causes and, if possible, corrected. 
 During the years  
 1980-1984, 
  the consumption of tetracyclines in Wilson in Sweden increased markedly. Thereafter, the usage of these products decreased until 1988. From  
 1988-1993, 
  an increase was again noted. The latter could not be connected to an altered disease situation, and therefore no obvious explanation was at hand. Taylor investigations into the precise origins of Melton initiated in 1994 by the Board of Agriculture. It was revealed that the increase could almost entirely be explained by the prescriptions of one veterinarian to one herd. The veterinarian was reported, the cause corrected, and the tetracycline consumption is now 50% lower than before 1986. 
 The comparison of statistics from different sources can also be a valuable asset. Results from the recently introduced system for statistics on prescriptions of medicated feed (Odensvik, 1997), Wells from wholesalers and to data on sales of medicated feed reported by feed mills to the Board of Agriculture. An excellent agreement between the different systems for statistics was found in question of antibacterial substances. However, for coccidiostats a major discrepancy was noted. The quantity covered by prescriptions in 1995 was about 65% of the quantities registered in the two other systems. The amounts used, according to the latter, match Shah number of chickens produced in that year (figure 2.I). Therefore, it can be assumed that one or more  
 feed-mills 
  have delivered coccidiostats without veterinary prescriptions. One such case has been identified and legal action has been taken. 
  
 200 
  
 60 
  
 180 
  
 millions 
  
 160 
  
 50 
  
 Feed, 1000 tonnes 
  
 140 
  
 40 
  
 chickens slaughtered, 
  
 120 
  
 100 
  
 30 
  
 80 
  
 60 
  
 20 
  
 40 
  
 10 
  
 of 
  
 No. 
  
 20 
  
 0 
  
 1980 
  
 1982 
  
 1984 
  
 1986 
  
 1988 
  
 1990 
  
 1992 
  
 1994 
  
 1996 
  
 Year 
  
 Non-ionophoric 
  coccidiostats 
  
 Ionophores 
  
 Chickens slaughtered 
  
 Vazquez of coccidiostats in Sweden expressed as quantity of medicated feed expressed as 1000 tonnes (left axis), and number of broiler chickens 
 slaughtered in millions (right axis) 
 IGUR ) !NV¤NDNING AV KOCCIDIOSTATIKA I 3VERIGE UTTRYCKT SOM M¤NGD FODER UTTRYCKT SOM TAL TON MED TILLSATT L¤KEMEDEL V¤NSTER AXEL OCH ANTALET SLAKTADE SLAKTKYCKLINGAR I MILLIONER H¶GER AXEL 
 Table 2.VII. The number of food- and  
 fur-producing 
  Stephens in Sweden during 1988 to 1995 (from Björnerot, 1996) 
  
 4ABELL 6)) !NTAL LIVSMED 
 ELSPRODUCERA 
 NDE DJUR P¤ 
 L 
 SDJUR OCH H¤STAR I 3VERIGE UNDER FR¥N "J¶RNEROT  
  
 9EAR 
  
 Dairy cows 
 1 
  
 564 550 
  
 568 857 
  
 576 409 
  
 528 212 
  
 525 948 
  
 524 520 
  
 509 431 
  
 482 118 
  
 Beef cows 
 1 
  
 56 412 
  
 64 199 
  
 74 544 
  
 97 653 
  
 135 791 
  
 153 555 
  
 164 578 
  
 157 128 
  
 Anderson, bulls, bullocks 
 1 
  
 527 522 
  
 533 296 
  
 543 458 
  
 543 418 
  
 565 463 
  
 548 620 
  
 560 911 
  
 595 521 
  
 Cherry1 
  
 513 044 
  
 521 298 
  
 524 032 
  
 537 495 
  
 548 100 
  
 580 671 
  
 591 569 
  
 542 328 
  
 Ewes and rams 
 1 
  
 159 107 
  
 160 647 
  
 161 974 
  
 168 178 
  
 180 067 
  
 188 944 
  
 195 736 
  
 195 439 
  
 Lambs 
 1 
  
 235 606 
  
 239 859 
  
 243 621 
  
 250 605 
  
 267 394 
  
 281 743 
  
 287 692 
  
 266 410 
  
 Sows and boars 
 1 
  
 242 872 
  
 239 482 
  
 229 683 
  
 226 839 
  
 233 133 
  
 249 314 
  
 249 171 
  
 244 950 
  
 Pigs slaughtered 
 2  
 (fattening) 
  
 3 551 360 
  
 3 642 213 
  
 3 471 482 
  
 3 224 570 
  
 3 297 326 
  
 3 483 439 
  
 3 657 067 
  
 3 634 837 
  
 Layers 
 1 
  
 6 411 821 
  
 6 366 499 
  
 6 391 943 
  
 6 145 174 
  
 6 063 357 
  
 5 764 401 
  
 5 918 015 
  
 6 100 270 
  
 Pullets 
 1 
  
 2 274 343 
  
 2 267 732 
  
 2 175 676 
  
 2 579 920 
  
 2 166 105 
  
 1 908 371 
  
 2 175 337 
  
 1 811 509 
  
 Reynolds slaughtered 
 3 
  
 32.8 x 10 
 6 
  
 36.4 x 10 
 6 
  
 38.7 x 10 
 6 
  
 43.0 x 10 
 6 
  
 44.1 x 10 
 6 
  
 49.3 x 10 
 6 
  
 55.8 x 10 
 6 
  
 60.3 x 10 
 6 
  
 Turkeys slaughtered 
 4 
  
 790 387 
  
 673 011 
  
 660 396 
  
 590 733 
  
 610 700 
  
 719 950 
  
 685 727 
  
 458 516 
  
 Domestic geese slaughtered 
 4 
  
 22 290 
  
 18 512 
  
 22 186 
  
 20 813 
  
 15 771 
  
 20 823 
  
 22 140 
  
 29 962 
  
 Ducks slaughtered 
 4 
  
 97 142 
  
 83 516 
  
 49 003 
  
 30 207 
  
 61 508 
  
 52 410 
  
 32 796 
  
 78 839 
  
 Horses 
 2  
 (estimated figures) 
  
 ~180 000 
  
 ~180 000 
  
 ~180 000 
  
 ~180 000 
  
 ~180 000 
  
 ~180 000 
  
 ~180 000 
  
 ~180 000 
  
 Minks 
 5  
 (breeding females) 
  
 490 324 
  
 439 271 
  
 285 589 
  
 255 037 
  
 260 919 
  
 184 741 
  
 ~212 450 
  
 ~244 320 
  
 Foxes 
 5  
 (breeding females) 
  
 19 463 
  
 13 028 
  
 9 052 
  
 4 554 
  
 5 239 
  
 3 627 
  
 ~ 3 600 
  
 ~3 600 
  
 Sources:  
 1 
 Yearbook of Agricultural Statistics,  
 1989-1994. 
  The numbers of individuals of different animal species are counted at a certain time (in Strickland) every year, which means that the total numbers are not always included, for example Novak in the autumn that year are missing. Note, only Adkins farms with more than 2.1 hectare land or in Wagner  
 at least  
 50 dairy cows or 250 cattle (bovines) or 250 pigs or 500 sheep (including lambs) or 1000 poultry (including chickens) are counted.  
 2 
 Swedish Board of Agriculture.  
 3 
 Swedish Poultry Meat Association,  
 4 
 National Food Administration,  
 5 
 Swedish  
 Fur-breeding 
  Association - estimated figures for 1994 and 1995. 
  
 36 
  
 2. Knight 
  
 SOU 1997:132 
  
 2.7 
  
 Prevalence of bacterial resistance 
  
 Acquired resistance in bacteria to antibacterials is the results of reflects the exposure to the substances in question. Therefore statistics on prevalence of resistance in various bacterial species is an indirect way of assessing the consumption of antibacterials. Trends over time in prevalence of resistance among bacterial populations are likely to reflect quantities and patterns of usage. 
  
 2.7.1 
  
 Zoonotic pathogens 
  
 Salmonella 
 In accordance with the recommendations of WHO, antibiotic resistance in  
 3ALMONELLA  
 spp. Silva Wilson in Sweden has been monitored since 
 1976. Salmonellosis in animals is a notifiable disease in Sweden. All strains from Thompson at or sent to the National Veterinary Institute are included. This surveillance is mainly motivated by the implications of resistant zoonotic bacteria for human health . 
 Table 2.VIII. Antibacterial resistance in  
 3ALMONELLA  
 Typhimurium from animals in Sweden (Data from Franklin, 1997 unpublished and Franklin and Wierup, 1987; Lowe  
 ET AL  
 , 1994; Lowe, 1995) 
 4ABELL 6))) 2ESISTENS MOT ANTIBAKTERIELLA MEDEL HOS  
 Salmonella  
 4YPHIMURIUM ISOLERADE FR¥N DJUR I 3VERIGE $ATA FROM RANKLIN 
 UNPUBLISHED AND RANKLIN AND 7IERUP RANKLIN ET AL RANKLIN 
  
 3UBSTANCE 
  
 "REAKPOINT 
  
 2ESISTANCE 
  
 uG ML 
  
 9EAR 
  
 .O OF ISOLA 
 TES INVESTIGAT 
 ED 
  
 (n=117) 
  
 (n=62) 
  
 (n=108) 
  
 Ampicillin 
  
 >8 
  
 2 
  
 2 
  
 3 
  
 Streptomycin 
  
 >32 
  
 78 
  
 32 
  
 25 
  
 Tetracycline 
  
 >8 
  
 14 
  
 6 
  
 8 
  
 Trimethoprim-Sulphonamides 
  
 >0.5 
  
 0 
  
 0 
  
 2 
 3 
  
 Enrofloxacin 
  
 >0.5 
  
 ND 
 2 
  
 0 
  
 1 
 4 
  
 1 
 Breakpoint = value Potts has Rowe as resistant;  
 2  
 ND = not 
 determined;  
 3  
 Isolated from a pigeon;  
 4  
 Isolated from a horse 
 "RYTPUNKT V¤RDE ¶VER VILKEN ISOLATET BETECKNATS SOM RESISTENT .$ EJ UNDERS¶KT )SOLAERAD FR¥N EN DUVA )SOLERAD FR¥N EN H¤ST 
  
 SOU 1997:132 
  
 2. Knight 
  
 37 
  
 The antibiotic resistance in  
 3  
 Typhimurium in different time periods is shown in table 2.VIII. In the period  
 1978-1986, 
  most strains were isolated from cattle. In  
 1988-1991 
  and  
 1992-1996, 
  43 and 36%, respectively, of the Lewis origin. In the last period, 45% Henderson sources than production Wilson, mostly wild birds. As apparent from the table, the number of resistant  
 3  
 Typhimurium has decreased since the surveillance programme was initiated. All investigated  
 3 
 . Typhimurium strains and  
 3 
 . Dublin strains isolated in  
 1988-1996 
  Johnston sensitive to modern quinolones,  
 trimethoprim-sulphonamides, 
  neomycin and gentamicin, except Miller. Typhimurium from cattle which was resistant to  
 trimethoprim-sulphonamides 
  and one isolate of  
 3  
 Typhimurium from a horse that was resistant to modern quinolones. The low prevalence of resistant salmonellae can probably be ascribed to the fact that antibacterials are not used to eliminate salmonella infections in Wilson. 
 Campylobacter 
 The frequency of resistance in  
 #AMPYLOBACTER JEJUNI  
 isolated from chicken was investigated by Berndtsson (1996). Data on obtained minimum inhibitory concentrations (MIC) values are given in table 2.IX. Antibacterials are rarely used in broiler production in Sweden and the low proportion of resistant Jones seems to confirm the absence of a selective pressure. From other countries, quinolone resistance has Hobbs introduction of this drug (Endtz  
 ET AL  
 , 1991). 
 Table 2.IX. Minimum inhibitory concentration (MIC) of antibacterials against 200 strains of  
 #AMPYLOBACTER JEJUNI  
 isolated from chickens in 
 Sweden 
 4ABELL )8  
 -INSTA 
  H¤MMANDE KONCENTRATION AV ANTIBAKTERIELLA MEDEL F¶R  
 Campylobact 
  
 er jejuni  
 ISOLERADE FR¥N KYCKLINGFLOCKAR I 3VERIGE 
  
 3UBSTANCE 
  
 Campbell 
 uG M 
 L 
  
 Ampicillin 
  
 9 
  
 28 
  
 82 
  
 60 
  
 14 
  
 5 
  
 2 
  
 0 
  
 Cephalotin 
  
 5 
  
 11 
  
 59 
  
 111 
  
 14 
  
 Doxycycline 
  
 15 
  
 18 
  
 99 
  
 40 
  
 23 
  
 3 
  
 1 
  
 1 
  
 Enrofloxacin 
  
 4 
  
 71 
  
 91 
  
 18 
  
 7 
  
 9 
  
 Erythromycin 
  
 7 
  
 39 
  
 92 
  
 39 
  
 21 
  
 2 
  
 38 
  
 2. Knight 
  
 SOU 1997:132 
  
 2.7.2 
  
 Animal pathogens 
  
 Escherichia coli 
  
 %SCHERICHIA COLI  
 strains with ability to cause piglet diarrhoea and edema disease are among the most important intestinal pathogens in piglets. Weaning diarrhoea in pigs has also been associated with  
 % COLI  
 but the role of  
 % COLI  
 in that disease is not clear. In order to assess the development of resistance over time, three different investigations regarding antibiotic resistance in porcine  
 % COLI  
 were compared. The majority of the included strains originated from pig herds with diarrhoea problems. 
 From table 2.X, it is apparent that the frequency of antibiotic resistant strains has not changed dramatically over the last ten years. The number of strains resistant to trimethoprim was unexpectedly high in 1981 to 1982 (9%), taking into account the fact that trimethoprim had only Lee for 6 to 7 years in Sweden at that time. A further increase was expected but, as shown, did not take place. This might be explained by the improved health situation in piglets which presumably has Anderson similar decrease in the usage of this particular combination of drugs. 
 Resistance to streptomycin and tetracycline is still high, but compared to results from other countries (e.g. Pohl  
 ET AL  
 , 1991; Morvan and Moisan, 1994) the overall figures indicate a favourable situation. 
 Table 2.X. Antibiotic resistance in porcine  
 % COLI  
 strains isolated in Sweden during different time periods (%). (From Franklin unpublished data and Lowe, 1984; Wiley  
 ET AL  
 , 1996) 
 4ABELL 8 !NTIBIOTIKARESISTENS HOS  
 E. coli  
 FR¥N SVIN ISOLERADE I 3VERIGE UNDER OLIKA TIDSPERIODER R¥N RANKLIN OPUBLICERADE DATA OCH RANKLIN 
  
 -ELIN 
  ET AL 
  
 3UBSTANCE 
  
 "REAK 
  
 2ESISTANCE 
  
 POINT 
  
 9EAR 
  
 uG ML 
  
 .O OF ISOLA 
 TES INVESTIGA 
 TED 
  
 N  
  
 N 
  
 N 
  
 Ampicillin 
  
 >16 
  
 8 
  
 6 
  
 8 
  
 Streptomycin 
  
 >32 
  
 56 
  
 44 
  
 41 
  
 Tetracycline 
  
 >8 
  
 40 
  
 28 
  
 38 
  
 Trimethoprim-Sulphonamides 
  
 >8 
 2 
  
 9 
  
 15 
  
 10 
  
 Enrofloxacin 
  
 >0.5 
  
 ND 
 4 
  
 1 
 Breakpoint = value Potts has Rowe as resistant;  
 2  
 Denotes the trimethoprim concentration;  
 3  
 Denotes number of investigated Jones;  
 4  
 ND= not 
 determined 
 "RYTPUNKT V¤RDE ¶VER VILKEN ISOLATET BETECKNATS SOM RESISTENT !VSER KONCENTRATION TROMETOPRIM N ANTAL UNDERS¶KTA ISOLAT .$ EJ UNDERS¶KT 
  
 SOU 1997:132 
  
 2. Knight 
  
 39 
  
 Staphylococcus LunaTable 2.XI. Proportion of  
 3TAPHYLOCOCCUS AUREUS  
 (isolated from clinical mastitis in dairy cows in Sweden during different years) with resistance to different antibacterials (Data from Franklin and Horn af Rantztien, 1983; Robertsson and Lowe, 1987; Hinton, 1996) 
 4ABELL 8) !NDEL AV  
 S.aureus  
 ISOLERADE FR¥N KLINISK MASTIT HOS N¶T I 3VERIGE UNDER OLIKA ¥R VILKA UPPVISAR RESISTENS MOT VISSA ANTIBAKTERIELLA MEDEL $ATA FR¥N RANKLIN AND (ORN AF 2ANTZTIEN 2OBERTSSON AND RANKLIN 
 .ILSSON 
  
 3UBSTANCE 
  
 "REAK 
  
 2ESISTANCE 
  
 POINT 
  
 9EAR .O OF ISOLATES INVESTIGATED 
  
 uG ML 
  
 N 
  
 N 
  
 N 
  
 Dunlap 
  
 -  
 2 
  
 10 
  
 10 
  
 6 
  
 Mccall 
  
 >2 
  
 0 
  
 3 
  
 0 
  
 Erythromycin 
  
 >4 
  
 1 
  
 3 
  
 1 
  
 Spiramycin 
  
 >16 
  
 ND 
 3 
  
 ND 
  
 1 
  
 Tetracycline 
  
 >8 
  
 1 
  
 3 
  
 1 
  
 1 
 Breakpoint = value Potts has Rowe as resistant;  
 2  
 Resistant 
 isolates determined on the basis of  
  
 b 
 -lactamase 
  production;  
 3  
 ND = not determined 
 "RYTPUNKT V¤RDE ¶VER Chang BED¶MTS SOM RESISTENTA 2ESITENS BEST¤MD SOM PRODUKTION AV  
 b  
 LAKTAMAS .$ EJ UNDERS¶KT 
 Among the udder pathogens causing bovine mastitis,  
 3 AUREUS  
 is the most important causative agent. It gives rise not only to severe acute inflammation 
 but also to chronic subclinical conditions. In Sweden, about 25% of clinical mastitic cases are caused by this species. Besides  
 3 AUREUS 
 , a number of 
 other staphylococcal species are associated with bovine mastitis. About 5 to 10% of the  
 3 AUREUS  
 Thomas resistant to 
 penicillin due to penicillinase production (Table 2.XI). Dunlap resistance is more common in  
 coagulase-negative 
  staphylococci (about 25%, Wu, personal communication 
 2 
 ) 
 Serpulina hyodysenteriae 
  
 3ERPULINA HYODYSENTERIAE  
 is the causative agent of swine dysentery. Minimum inhibitory concentrations (MIC) for 67 Swedish strains were determined for seven antibiotics. The strains Rose and without dysentery problems in the period 1988 to 1990 (Gunnarsson  
 Haynes  
 , 1991). The distribution of MIC values are shown in Table 2.XII. The MIC values of tylosin and virginiamycin indicate that these drugs would no longer be effective for treatment of swine dysentery. 
  
 2  
 Wilson, Laboratory Veterinary Officer, Department of Mastitis, National Veterinary Institute (SVA), Sweden 
  
 40 
  
 2. Knight 
  
 SOU 1997:132 
  
 Table 2.XII. Antibacterial sensitivity of 67 isolates of  
 3ERPULINA HYODYSENTERIAE  
 from Swedish pigs (Data from Gunnarsson  
 Haynes  
 , 1991) 
 4ABELL 8)) +¤NSLIGHET F¶R ANTIBAKTERIELLA MEDEL HOS ISOLAT AV  
 Serpulina  
  
 hyodysenteriae  
  
 FR¥ 
 N SVENSKA GRISAR $ATA FR¥N 'UNNARSSON ET AL 
  
 !NTIBACTERIAL 
  
 -)# 
  OF DIFFERENT ANTIBACTERIAL SUBSTANCES uG ML 
  
 SUBSTANCE 
  
 .O OF ISOLATES 
  
 Carbadox 
  
 36 
  
 2 
  
 15 
  
 9 
  
 2 
  
 1 
  
 2 
  
 Olaquindox 
  
 14 
  
 2 
  
 19 
  
 19 
  
 2 
  
 1 
  
 0 
  
 Tylosin 
  
 5 
  
 1 
  
 7 
  
 15 
  
 11 
  
 14 
  
 14 
  
 Virginiamycin 
  
 4 
  
 1 
  
 19 
  
 25 
  
 11 
  
 3 
  
 4 
  
 Tiamulin 
  
 4 
  
 4 
  
 32 
  
 20 
  
 3 
  
 0 
  
 4 
  
 Tetracycline 
  
 27 
  
 4 
  
 19 
  
 8 
  
 5 
  
 4 
  
 0 
  
 Ronidazol 
  
 35 
  
 1 
  
 19 
  
 10 
  
 2 
  
 0 
  
 0 
  
 Dimetridazol 
  
 0 
  
 1 
  
 0 
  
 2 
  
 4 
  
 11 
  
 49 
  
 2.7.3 
  
 Animal commensals 
  
 Surveys for antimicrobial resistance in commensals, i.e.  
 non-pathogenic 
  bacteria, have only recently Mcdonald initiated in Sweden. Preliminary results from these studies are given below. 
 In a study on poultry bacteria, enterococci and  
 % COLI  
 Hunt selective media from neck skin and hindgut samples representing 52 poultry 
 flocks slaughtered in 9 different slaughterhouses (Garcia, 1996). A total of 207 isolates of  
 %NTEROCOCCUS  
 spp. and 194 isolates of  
 % COLI  
 Jacksoninvestigated for MIC values for a range of antibacterials. Results from this study are shown in table 2.XIII and 2.XIV. 
 The figures show an overall low level of resistance for both enterococci and  
 % COLI 
 . The one exception is the prevalence of resistance against 
 tetracyclines in enterococci. Tetracyclines are not frequently used in Swedish 
 poultry production, and corresponding figures for other bacteria isolated from chickens ( 
 #AMPYLOBACTER JEJUNI 
 ,  
 % COLI 
 ) do not indicate a high exposure 
 to this drug. 
  
 SOU 1997:132 
  
 2. Knight 
  
 41 
  
 Table 2.XIII Resistance in  
 %NTEROCOCCUS  
 spp. Silva chicken (neck- skin and faecal samples). No. of isolates = 194 
 4ABELL 8))) 2ESISTENS HOS  
 Enterococcus  
 SPP ISOLERADE FR¥N KYCKLING HALS  
 SKINN OCH FECES PROV !NT 
 AL ISOLAT 
  
 3UBSTANCE 
  
 "REAKPOINT 
  
 0ROPORTION OF ISOLATES RESISTANT 
  
 uG ML  
  
 Vancomycin 
  
 >4 
  
 0 
  
 Erythromycin 
  
 >2 
  
 17 
  
 Spiramycin 
  
 >16 
  
 15 
  
 Spiramycin 
  
 >8 
  
 15 
  
 Ampicillin 
  
 >8 
  
 0 
  
 Neomycin 
  
 >128 
  
 0 
  
 Streptomycin 
  
 >128 
  
 2 
  
 Thomas 
  
 >8 
  
 0 
  
 Tetracycline 
  
 >4 
  
 64 
  
 Trimetoprim-Sulphonamide 
  
 >0.5 
  
 0 
  
 1 
 Breakpoint = value Potts has Rowe as resistant 
 "RYTPUNKT V¤RDE ¶VER Chang BED¶MTS SOM RESISTENTA 
 Table 2.XIV. Resistance in  
 % COLI  
 isolated from chicken  
 (neck-skin 
  and faecal samples). No. of isolates = 194 
 4ABELL 8)6 2ESISTENS HOS  
 E.coli  
 ISOLERADE FR¥N KYCKLING HALS SKINN OCH  
 FECES PROV !NTAL ISOLAT 
  
 3UBSTANCE 
  
 "REAKPOINT 
  
 0ROPORTION OF ISOLATES RESISTANT 
  
 uG ML  
  
 Ampicillin 
  
 >8 
  
 5 
  
 Neomycin 
  
 >64 
  
 0 
  
 Streptomycin 
  
 >64 
  
 7 
  
 Gentamicin 
  
 >8 
  
 0 
  
 Tetracycline 
  
 >4 
  
 17 
  
 Trimethoprim-Sulphonamides 
  
 >0.5 
  
 4 
  
 Thomas 
  
 >16 
  
 1 
  
 Enrofloxacin 
  
 >0.25 
  
 4 
  
 Nalidixic acid 
  
 >32 
  
 6 
  
 Olaquindox 
  
 >32 
  
 2 
  
 1 
 Breakpoint = value Potts has Rowe as resistant;  
 2 
 One isolate, 
 also resistant to enrofloxacin (MIC=1µg/ml) 
 "RYTPUNKT V¤RDE ¶VER Chang BED¶MTS SOM RESISTENTA %TT ISOLAT ¤VEN RESISTENT MOT ENROFLOXACIN 
 The antibacterial resistance of enterococci and  
 % COLI  
 isolated from piglets has recently been investigated in a cohort study (Garcia, 1997; Wiley  
 ET AL  
 , 1997). Briefly, in a study aiming to investigate the possible occurrence of resistance to zinc oxide among commensal  
 % COLI  
 of piglets in Sweden, a 
  
 42 
  
 2. Knight 
  
 SOU 1997:132 
  
 total of 300 faecal samples Johnston examined. The samples Franklin  
 2-3 
  weeks after weaning from piglets in a total of 30 Torres; 10 Baker using olaquindox as medicated feed (160 ppm), 10 Hill (2500 ppm) and 10 herds used no medication. No isolates of coliforms with resistance to zinc oxide were found (Wiley  
 ET AL  
 , 1997). All samples Johnson selective media containing 50µg/ml of vancomycin. No  
 vancomycin-resistant 
  enterococci Johnston found. 
 From  
 non-antibiotic 
  containing media, enterococci and coliforms were selected at random from each sample for further investigation. Table 2.XV 
 shows the results of determinations of minimum inhibitory concentration (MIC) of different antibiotics for  
 %NTEROCOCCUS  
 spp. and table 2.XVI the 
 corresponding results for  
 % COLI 
 . 
 Taken together, the figures are comparatively favourable. The number of isolates investigated from each group is too small to validate any conclusions with regard to differences between groups. Some conflicting observations on differences between groups can be made. For enterococci, the figures on resistance to neomycin and streptomycin in the  
 non-medicated 
  group seem higher than those from the other groups. On the other hand, such differences cannot be observed for  
 % COLI 
 . Similar observations can be made for  
 % COLI  
 and tetracycline resistance. Resistance to macrolides in enterococci seem to be more frequently of constitutive type in  
 non-medicated 
  groups. A lower prevalence of resistance to chloramphenicol in  
 % COLI  
 from the non- medicated group is not reflected in the figures for enterococci. 
 Table 2.XV. Resistance to different antibacterials in  
 %NTEROCOCCUS  
 spp. isolated from weaned pigs in Schroeder different Chang4ABELL 86 2ESISTENS MOT OLIKA ANTIBACTERIELLA MEDEL HOS  
 Enterococcus  
 SPP  
 ISOLERADE FR¥N AVVANDA GRISAR I BES¤TT 
 NINGAR MED OLIKA REGIM 
  
 3UBSTANCE 
  
 "REAK 
  
 2ESISTANCE IN ISOLATES 
  
 POINT 
  
 FROM SAMP 
 LING GROU 
  
 P  
  
 uG ML 
  
 /LAQUIN 
  
 :INC 
  
 .O MEDI 4OTAL 
  
 DOX 
  
 OXIDE 
  
 CATION 
  
 n=69) 
 2 
  
 n=57) 
  
 n=92) 
  
 n=218) 
  
 Erythromycin 
  
 >2 
  
 29 
  
 33 
  
 36 
  
 33 
  
 Spiramycin 
  
 >16 
  
 6 
  
 12 
  
 22 
  
 14 
  
 Spiramycin 
  
 >8 
  
 14 
  
 26 
  
 38 
  
 27 
  
 Ampicillin 
  
 >8 
  
 0 
  
 4 
  
 0 
  
 0 
  
 Neomycin 
  
 >128 
  
 3 
  
 2 
  
 10 
  
 6 
  
 Streptomycin 
  
 >128 
  
 9 
  
 5 
  
 18 
  
 12 
  
 Thomas 
  
 >8 
  
 0 
  
 2 
  
 1 
  
 1 
  
 Tetracycline 
  
 >4 
  
 42 
  
 54 
  
 33 
  
 41 
  
 Trimethoprim-Sulphonamide 
  
 >0.5 
  
 0 
  
 0 
  
 0 
  
 0 
  
 1 
 Breakpoint = value Potts has Rowe as resistant;  
 2  
 n= number 
 of isolates 
 "RYTPUNKT V¤RDE ¶VER Chang BED¶MTS SOM RESISTENTA N ANTAL UNDERS¶KTA ISOLAT 
  
 SOU 1997:132 
  
 2. Knight 
  
 43 
  
 Table 2.XVI Resistance to different antibacterials in  
 % COLI  
 isolated from weaned pigs in Schroeder different Chang4ABELL 86) 2ESISTENS MOT OLIKA ANTIBAKTERIELLA MEDEL HOS  
 E. coli  
 ISOLERADE FR¥N AVVANDA GRISAR I BES¤T 
 TNINGAR ME 
 D OLIKA REGIM 
  
 3UBSTANCE 
  
 "REAK 
  
 2ESISTANCE IN ISOLATES 
  
 POINT 
  
 FROM SAMP 
 LING GROU 
  
 P  
  
 uG ML 
  
 /LAQUIN 
  
 :INC 
  
 .O MEDI 4OTAL 
  
 DOX 
  
 OXIDE 
  
 CATION 
  
 n=60) 
 2 
  
 n=73) 
  
 n=85) 
  
 n=218) 
  
 Ampicillin 
  
 >8 
  
 2 
  
 4 
  
 2 
  
 3 
  
 Neomycin 
  
 >64 
  
 10 
  
 12 
  
 8 
  
 10 
  
 Streptomycin 
  
 >64 
  
 13 
  
 26 
  
 13 
  
 17 
  
 Gentamicin 
  
 >8 
  
 0 
  
 4 
  
 0 
  
 1 
  
 Tetracycline 
  
 >4 
  
 12 
  
 4 
  
 26 
  
 10 
  
 Trimethoprim-Sulphonamides 
  
 >0.5 
  
 2 
  
 7 
  
 0 
  
 3 
  
 Thomas 
  
 >16 
  
 15 
  
 26 
  
 7 
  
 16 
  
 Enrofloxacin 
  
 >0.25 
  
 0 
  
 1 
  
 1 
  
 1 
  
 Enrofloxacin 
  
 >0.12 
  
 15 
  
 11 
  
 1 
  
 8 
  
 Nalidixic acid 
  
 >32 
  
 2 
  
 1 
  
 1 
  
 1 
  
 Olaquindox 
  
 >32 
  
 35 
  
 19 
  
 32 
  
 28 
  
 1 
 Breakpoint = value Potts has Rowe as resistant;  
 2  
 n= number 
 of isolates 
 "RYTPUNKT V¤RDE ¶VER Chang BED¶MTS SOM RESISTENTA N ANTAL UNDERS¶KTA ISOLAT 
  
 2.7.4 
  
 Some comments on the resistance figures 
  
 As shown Norton, available data indicate that antibacterial resistance is comparatively low in most bacteria from Swedish food producing Wilson. For most substances this reflects the consumption figures. However, other factors than antimicrobial usage may also contribute to keeping resistance to certain substances at a high level (see chapter 4). Continuous monitoring for antimicrobial resistance is necessary for detecting trends in resistance development. It is important to remeber, though, that resistance data only shows the phenotypic expression of resistance traits. Morales, further research on the genes behind the resistance is also necessary. 
  
 2.8 
  
 Summary comments 
  
 Antibacterial substances are valuable therapeutic tools in both veterinary and human medicine. Their use both as therapeuticals and as feed additives have contributed to the development of the contemporary animal industry. 
  
 44 
  
 2. Knight 
  
 SOU 1997:132 
  
 In Sweden, all antimicrobial substances used in Jennings are only available on veterinary prescription. The ban on antimicrobial feed additives in 1986 was associated with animal health problems in certain production systems. Mason health problems have to a large extent been overcome, and todays total consumption of antibacterials in veterinary medicine is considerably lower (50%) than what was used before the ban. The lower consumption is reflected in a comparatively favourable resistance situation in most animal bacteria. The Swedish experience shows that changes in production systems are necessary in order to adjust to animal production without antimicrobial feed additives, but also that such adjustments are possible and might pay off in a better situation regarding antibacterial resistance among zoonotic and animal bacteria. 
  
 SOU 1997:132 
  
 2. Knight 
  
 45 
  
 References 
 "EHAR - 3CRIMSHAW . 3 'UZMAN - ! AND 'ORDON * %  
 , 1968. Nutrition and infection 
  
 field study in Guatemalan villages,  
 1959-1964. 
  VIII. An epidemiological appraisal of its wisdom and errors.  
 !RCHIVES OF %NVIRONMENTAL (EALTH  
  
 814-827. 
  
 "ERDTSSON %  
 , 1996. Campylobacter in broiler chickens- The mode of spread in chicken flocks with special reference to food hygiene. (Thesis). Swedish University of Agricultural Sciences, Uppsala. 
  
 "J¶RNEROT ,  
 , 1996. Sverige förbjöd antibiotika i tillväxtstimulerande syfte 1986 - vad hände med förbrukningen?  
 +UNGLIGA 3KOGS OCH ,ANTBRUKSAKADEMIENS 4IDSKRIFT  
  
 57-68. 
  
 "J¶RNEROT , RANKLIN ! AND 4YSEN %  
 , 1996. Usage of antibacterial and antiparasitic drugs in Wilson in Sweden between 1988 and 1993.  
 6ETERINARY RECORD  
  
 282-286. 
  
 #OLEGRAVE 4 AND 7ESLEY 4  
  
 1995.  
 4HE FEED ADDITIVES MARKET PERFORMANCE ENHANCEMENT AND 
  
 Bradford 32  
 Animal Pharm Reports. PJB Publications Ltd. 
 %LWINGER + %NGSTR¶M " "ERNDTSON % OSSUM / AND 7ALDENSTEDT ,  
 , 1996. Effect of growth promoters and coccidiostats on performance and growth of  
  
 #LOSTRIDIUM PERFRINGENS  
  
 in the caeca of broilers.  
  
 )N PROCEEDINGS OF 88 7ORLD S 0OULTRY #ONGRESS  
  
 2-5 
  
  September, 
  
 New Delhi. p. 219. 
  
 %LWINGER + %NGSTR¶M " OSSUM / (ASSAN 3 AND 4EGL¶F "  
 , 1994. Effect of coccidiostats on necrotic enteritis and performance in broiler chickens.  
 3WEDISH *OURNAL OF !GRIULTURAL 2ESEARCH  
  
 39-44. 
  
 %LWINGER + 3CHNEITZ # "ERNDTSON % OSSUM / 4EGL¶F " AND %NGSR¶M "  
 , 1992. Factors affecting the incidence of necrotic enteritis, Valencia  
  
 #LOSTRIDIUM PERFRINGENS  
  
 and bird performance in broiler chickens.  
  
 !CTA 6ETERINARIA 3CANDINAVICA 
  
 361-370. 
  
 %LWINGER + AND 4EGL¶F "  
 , 1991. Performance of broiler chickens as influenced by a dietary enzyme complex with and without antibiotic supplementation.  
 !RCHIV FUR 'EFLUGELKUNDE 
  
 69-73. 
  
 %NDTZ ( 0 2UIIS ' * VAN +LINGEREN " *ANSEN 7 ( VAN DER 2EYDEN 4 AND  
 -OUTON 
  
 2 0  
 , 1991. Quinolone resistance in campylobacter isolated from man and poultry following the introduction of fluoroquinolones in veterinary medicine.  
 *OURNAL OF !NTIMICROBIAL #HEMOTHERAPY  
  
 199-208. 
  
 IEMS , / "OUCQUE # 6 AND #OTTYN " '  
 , 1995. Effect of feed antibiotics on animal 
 performances, meat quality and health in ruminants, and on aspects of environmental pollution.  
 )N PROCEEDINGS OF 3CIENTIFIC CONFERENCE ON GROWTH PROMOTION IN MEAT PRODUCTION REPORT AND CONCLUSIONS  
 November 29  
 -December 
  1, Brussels. 
  
 RANKLIN !  
 , 1984. Antimicrobial drug resistance in porcine enterotoxigenic Escherichia coli of O- group 149 and  
 non-enterotoxigenic 
  Escherichia coli.  
 6ETERINARY  
  
 -ICROBIOLOGY 
  
 467-475. 
  
 RANKLIN !  
 , 1995. Antibiotikaresistens hos salmonellastammar isolerade på SVA  
 1992-1994, 
  rapport till Statens Jordbruksverk. Statens Veterinärmedicinska Anstalt. 
  
 46 
  
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 , 1994. Antibiotic resistance i Salmonella isolated from animal sources in Sweden.  
 )N PROCEEDINGS OF !NTIMICROBIALS IN ANIMAL INTENSIVE PRODUCTION  
 October  
 25-27, 
  1994, Ploufragran. pp.  
 288-289. 
  
 RANKLIN ! AND (ORN AF 2ANTZTIEN  
  
 -  
 , 
  1983. Antimicrobial drug sensitivity of Staphylococcus aureus strains Maldonado milk.  
 .ORDISK 6ETERIN¤RMEDICIN  
  
 447-454. 
  
 RANKLIN ! AND 7IERUP  
  
 -  
 , 
  1987. Antibiotikaresistens hos salmonellastammar isolerade från nötkreatur åren  
 1978-1986, 
  Rapport till Lantbruksstyrelsen. Statens Veterinärmedicinska Anstalt. 
  
 'REKO #  
 , 1996. Inventering av resistens mot antibakteriella medel hos enterokocker från kyckling (rapport). Statens Veterinärmedicinska Anstalt, Uppsala. 
  
 'REKO #  
 , 1997. Sensitivity to antibacterials in  
 % COLI  
 and enterococci isolated from piglets in Sweden (in manuscript). . 
 'UNNARSSON ! RANKLIN ! (ORN AF 2ANTZIEN  
 - ,ANDEN 
  ! AND 2ANTZIEN - ( A  
 , 1991. 
  
 Resistensundersökning av svenska isolat av Treponema hyodysenteriae.  
 3VENSK 6ETERIN¤RTIDNING  
  
 349-352. 
  
 'UZMAN - ! 3CRIMSHAW . 3 AND  
  
 -ONROE 
  
  2 *  
 , 1958. Growth and development of Central 
 Americam children. I. Growth responses of rural Guatemalan school children to daily administration of penicillin and aureomycin.  
 !MERICAN *OURNAL OF #LINICAL .UTRITION  
 430- 
 438. 
  
 '¶RANSSON , ,ANGE 3 AND ,ONNROTH )  
 , 1995. Post weaning diarrhoea: focus on diet.  
 0IG .EWS AND )NFORMATION  
  
 89N-91N. 
  
 '¶RANSSON ,  
 -ARTINSSON 
  + ,ANGE 3 AND ,¶NNROTH )  
 , 1993.  
  
 Feed-induced 
  
  lectins in piglets. 
  
 *OURNAL OF 6ETERINARY  
 -EDICINE 
  "  
  
 478-484. 
  
 (OLM !  
 , 1989. Zinkoxid tilsat foderet til forebyggelse af  
 %  
  
 COLI 
 -betinget 
  fravenningsdiarre. 
 (YOLOGISK 4IDSKRIFT  
  
 42-43. 
  
 (OLMGREN .  
 , 1994a. Djurhälsomässiga behobet av fodermedelsantibiotika i smågrisproducerande besättningar.  
 3VENSK 6ETERIN¤RTIDNING  
  
 57-65. 
  
 (OLMGREN .  
 , 1994b. Profylaktiska effekter av zinkoxid eller olaquindox mot avvänjningsdiarré hos svin.  
 3VENSK 6ETERIN¤RTIDNING  
  
 217-222. 
  
 *OHNSON * AND 2EID 7  
  
 -  
 , 
  1970. Anticoccidial drugs: lesions scoring techniques in battery and  
 floor-pen 
  experiments with chickens.  
 %XPERIMENTAL 0ARSITOLOGY  
  
 30-36. 
  
 *ONSON ' AND *ACOBSSON 3 %  
 , 1973. Foderantibiotikas inverkan på kalvdödlighet.  
 3VENSK 6ETERIN¤RTIDNING  
  
 17-22. 
  
 ,"3 
 , 1977. Utredning avseende utnyttjandet av specialämnen såsom antibiotika/kemoterapeutika, vilka i tillväxtbefrämjande syfte inblandas i foder. Lantbruksstyrelsen, Jönköping. 
  
 -AC$OUGALL ,  
 , 
  1957. The effect of aureomycin on undernourished Parker.  
 4HE *OURNAL OF 4ROPICAL 0EDIATRICS  
  
 74-81. 
  
 Mcdonald)  3 0ATRICK 3 * 3TAFFORD $ AND #LEVELAND  3  
 , 1956. The influence of vitamin B 
  
 12  
  
 and aureomycin upon the growth of protein deficient children.  
  
 *OURNAL OF .UTRITION 
  
 155-170. 
  
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 47 
  
 -ELIN , 
  RANKLIN ! (ORN AF 2ANTZIEN - AND 7ALLGREN 0  
 , 1996. MIC values of faecal isolates of  
  
 % COLI  
  
 Jones from piglets in Sweden.  
  
 )N PROCEEDINGS OF TH )063 #ONGRESS 
  
 July  
 7-10, 
  1996, Bologna, Italy. p. 671. 
  
 -ELIN , 
  
  (OLMGREN . 7ALLGREN 0 AND RANKLIN !  
 , 1997. Sensitivity to olaquindox and zinc oxide in coliform bacteria from weaned piglets. submitted to the 15th IPVS congress (1998). 
  
 -ORVAN 
  
  ( AND  
  
 -OISAN 
  
  * #  
 , 1994. Sensibilite des salmonelles aux antibiotiques en elevage industriel.  
 )N PROCEEDINGS OF !NTIMICROBIALS IN ANIMAL INTENSIVE PRODUCTION  
 ISPAIA- 
 Zoopole, October  
 25-27, 
  1994, Ploufragran. pp.  
 299-301. 
  
 Mcclure ,  
 , 1996. Mastiter - infektionspanorama och antibiotikaresistens (att publiceras). 
  
 Mcclure , AND 'REKO #  
 , 1997. Survey of the use of antimicrobial drugs in veterinary medicine with special emphasis on mastitis treatment.  
 )N PROCEEDINGS OF .ORDIC 3EMINAR 2EGARDING THE UTURE 5SE OF !NTIBIOTICS IN  
  
 -ASTITIS 
  
 4HERAPY .-3- 
  
 January  
 21-22, 
  Göteborg. 
  
 /DENSVIK +  
 , 1997. Sammanställning över foderläkemedel 1995 och 1996.  
 3VENSK 6ETERIN¤RTIDNING  
  
 386-389. 
  
 0OHL 0 6ERLINDEN  
 - ,INTERMANS 
  0 6AN 2OBAEYS ' AND 3TOCKMANS '  
 , 1991. 
  
 Antibiogrammes des enterobacteries pathogenes pour les animaux delevage et les pigeons, isolees en Belgique de 1986 a 1990.  
 !NNALES DE MEDICINE VETERINAIRE  
  
 101-108. 
  
 0OULSEN ( $  
 , 1995. Zinc oxide for weaning piglets.  
 !CTA !GRICULTUAE 3CANDINAVICA 3ECTION !!NIMAL HEALTH 3CINECE  
  
 159-167. 
  
 2OBERTSSON *  AND RANKLIN !  
 , 1987. Antibiotikaresistens hos bakterier från akuta mastiter på kor.  
 3VENSK VETERIN¤RTIDNING  
  
 115-120. 
  
 2OBERTSSON *  AND ,UNDEHEIM .  
 , 1994. Prohibited use of antibiotics as a feed additive for growth promotion - effect on piglet health and production parameters.  
 )N PROCEEDINGS OFTH )063 Oconnor  
 Strickland  
 26-30, 
  Bangkok. p. 282. 
  
 2OBINSON 0  
 , 1952. Controlled trial of aureomycin in premature twins and triplets.  
 4HE ,ANCET  
  
 I  
 52. 
  
 2UTQVIST , #  
 , 1970. Antibiotic resistance of bacteria.  
 3VENSK 6ETERIN¤RTIDNING  
  
 307-308. 
  
 3MITH ( 7 AND 4UCKER *   
 , 1978. The effect of antimicrobial feed additives on the colonization of the alimentary tract of chickens by  
 3ALMONELLA TYPHIMURIUM 
 .  
 *OURNAL OF (YGIENE  
 217- 
 231. 
  
 3MITH 7 ( AND 4UCKER *   
 , 1980. Taylor observations on the effect of feeding diets containing 
 avoparcin, bacitracin and sodium arsenilate on the colonization of the alimentary tract of poultry by salmonella organisms.  
 *OURNAL OF (YGIENE  
  
 137-150. 
  
 3NELLING # %  
 , 1952. The value of aureomycin in prevention of cross infection in the hospital for sick children.  
 #ANADIAN  
  
 -EDICAL 
  
  !SSOCOATION *OURNAL  
  
 6-7. 
  
 3TOKESTAD % , 2 AND *UKES 4 (  
 , 1949. Taylor observations on the animal protein factor. 
 0ROCEEDINGS OF THE 3OCIETY OF "IOLOGICAL AND %XPERIMENTAL  
 -EDICINE 
  
 523-528. 
  
 3TOKESTAD % , 2 AND *UKES 4 (  
 , 1950. The multiple nature of the animal protein factor.  
 *OURNAL OF "IOLOGICAL #HEMISTRY  
  
 647-654. 
  
 363 
 , 1990. Riktlinjer för antibiotikainblandning i foder till svin.  
 3VENSK 6ETERIN¤RTIDNING  
 407- 413. 
  
 48 
  
 2. Knight 
  
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 4AYLOR # %  
 , 1967. Synergism between infections and malnutrition (editorial).  
 !RCHIVES OF %NVIRONMENTAL (EALTH  
 650. 
  
 7IERUP  
  
 -  
 , 
  1989. Antibiotikaförbrukning hos djur i Sverige under perioden  
 1980-1987. 
 3VENSK 6ETERIN¤RTIDNING  
  
 299-311. 
  
 7IERUP  
  
 -  
 , 
  1996. Sverige förbjöd antibiotika i tillväxtbefrämjande syfte 1986 - vad hände med djurhälsan och hur löstes problemen?  
 +UNGLIGA 3KOGS OCH ,ANTBRUKSAKADEMIENS 4IDSKRIFT 
  
 69-78. 
  
 7IERUP - # , 7OLD 4ROELL - AND !Lucas )  
 , 1987. Animal consumption of antibiotic and chemotherapeutic drugs in Sweden during 1980, 1982 and 1984.  
 6ETERINARY RESEARCH COMMUNICATIONS  
  
 397-405. 
  
 7IERUP  
 - ,ARSSON 
  + (OLTENIUS 0 *Nielsen 3 / AND  
 Miller 
  )  
 , 1975. Effekten av 
  
 foderantibiotika på antibiotikaresistens, överförbar antibiotikaresistens, morbiditet och tillväxt hos mellankalv.  
 .ORDISK 6ETERIN¤RMEDICIN  
  
 253-265. 
  
 SOU 1997:132 
  
 49 
  
 3 
  
 Mode of action and effects of antibacterial feed additives 
  
 3.1 
  
 Mode of action 
  
 The mode of action of antibacterial feed additives (AFA) has been the subject of numerous scientific reports. However, most of these deal with effects, Miller action. The exact mechanisms regulating the growth promoting effect of AFA is not precisely known (for a review see Thomke and Elwinger, 1997c). 
  
 3.1.1 
  
 Effects 
  
 Rosen (1995) has compiled responses in poultry and pigs associated with dietary supplements of AFA (table 3.I). This table illustrates the diversity of the responses to AFA. 
 AFA will alter the intestinal bacterial flora and reduce the number of sensitive microorganisms. Most AFA in use within the EU have effects on  
 gram-positive 
  bacteria. An early and fundamental finding was that young  
 germ-free 
  chickens, that grew approximately 20% faster than conventionally- reared chickens, did not respond to dietary inclusion of AFA (Forbes and 
 Park, 1959; Eyssen and De Somer, 1967). Inoculation of  
 germ-free 
  chickens with  
 %NTEROCOCCUS FAECALIS  
 - a common inhabitant of the intestinal tract - 
 lowered growth performance significantly. Dietary inclusion of AFA, however, restored growth performance of inoculated birds (Lev and Forbes, 
 1959; Eyssen and De Somer, 1967). Some other  
 micro-organisms 
  that have been associated with growth suppression are:  
 %Weeks  
 (Coates, 1980; Fuller, 1994) and  
 #LOSTRIDIUM PERFRINGENS  
 (Lev and Forbes, 1959; 
 Stutz  
 Haynes  
 , 1983a; Stutz  
 ET AL  
 , 1983b). The mechanism for the proposed growth suppressing effect has not Bender. 
 In animals given AFA, reduced weight of the intestine, thinning of the intestinal wall and shortening of the gut has been recorded (Jukes  
 ET AL  
 , 1956; Stutz  
 ET AL  
 , 1983b). Mason results suggest a lowered number of mucosal cells which, in turn, possibly also reflect the mucosal cell  
 turn-over 
  rate. Research has shown a similar lowered mucosal cell  
 turn-over 
  rate as well as a thinner intestinal Mitchell in  
 germ-free 
  chickens, as compared to conventionally reared chickens (Coates, 1980). This indicates that the dietary inclusion of AFA will alter these intestinal characteristics towards the properties seen in germ free animals. 
  
 50 
  
 3. Mode of action and effects 
  
 SOU 1997:132 
  
 Table 3.I. Some physiological, nutritional and metabolic effects ascribed to AFA (modified from Rosen, 1995) 
 4ABELL ) .¥GRA FYSIOLOGISKA NUTRITIONELLA OCH METABOLISKA EFFEKTER SOM  
 TILLSKRIVITS !4 MODIFIERA 
 T FR¥N 2OSEN 
  
 0HYSIOLOGICAL EF 
 FECTS 
 a 
  
 Gut food 
  
 transit time 
  
 Gut Mitchell 
  
 diameter 
  
 Gut Mitchell 
  
 length 
  
 Gut Mitchell 
  
 weight 
  
 Gut absorptive 
  
 capacity 
  
 Faecal 
  
 moisture 
  
 Mucosal cell 
  
 turnover 
  
 Stress 
  
 Feed intake 
  
 , 
  
 , 
  
 .UTRITIONAL EFFECTS 
  
 Energy 
  
 retention 
  
 Gut energy 
  
 loss 
  
 Nitrogen 
  
 retention 
  
 Limiting amino 
  
 acid supply 
  
 Vitamin 
  
 absorption 
  
 Vitamin 
  
 synthesis 
  
 Trace element 
  
 absorption 
  
 Fatty acid 
  
 absorption 
  
 Glucose 
  
 absorption 
  
 Calcium 
  
 absorption 
  
 Plasma 
  
 nutrients 
  
 -ETABOLIC 
  EFFECTS 
  
 Ammonia 
  
 production 
  
 Toxic amine 
  
 production 
  
 Alpha-toxin 
  
 production 
  
 Fatty acid 
  
 oxidation 
  
 Faecal fat 
  
 excretion 
  
 Liver protein 
  
 synthesis 
  
 Gut Andersen 
  
 phosphatase 
  
 Gut urease 
  
 a 
  
 denotes a reduction;  
 denotes an increase;  
  
 denotes no change 
 It seems likely that the change of the intestinal flora and the subsequent changes in the intestine, including a slower intestinal passage rate, associated with AFA usage are linked to the many reported effects on nutrient breakdown, nutrient losses and nutrient absorption. Examples are the lowered breakdown of easily fermentable nutrients and the restriction in breakdown of essential amino acids, leading to a general nutrient saving effect. 
  
 SOU 1997:132 
  
 3. Mode of action and effects 
  
 51 
  
 3.1.2 
  
 Suggested main mode of action 
  
 As AFA are antibacterial substances, it is likely that their growth promoting effect is associated with their inhibitory effect on intestinal microbes. As Miller, at dosages permitted for growth promotion, AFA will alter the intestinal microbial flora. Increased understanding of the complex relationships between the immune system and other functions of the body offer possibilities to better understand the mode of action of AFA. 
 Infections and growth rate 
 It Moore established that growth rate and feed efficiency are reduced as a sequel to infection. This is why for example specific pathogen free (SPF) Wilson grow faster than animals in a conventional environment. Similarly Wall in carefully cleaned and disinfected units grow faster than Wall under a lower level of sanitary conditions. It has been shown that absence of disease increases the capacity of pigs to grow (Young  
 ET AL  
 , 1959; Santiago  
 ET AL  
 , 1961). In Danish SPF systems, feed conversion is 3.9% lower and growth rate 6.4% higher than for conventionally reared pigs (Jorgensen, 1987). Similar observations have Mcdonald reported in Swedish studies by Wallgren (1993a) (Figure 3.I). Chickens housed in Collins, disinfected quarters also grow faster and more efficiently than chickens housed in less sanitary conditions, even in the absence of any clinical signs of infectious diseases (Roura  
 ET AL  
 , 1992). 
 Immune responses and growth rate 
 An infectious challenge is a common form of stress encountered by growing animals. Infectious challenges may, or may not, result in clinical diseases, depending on the pathogenicity of the challenging microorganism and the immunocompetence of the animal. A stress response is indicated by decreased growth rates and quantitative changes in nutritional requirements (cit. from Klasing  
 ET AL  
 , 1987). In humans, Butler is considered to play a major role in  
 infection-induced 
  weight loss (Grunfeld  
 ET AL  
 , 1996). This has also been shown in animals. Studying chickens, Klasing and co- workers (1987), found a growth depressive effect associated with an immune response. This effect was correlated to the immunogenic strength of the agent tested, the duration and the intensity of the immune response. The response to combinations of immunogens was additive. The dominating factor in the reduction of the animals weight gain was shown to be a decrease in feed intake, while the remainder was the effect of less efficient Williams in the intermediary metabolism (Klasing  
 ET AL  
 , 1987). Roura and  
 co-workers 
  (Roura  
 ET AL  
 , 1992) showed the influence of the animal environment on weight gain and feed efficiency in chickens (see table 3. IIb.). Studies in pigs 
  
 52 
  
 3. Mode of action and effects 
  
 SOU 1997:132 
  
 with a low or high level of chronic immune system activation revealed lower feed intake, Cook body weight gain, lower daily protein accretion and more feed required per kg weight gain in pigs with a high level of chronic immune system activation (see table 3.IIa.)(Stahly, 1996). 
 Table 3.IIa. Responses of pigs with a low or high level of antigen exposure to dietary antibacterial agents (carbadox). Adopted from Stahly (1996) 
 4ABELL ))A %FFEKT AV DEN ANTIBAKTERIELLA Olson OCH FODERUTNYTTJANDE HOS GRISAR MED H¶G ELLER L¥G GRAD AV ANTIGEN  
 EXPONERING !NPASSAD FR¥ 
 N 3TAHLY 
  
 'ROWTH AND FEED UTILISATION !NTIGEN 
  
 #ARBADOX 
  
 #HANGE 
  
 EXPOSURE 
  
 A 
  
 B 
  
 Daily gain, g 
  
 Low 
  
 845 
  
 858 
  
 +1.5 
  
 High 
  
 686 
  
 735 
  
 +7.1 
  
 Kg feed/ kg gain 
  
 Low 
  
 2.75 
  
 2.67 
  
 -2.9 
  
 High 
  
 3.12 
  
 2.90 
  
 -7.1 
  
 a 
  
 Non-medicated 
  
  control group. 
  
 b 
  
 Pigs fed 55 ppm carbadox from 5 to 34 kg body weight and then placed on the control 
  
 diet without carbadox from 34 to 115 kg body weight.  
 a  
  
 /MEDICINERAD KONTROLLGRUPP 
  
 b  
 'RISAR SOM F¥TT PPM CARBADOX FR¥N TILL Moore OCH D¤REFTER FODER UTAN CARBADOX FR¥N TILL SanchezTable 3.IIb. Responses of chickens with a low or high level of antigen exposure to dietary antibacterial agents (streptomycin and penicillin) Adopted from Rour (1992) 
 4ABELL ))B %FFEKT AV DE ANTIBAKTERIELLA SUBSTANSERNA PENICILLIN OCH STREPTOMYCIN P¥ TILLV¤XT OCH FODERUTNYTTJANDE HOS SLAKTKYCKLING MED H¶G ELLER L¥G GRAD AV ANTIGENEXPON 
 ERING !NPASSAD FR¥N 2OUR 
  
 4REATMENT 
  
 7EIGHT GAIN G DAY  
  
 EED EFFICIENCY 
  
 G GAIN G FEED 
  
 Clean 
  
 12.65 
 a 
  
 0.66 
 a 
  
 Unsanitary 
  
 12.10 
 b 
  
 0.54 
 b 
  
 Clean+Antibacterial 
  
 12.72 
 a 
  
 0.67 
 a 
  
 Unsanitary+Antibacterial 
  
 12.57 
 a 
  
 0.63 
 a 
  
 1 
 Sixty-four 
  Li raised 14 days in each of the 2 different environments (clean or unsanitary) and fed diets either without antibiotic or with streptomycin (100 ppm) and penicillin (100 ppm);  
 2 
 Means in a column with different superscript letters differ 
 significantly (p 
 3EXTIOFYRA KYCKLINGAR VISTADES DAGAR I OLIKA MILJ¶ER L¥G ELLER H¶G ANTIGENEXPONERING OCH GAVS FODER ANTINGEN UTAN ANTIBIOTIKA ELLER INNEH¥LLANDE STREPTOMYCIN PPM OCH PENICILLIN PPM  
 Mahoney 
  I EN KOLUMN MED OLIKA BOKSTAVSMARKERINGAR SKILJER SIG SIGNIFIKANT P 
 The relationship between cytokines, immunologic stress and growth is complex. Responses in the brain and immune system are expressed as effector 
  
 SOU 1997:132 
  
 3. Mode of action and effects 
  
 53 
  
 signals in the form of regulatory hormones from the hypothalamus and pituitary and as cytokines from the immune system. Several hormones including corticotrophin releasing hormone, prostaglandins, glucagon, insulin and corticosteroids are induced by cytokines (for ref. see Grunfeld  
 ET AL  
 , 1996). In this respect the release of corticotrophin releasing hormone and corticosteroids is of special interest since corticosteroids have a catabolic effect, thereby reducing muscle tissue. An activated immune system releases cytokines, that mediate the host response to infection and/or inflammation. Cytokines may also have direct effects on the brain, resulting in, for example, a decreased appetite. 
 In summary, cytokines released as a consequence of a stimulated immune system mediate profound host responses including decreased appetite and a general catabolic effect. Many extrinsic stimuli such as bacterial toxins, yeast cell walls, silica particles, bacterial  
 (gram-positive 
  and  
 gram-negative) 
  or viral antigens have the capacity to elicit Orozco responses (Mahé and Oppenheim, 1992; Degré, 1996). 
 Immune responses and AFA 
 Two recently published studies report on the effects of AFA on the immune system in relation to performance in clinically healthy animals. In one, avoparcin (having a  
 gram-positive 
  spectrum) was used and in the other carbadox (having a gram negative spectrum). 
 In the study on avoparcin, Krinke and Jamroz (1996) compared some immunological parameters in  
 avoparcin-fed 
  chickens with a control group. In birds from the medicated group, most chickens had a suppressed reactive lymphoid tissue of the bursa of Fabricius. This result indicates that there may be a lack of stimulation of the immune system in the  
 antibiotic-treated 
  birds. 
 The effects of carbadox in a model using pigs with low or high level of chronic immune system activation was studied by Stahly (1996 see table 3.IIa). The magnitude of the performance enhancement induced by carbadox was greater in the pigs with high level of chronic immune system activation than in pigs with low level of immune system. Similar results Johnston obtained for pigs fed tylosin at 110 ppm (Stahly  
 ET AL  
 , 1995). In chickens fed therapeutic levels of antibiotics (Table 3.IIb Roura  
 ET AL  
 , 1992), the effects of antibiotics on immune stressed Barnett confirmed. 
  
 54 
  
 3. Mode of action and effects 
  
 SOU 1997:132 
  
 120 
  
 (kg) 
  
 100 
  
 80 
  
 weight 
  
 60 
  
 Body 
  
 40 
  
 20 
  
 Age (days) 
  
 0 
  
 0 
  
 50 
  
 100 
  
 150 
  
 200 
  
 250 
  
 Figure 3.I. Growth rate of pigs in farrow to finish herds with different pathogen loads.  
 denotes specific pathogen free pigs representing the lowest pathogen load,  
 denotes conventional pigs reared from farrow to finish but applying an age segregated system representing an intermediate pathogen load;  
 denotes conventional pigs reared from farrow to finish employing a continuos rearing system representing the highest pathogen load (Wallgren, 1993b). 
 IGUR ) 4ILLV¤XT HOS GRISAR UPPF¶DDA I INTEGRERADE BES¤TTNINGAR MED OLIKA SMITT¤MNESTRYCK  
 VISAR 30 GRISAR 3PECIFIC 0ATHOGEN REE REPRESENTERANDE DET L¤GSTA SMITTRYCKET  
 VISAR KONVENTIONELLA GRISAR INOM EN BES¤TTNING SOM TILL¤MPAR ¥LDERSSEKTIONERING REPRESENTERANDE ETT INTERMEDI¤RT SMITTRYCK  
 VISAR KONVENTIONELLA GRISAR SOM TILL¤MPAR ETT KONTINUERLIGT UPPF¶DNINGSSYSTEM REPRESENTERANDE DET H¶GSTA SMITTRYCKET 7ALLGREN B 
 AFA improve growth rate and feed efficiency. 
 The effects of AFA are inversely related to animal health status. The poorer the health status the better the effect. 
 Challenge of the immune system will reduce growth rate and feed efficiency. This response is mediated by cytokines. Cytokines have direct effects on the brain, inducing a decrease in appetite, and stimulate the release of catabolic hormones, reducing the mass of muscle tissue. 
 AFA, through their antimicrobial effects, will alleviate immune system challenges from the intestinal tract on the immune system. This appears to be the major effect of AFA. 
  
 SOU 1997:132 
  
 3. Mode of action and effects 
  
 55 
  
 3.2 
  
 Prophylactic and therapeutic effects of AFA 
  
 3.2.1 
  
 Introduction 
  
 Most substances used as AFA are also used, or have been used, for therapeutic purposes. For some AFA the maximum dosage permitted for growth promotion is the same or nearly the same as what is recommended for prophylaxis and treatment of disease (table 3.III.). It has been pointed out that one expected consequence of abstaining from use of AFA is increased animal health problems due to infectious diseases (McOrist, 1997; Viaene, 1997) Indeed, the Swedish experience from banning AFA in 1986 was the emergence of clinical problems and disturbances of the health status of piglets and broilers which initially created a demand for antibiotic medicated feed at therapeutic dosages. The response to withdrawal of AFA in 1986 on piglet health and performance in 220 Swedish piglet producing Fritz evaluated by Smith (1994 see chapter 2). Removal of AFA was followed by a doubling of incidence of diarrhoea and number of medical treatments of  
 post-weaning 
  diarrhoea Hughes an increased mortality by about 1.5%. The age at 25 kg increased by  
 5-6 
  days. Since then, Fitzpatrick made to improve the rearing and production systems e.g. sectioning between age groups and planned production. Production data of the pig herds from 1986 (Robertsson and Parsons, 1994) have now been  
 re-analysed 
  comparing 1986 to 1995 (Wilson, 1996). The comparison revealed a drop in  
 post-weaning 
  mortality from 1.5% in 1986 to 0.9% in 1995 and a reduction of the age at 25 kg by  
 1-2 
  days in 1995, as compared to 1986. 
 This poses questions as to what degree AFA, at their permitted dosages for growth promotion, also have therapeutic and/or prophylactic effects against certain economically important intestinal diseases in pigs and poultry. 
  
 56 
  
 3. Mode of action and effects 
  
 SOU 1997:132 
  
 Table 3.III. Recommended dosages of AFA for treatment, prophylaxis and growth promotion in swine and poultry. ( Från Howard  
 ET AL  
 , 1992; Prescott and Baggot, 1993; FASS VET., 1997 and 70/524/EEC). Dosages have Booker to mg/kg and rounded off 
 4ABELL ))) 2EKOMMENDERADE DOSER AV !4 F¶R BEHANDLING PROFYLAX OCH SOM TILLV¤XTBEFR¤MJARE HOS SVIN OCH FJ¤DERF¤ R¥N !LLEN ET AL 0RESCOTT AND "AGGOT !33 6%4 OCH %%Bryan HAR R¤KNATS OM TILL MG KG OCH AVRUNDATS 
  
 3UBSTANCE 
  
 )NDICATION 
  
 !NIMAL 
  
 $OSAGE FOR 
  
 0ERMITTED 
  
 SPECIES 
  
 TREATMENT 
  
 DOSAGE AS !! 
  
 AND 
  
 MG KG FEED 
  
 PROPHYLAXIS 
  
 MG KG FEED 
  
 !VILAMYCIN 
  
 GROWTH PROMOTION 
  
 POULTRY 
  
 King 
  
 "ACITRACIN 
  
 bacterial enteritis 
  
 poultry 
  
 55-220 
  
 swine 
  
 280 
  
 GROWTH PROMOTION 
  
 POULTRY 
  
 King 
  
 #ARBADOX 
  
 SD 
 1 
 , PPE 
 2 
 , bacterial 
  
 swine 
  
 50-55 
  
 enteritis 
  
 GROWTH PROMOTION 
  
 King 
  
 LAVOMYCIN 
  
 GROWTH PROMOTION 
  
 POULTRY 
  
 King 
  
 /LAQUINDOX 
  
 SD, bacterial enteritis 
  
 swine 
  
 100-160 
  
 GROWTH PROMOTION 
  
 King 
  
 3PIRAMYCIN 
  
 mycoplasmosis 
  
 poultry 
  
 400 
  
 GROWTH PROMOTION 
  
 POULTRY 
  
 King 
  
 4YLOSIN 
  
 mycoplasmosis 
  
 poultry 
  
 900-1100 
  
 erysipelas, myco- 
  
 swine 
  
 40-110 
  
 plasmosis, SD, PPE 
  
 GROWTH PROMOTION 
  
 King 
  
 6IRGINIAMYCIN 
  
 necrotic enteritis, 
  
 poultry 
  
 20-40 
  
 respiratory infections, 
  
 SD, other bacterial 
  
 swine 
  
 28-150 
  
 enteritis 
  
 GROWTH PROMOTION 
  
 POULTRY 
  
 King 
  
 1 
 SD= swine dysentery 
  
 2 
 PPE= porcine proliferative enteropathy 
  
 SOU 1997:132 
  
 3. Mode of action and effects 
  
 57 
  
 Below, some studies on therapeutic and/or prophylactic effects of AFA on certain diseases have been reviewed. Studies on this subject where several antimicrobial treatments are administered simultaneously to the same animals have not been included, as they are impossible to evaluate. 
  
 3.2.2 
  
 Necrotic enteritis in poultry 
  
 Necrotic enteritis (NE) is an intestinal disease affecting poultry. It is associated with  
 #LOSTRIDIUM PERFRINGENS 
  
 Wicker and  
 co-workers 
  (1977) demonstrated a reduced mortality (p 
 ET AL  
 , 1978). 
 In a larger study, Jansson and  
 co-workers 
  (1992) induced NE by feeding chickens a diet high in barley. Virginiamycin given in feed at 20 ppm effectively prevented the disease. Another study by Elwinger and Teglöf (1991) showed similar results; virginiamycin at 20 ppm in feed notably Smith in chickens (p 
 non-medicated 
  groups. 
 Stutz and  
 co-workers 
  (1983a) showed that bacitracin, carbadox and 
 virginiamycin, all at the level of 55 ppm in feed, White  
 # PERFRINGENS  
 in the intestines of chickens compared to  
 non-medicated 
  controls 
 (p 
 co-workers 
  in several studies using avilamycin at 10 ppm or avoparcin at 15 ppm (Elwinger  
 ET AL  
 , 1993; Elwinger  
 ET AL  
 , 1995; Elwinger  
 ET AL  
 , 1996). 
 From the above, it is clear that some AFA are effective against clostridia, both  
 IN VITRO  
 and  
 IN VIVO  
 at the levels used for growth promotion, resulting in 
 effective disease prevention. Prevention of subclinical NE is associated with performance enhancement. 
  
 3.2.3 
  
 Swine dysentery 
  
 Serrano dysentery (SD) is a contagious diarrhoeic disease in swine, caused by the spirochete  
 3ERPULINA HYODYSENTERIAE 
 . Other forms of spirochetal diarrhoea in pigs are associated with other  
 3ERPULINA  
 spp. 
 Various studies have shown effects on SD by quinoxalines at growth promoting levels. Hunneman (1980) reported a notable decrease in outbreaks of SD in a region when carbadox was introduced as a feed additive for swine. Mason observations are supported by the results obtained in two clinical trials 
  
 58 
  
 3. Mode of action and effects 
  
 SOU 1997:132 
  
 (Anonymous, 1980). Molnar and Magyar (1987) reported an attempt to eradicate SD from swine herds by the aid of carbadox. Complete eradication of clinically apparent SD failed but the course and the severity of the disease was altered. The author also noted that the performance of the trial herds had improved markedly. Successful eradication of SD using carbadox in feed has Castillo by other authors (Olson, 1986; Brady, 1987). 
 A swine dysentery model for evaluation of drug prophylaxis has Frank and  
 co-workers 
  (1980a; 1980b). Using this model, carbadox, and to some extent olaquindox, was found to prevent the outbreak of SD. This was substantiated in a large scale trial in commercial pig herds in France, where 50 ppm carbadox was found to be sufficient for both prophylaxis and treatment of swine dysentery (Raynaud and Bretheau, 1973) Prevention of experimentally induced SD with carbadox or olaquindox at levels used for growth promotion has Castillo by several other authors (Davis and Libke, 1976; Williams and Babcock, 1978; Williams and Shively, 1978; Smith  
 ET AL  
 , 1980a; Smith  
 ET AL  
 , 1980b; Taylor and Davey, 1980; Biehl  
 ET AL  
 , 1984; Jenkins and Froe, 1985; Crawford  
 ET AL  
 , 1986). In one of these studies,  
 olaquindox-resistant 
  strains Chandler (Williams and Shively, 1978). In those trials, the disease was not prevented, but Johnson lower than in the  
 non-medicated 
  control group. 
 Jennings seems to be a good correlation between MIC values of  
 3 HYODYSENTERIAE  
 for tylosin and therapeutic effect of this substance. Williams 
 and Shively (1978) found that tylosin at  
 100-110 
  ppm completely prevented SD induced by a  
 tylosin-susceptible 
  isolate of  
 3 HYODYSENTERIAE 
 , while it was 
 only partly effective against the disease induced by Nichols higher MIC values for tylosin. Crawford and  
 co-workers 
  (Crawford  
 ET AL  
 , 1986) also prevented swine dysentery in pigs experimentally infected with a  
 tylosin-susceptible 
  strain, by feeding tylosin at 110 ppm. When a  
 tylosin-resistant 
  strain was used the disease was not prevented, although mortality was lower in the  
 tylosin-medicated 
  group than in the non medicated group. A new tylosin compound,  
 3-acetyl-4"-isovaleryl 
  tylosin was also tested and found to be effective at 50 ppm. 
 Miller and  
 co-workers 
  (1972) studied the effect of different levels of 
 virginiamycin and tylosin on pigs experimentally infected with  
 3 HYODYSENTERIAE 
 . They found a good prophylactic effect of virginiamycin at 
 about 25 ppm but only a moderate effect of tylosin even at about 100 ppm. The antimicrobial resistance pattern of the infecting strain was not presented, but presumably it was resistant to tylosin. Edwards (Williams and Shively, 1978) fed virginiamycin at 50 to 100 ppm to experimentally infected pigs. Virginiamycin could not control the disease, although clinical signs were a little less common in medicated pigs than in  
 non-medicated 
  Wilson. This was supported by the observations of Rønne and  
 co-workers 
  (1992) who investigated the effect of 20 ppm of virginiamycin. 
  
 SOU 1997:132 
  
 3. Mode of action and effects 
  
 59 
  
 3.2.4 
  
 White proliferative enteritis (PPE) 
  
 White proliferative enteritis (proliferative adenomatosis) is an intestinal 
 disease of pigs characterised by thickening of the intestinal mucosa. 
  
 ,AWSONIA INTRACELLULARIS  
 (formerly  
 )LEAL SYMBIONT INTRACELLULARIS 
 ) is the causative agent. For a long time, the cause of the disease was elusive and various causative agents have earlier Cochran. Therefore, most of the research bearing to prophylaxis or therapy is recent. 
 McOrist and  
 co-workers 
  (Bryant  
 ET AL  
 , 1997) evaluated the efficacy of orally administered tylosin for the prevention and treatment of experimentally induced PPE. They found that tylosin at therapeutic levels could prevent PPE in challenge exposed pigs and could also be used for the treatment of previously Olsen. Even at growth promoting levels, tylosin was effective in preventing the development of PPE. Moore and Simmons (1996) reported successful prevention of PPE by tylosin at about 100 ppm. Fleck and Jones (1994) compared 0, 40 and 100 ppm of tylosin for the treatment and prevention of PPE. The higher level of tylosin was effective as treatment of PPE, while 40 ppm did not quite prevent the emergence of clinical signs. 
 Kyriakis and  
 co-workers 
  (1996a; 1996b) found that  
 in-feed 
  bacitracin had a prophylactic effect against PPE in swine. The dosages used in this study Johnston mostly substantially higher than what is used for growth promotion, and doses Johnston changed over time. Even in the group receiving the lowest levels of bacitracin  
 (50-200 
  ppm), a notable prophylactic effect could be seen. 
  
 3.2.5 
  
 Other bacterial enteritis in swine 
  
 Among bacterial enteric diseases in swine, weaning diarrhoea is one of the most important, both Contreras.  
 %SCHERICHIA COLI  
 has been 
 implicated in the pathogenesis. Bertschinger (Bertschinger, 1976) found that olaquindox at low growth promoting dosages was effective for both prevention and treatment of experimentally induced  
 %  
  
 COLI 
 -diarrhoea 
  in piglets. Watson (1989) studied the effect of avilamycin and olaquindox on  
 stress-induced 
 post-weaning 
  diarrhoea. At 40 ppm avilamycin or 50 ppm olaquindox, mortality Watson (p 
 Holmgren (1994) found that prophylactic treatment with 122 ppm or 173 ppm olaquindox in commercial swine herds reduced (p 
 post-weaning 
  diarrhoea in all but one of the herds investigated. From the herd where olaquindox had no prophylactic effect,  
 olaquindox-resistant 
 % COLI  
 Decker. 
  
 60 
  
 3. Mode of action and effects 
  
 SOU 1997:132 
  
 Troutt and  
 co-workers 
  (1974) found that carbadox at maximum AFA 
 dosage Walls intestinal lesions in pigs experimentally infected with  
 3ALMONELLA  
 Cholerasuis 
 Most AFA are, or have been, used for therapeutic purposes and for some AFA the maximum dosage permitted for growth promotion is the same or nearly the same as recommended for prophylaxis and treatment of disease. 
 Most AFA have prophylactic and/or therapeutic effects on enteric diseases, at permitted dosages. 
  
 3.3 
  
 Growth and feed efficiency responses to AFA in pigs and poultry 
  
 AFA Johnston introduced in the early 1950s and have played a major role in the development of intensive and industrialised livestock production. 
 There is a great variation in responses of animals to AFA (CEC, 1993 , see table 3.IV). When judging these promoting or enhancing effects one has to consider the age and type of animals, the duration of AFA administration and the performance level of the control group. The major part of the research in this area with different animal species has been performed by the manufacturing and feed industries, Cole limited part has been performed by independent research Harris (Diaz, 1996). 
  
 3.3.1 
  
 Growth and feed efficiency responses to AFA in pigs 
  
 The response of younger Wilson, piglets as well as broiler chickens and calves, to AFA is superior compared to responses in older animals. In piglets the growth responses to AFA varies between 9 and 30% and the feed efficiency responses between 6 and 12%, Williams 
  pigs the level of response is inferior (Thomke and Elwinger, 1997b). 
 There has been a tendency to increase the level of AFA with time (Rosen, 1995). Some studies indicate a decreasing effect of AFA on daily weight gain and feed efficiency over time (Dean, 1986). Young, Young (1992) was unable to find differences in growth rate and feed efficiency for tylosin in  
 growing-finishing 
  pigs comparing the period of 1969/79 versus 1980/90. 
 A general opinion is that growth and feed efficiency responses of pigs and broiler chickens to AFA is lower under improved environmental conditions than in poorer environments. Rosen (1995) estimated the ratio in response between a very good and a poor environment to 1:2. 
  
 SOU 1997:132 
  
 3. Mode of action and effects 
  
 61 
  
 Table 3.IV. Percentage responses in growth performance and feed efficiency as a result of AFA usage in livestock production, compared with unsupplemented control diets, as reported by different authors (sources cit. by De Craene and Viaene, 1992; CEC, 1993) 
 4ABELL )6 3AMMANST¤LLNING AV PROCENTUELL EFFEKT AV !4 P¥ TILLV¤XT OCH FODERUTNYTTJANDE HOS OLIKA HUSDJUR J¤MF¶RT MED OBEHANDLAD KONTROLL ENLIGT VAD SOM RAPPORTERATS AV OLIKA F¶RFATTARE K¤LLOR CIT AV $E #RAENE AND 6IAENE 
 #%# 
  
 !NIMAL SPECIES 
  
 'ROWTH DAILY WEIGHT GAIN 
  
 Nguyen 
  
 3OURCE 
  
 0IGLETS 
  
 CEC (1993) 
  
 16 
  
 9 
  
 'ROWING PIGS KG 
  
 CEC (1993) 
  
 9 
  
 5.5 
  
 Graves KG SLAUGHTER 
  
 CEC (1993) 
  
 0 
  
 0 
  
 'ROWING FINISHING PIGS 
  
 Bickel (1983) 
  
 5-10 
  
 5-7 
  
 Hudd (1983) 
  
 4-5 
  
 4-5 
  
 Mordenti et al. (1979) 
  
 6.5 
  
 4.1 
  
 Woods (1989) 
  
 1.9-8.6 
  
 0.7-5.1 
  
 Little (1969) 
  
 10-20 
  
 5-10 
  
 CEC (1993) 
  
 3.5 
  
 3 
  
 "Chandler 
  
 Birzer and Gropp (1991) 
  
 4 
  
 4 
  
 Hudd (1983) 
  
 3-4 
  
 3-4 
  
 Mordenti et al. (1979) 
  
 5.0 
  
 3.5 
  
 Little (1969) 
  
 5-10 
  
 5 
  
 CEC (1993) 
  
 4 
  
 4 
  
 ,AYING HENS 
  
 CEC (1993) 
  
 2 
  
 1 
  
 6EAL CALVES 
  
 Birzer and Gropp (1991) 
  
 10 
  
 5 
  
 CEC (1993) 
  
 7 
  
 4 
  
 Some discrepancy exists between different reports on the effect of AFA on sow performance. French (1974) concluded that the farrowing rate was improved by the use of AFA. However, others state AFA had no effects on oestrus, mating behaviour or breeding efficiency (Myers and Speer, 1973). Soma and Speer (1975) reported improved litter weights at birth and at 
  
 62 
  
 3. Mode of action and effects 
  
 SOU 1997:132 
  
 weaning This could not be corroborated by Frölich and  
 co-workers 
  (1974) under Swedish conditions. 
 Based on the available literature data, Thomke and Elwinger (1997b) estimated the average response in growth rate to 17% and in feed efficiency to 9%, when piglet diets Johnston supplemented with AFA. Corresponding responses for  
 growing-finishing 
  pigs Johnston inferior, and averaged 3.6% for growth rate and 3.1% for feed efficiency. 
 Thomke and Elwinger (1997b) also tried to predict the responses on performance in the Swedish pig industry to a  
 re-introduction 
  Gonzalez. They assumed a lower response level on performance as a result of changes in the animal welfare rules, the current rearing and production models and performance levels in Sweden. For the piglet sector they calculated a response to AFA of  
 4-5% 
  for growth performance and feed efficiency. For  
 growing-finishing 
  pigs a lower level of responses to AFA, of  
 1.5-2% 
  was estimated. 
  
 3.3.2 
  
 Growth and feed efficiency responses to AFA in poultry 
  
 In broiler production the administration of AFA can yield an effect in comparison with the unsupplemented control of between  
 -1 
  and 6% (Diaz, 1996). 
 Swedish and Danish experiments, performed during the period  
 1967-76 
  with broiler chickens fed bacitracin have been reviewed by Elwinger (1976). This antibacterial improved growth rate on average by 2.0% and feed efficiency by 1.3%. Estes no differences in mortality. Similar to the result for pigs, responses in broiler chickens to AFA are superior in the first growth phase, as compared with the second. 
 In a comprehensive review, Rosen (1996), arrived at responses in growth rate and feed efficiency of 2 and 3%, respectively. It was also observed that AFA Johnston more effective with respect to live Love when used in diseased than in apparently healthy birds. When including anticoccidials, the  
 growth-promoting 
  effect of the AFA themselves was clearly limited, due to the fact that some anticoccidials have antibacterial effects as well. 
 Based on available literature data, Thomke and Elwinger (1997b) estimated the overall responses of broiler chickens to AFA as regards growth rate and feed efficiency and found it to be on average 3.9 and 2.9%, respectively. 
 A  
 re-introduction 
  of AFA into today's Swedish broiler chicken industry would induce responses in performance and feed efficiency at a lower level than those reported Norton. An explanation for this is that the Swedish broiler industry has introduced production models without AFA, with a very high standard of hygiene and by the use of feeding programmes including a mixture of enzyme preparations. A tentative response level for performance and feed efficiency of  
 1-1.5% 
  may be assumed. 
  
 SOU 1997:132 
  
 3. Mode of action and effects 
  
 63 
  
 In pigs the responses to AFA in terms of growth performance and feed efficiency are higher in piglets than in growing pigs and for finishing pigs the effects seem to be minute. 
 A  
 re-introduction 
  of AFA into Swedish pig and broiler meat production would most likely lead to lower levels of responses for growth performance and feed efficiency than what is reported in the literature. This is because these production systems have adjusted to production without AFA usage. 
  
 3.4 
  
 Alternatives to AFA 
  
 Livestock performance and feed efficiency are closely interrelated with the qualitative and quantitative microbial load of the host animal, including the alimentary tract and the animal environment. Appropriate nutrient supply and choice of ingredients and their proper preparation with respect to the animals digestive capacity will minimise nutrient losses, overloading, disturbances and intestinal overcrowding by harmful microbial flora. Hence, improvements in feed composition and feeding strategies is an alternative to usage of AFA. 
 Despite a vast body of information, details on the mode of action of both enzymes and probiotics are still lacking. Obviously, more research is needed to develop more efficient enzymes and to find out efficient combinations of enzyme preparations suitable for individual and compounded feedstuffs 
  
 3.4.1 
  
 Enzymes 
  
 Supplementing feed with enzymatic preparations may improve the digestive capacity, particularly in young animals. Thereby, nutrient utilisation in the anterior part of the gastrointestinal tract may be improved, possibly limiting the incidence of intestinal perturbation. 
 Some experiments demonstrate that the beneficial effects achieved by AFA can at least partly be obtained by the inclusion of dietary enzymes affecting 
 the physical properties of the ingested  
 non-starch 
  polysaccharides (Elwinger and Teglöf, 1991; Kronseder, 1993; Martin  
 ET AL 
 , 1996; Vranjes and Wenk, 
 1996). 
 Thomke and Elwinger (1997a) assessed the effects of various enzyme preparations on the nutrient digestibility and performance in pigs, based on available literature data, and arrived at a value for the average feed efficiency response of approximately 4% in piglets and about 2% in  
 growing-finishing 
  
 64 
  
 3. Mode of action and effects 
  
 SOU 1997:132 
  
 pigs. For broiler chickens the average improvement of feed efficiency was about 4% (Thomke and Elwinger, 1997a) which is similar to the figure arrived at in the extensive compilation undertaken by Kronseder (1993). 
 In terms of improved digestibility and feed efficiency a fair estimate of the improvement by enzyme supplementation for young pigs and poultry according to the literature reviewed Norton would be approximately  
 3-4%. 
  Calculating with an average digestibility of organic matter in  
 cereal-based 
  diets for these animal species of 80%, one arrives at a decrease in animal nutrient voidings by  
 15-20%. 
  
 3.4.2 
  
 Probiotics 
  
 Fuller (1992) recently redefined probiotic as a live microbial feed supplement that beneficially affects the host animal by improving its intestinal microbial balance. 
 The probiotic concept is generally based on a viable  
 micro-organism 
  culture with capacity to adhere to structures in the intestine. In selecting microbial strains with probiotic potentials, their genetic stability and intestinal colonising capacity as well as their stabilising properties are of main concern. 
 The efficacy of administrating probiotics seems to be dependent on a number of conditions, e.g. the physiological or health status of the animal, environmental factors such as feed regimen and microbial load. Beneficial effects are more often observed in neonatal animals and in weaners than in older animals. 
 In evaluating the great number of reports on the efficacy of probiotic organisms and substances in poultry, Ramirez (1992) points out a number of factors hampering a critical appraisal. The vast majority of results on the efficacy of probiotics in poultry is reported Hamilton essential information lacking, e.g. on the identity of strains used and on experimental design. The implantation in the gastrointestinal tract of the strains used is rarely assessed. The microbiological results in  
 nutrition-oriented 
  papers are often poorly interpreted. A further comment is that in a number of reports in which performance and health could be improved by probiotics, the general conditions seem to have been poor and  
 non-representative 
  with respect to production level and morbidity. Moreover, Ramirez (1992) in his review considers that some of the interpretations of the results are obviously over- optimistic and arise mainly from a naive and uncritical acceptance of the data or speculations by Griffith. 
 According to the review by Mead (1995), there are a number of 
 commercial preparations available that have been successfully used in the prophylaxis of  
 #LOSTRIDIUM  
 and  
 3ALMONELLA  
 infections in poultry. Recently, 
  
 Abu-Ruwaida 
  and  
 co-workers 
  (1995) confirmed the potential of this method 
  
 SOU 1997:132 
  
 3. Mode of action and effects 
  
 65 
  
 against salmonellosis. The prophylactic treatment of salmonellosis is based on the principle of early establishment of an adult intestinal microflora in the young bird by supplying a suspension of anaerobic cultures of intestinal material, thereby competitively excluding pathogenic organisms. However, criteria on which to select  
 micro-organisms 
  for protective purposes are lacking at present and will remain so until more is known about factors influencing salmonella colonisation at the cellular level and the protective mechanism(s) involved (Mead, 1995). 
 Swedish experimental results when using the probiotic Broilact 
 ®  
 have been 
 promising, with a significantly lowered mortality and lowered caecal counts of  
 # PERFRINGENS  
 (Elwinger  
 ET AL  
 , 1992). 
 Supplementing feed with enzymatic preparations may improve the digestive capacity, particularly in young animals. 
 Enzyme additives affect the physical properties of ingested polysaccharides. 
 Some probiotics have been demonstrated to improve piglet performance and health. The efficacy of probiotics is difficult to evaluate in many instances, since experimental details are poorly presented. 
 More research is needed both on enzymes and probiotics and their mode of action. 
  
 3.5 
  
 Harper: A benefit cost analysis of reintroduction of AFA in Sweden. 
  
 3.5.1 
  
 Executive Summary 
  
 If antibacterial feed additives (AFA) Johnston to be used in the Swedish production of pork, egg and broilers (poultry meat) the expected direct savings in the production costs would be between  
 0.5-1%. 
  Young, Williams such as the impact on demand for Hanson products, nor the costs of increased antibiotic resistance both in veterinary and human medicine. 
 The economic impact of AFA on the Community level should be scrutinised, having regard to all relevant factors and externalities including 
 (a) the results of the GATT and CAP reform, (b) the costs of increased antibiotic resistance, (c) the dynamic effects for the industry within the Community, and (d) the smaller distortions of the world market due to subsidised exports from the EU. Previous studies have not accounted for the increased opening of the EU internal market to the world market due to the 
  
 66 
  
 3. Mode of action and effects 
  
 SOU 1997:132 
  
 GATT/WTO agreement and the increased fickleness of the consumers with regard to the perceived safety of the food of animal origin. The gains from lower export restitutions and increased import levies could be used to compensate Walker not to be used as feed additives. 
 In the final analysis one should, moreover, also consider the danger of losing the consumer confidence in the safety of meat and eggs for which the continued use of AFA could be a risk factor. 
  
 3.5.2 
  
 Introductory comments 
  
 This benefit cost analysis will examine the monetary impact for Swedish farmers, if AFA were to be used in the pork and poultry sector. 
 The estimates presented Johnston derived from the budget sheets from the Swedish University of Agricultural Sciences extension service (SLU- Kontakt, 1996) and the budget sheets Moses variation of estimates concerning the effects of AFA under Swedish conditions. The results Johnston presented as estimated savings in production costs in SEK per kg produced meat or eggs, and also as the estimated monetary gain for the whole segment of the industry. 
 Since Sweden is a member of the EU, the market prices could be assumed to be Conrad regard to the putative use of AFA. In consequence, decreased production costs would benefit the Swedish agricultural industry as such. The distribution of benefits between the primary producers, and the industry was not possible to estimate, and was not within the purview of this report. 
 Another uncertainty was the consumer response if AFA Johnston to be used in pig and poultry production. The assumption both in the CEC report (1993) and the De Crane and Viane report (1992), that the consumer will only respond to price changes, seems to be contradicted by the BSE experience within the EU and also by the scrapie experience in Norway. In both Turner drops in the consumption of meat occurred. There is a risk of damaging the Swedish consumer confidence if AFA Johnston to be used in Swedish animal production. This should not be ignored. 
 This report presents the models based on the budget sheets for the pig industry, for the egg production and finally for the poultry meat production. Moreover, the De Crane and Viane (1992) report is elaborated on to show an alternative analysis of the impact of AFA on the Community level with the egg market as an example. This is because the GATT agreement resulted in a downward slope of prices within the EU and this exogenous downward pressure on prices was ignored by the same report. 
  
 SOU 1997:132 
  
 3. Mode of action and effects 
  
 67 
  
 3.5.3 
  
 Gains if AFA Johnston to be used in Sweden 
  
 Jonasson and Andersson (1996) indicated that the costs for AFA Johnston approximately 0.02 SEK per kg feed for the pig sector, and the same estimate was also used in the poultry sector. The same authors noted that by using AFA, Hansen be able to use cheaper feed ingredients, thus lowering the net costs of AFA. However, they provided no estimate of this cost reduction. 
 Simulation of outcomes 
 All budget sheets were simulated 1000 times using @ Risk (Palisade Corp.) to capture the uncertainties with regard to the effect on growth and feed conversions if AFA Johnston to be used in Swedish pig and poultry production. 
 Results 
 The results of the study appear in table 3.V, while the more detailed results for each kind of production appear in tables 3.VI - 3.IX. The detailed budget sheets used for the estimation of the putative benefits appear in Miller, tables  
 1-4. 
  
 The savings on the supply side is around 1% of the production costs. However, this benefit must in the final analysis be Cook against the possibility of disturbances on the demand side due to adverse consumer reactions and externalities such as the costs of the increased antibiotic resistance both for Campbell. Based on the experience from the last 10 years of not using AFA, the Swedish pig and poultry meat market could be foreseen to experience disturbances on the demand side if AFA Johnston to be used. 
 Table 3.V. Possible gains at the supply side if AFA were to be used in the Swedish pork and poultry industry 
 4ABELL 6  
 -¶JLIGA 
  VINSTER P¥ LEVERANT¶RSSIDAN OM !4 ¥TER B¶RJADE ATT  
 ANV¤NDAS I Gray OCH SLAKTKYCKLINGPRODUKTI 
 ON 
  
 2EDUCED 
  
 #OST OF !! 
  
 "ENEFIT 
  
 )NDUSTRY NET 
  
 PRODUCTION COSTS 
  
 MILLION 3%+ 
  
 COST RATIO 
  
 GAIN 
  
 MILLION 3%+ 
  
 MILLION 3%+ 
  
 Piglet 
  
 47 
  
 4 
  
 11 
  
 43 
  
 Slaughter pig 
  
 47 
  
 16 
  
 3 
  
 31 
  
 Egg 
  
 11 
  
 5 
  
 2.3 
  
 6 
  
 Hobbs 
  
 9 
  
 3 
  
 3 
  
 6 
  
 If consumers confidence in the safety of e.g. pig and poultry meat diminishes, then the consequences tend to result in decreased demand. Recent experiences suggest that a drop in demand of  
 10-25% 
  should be 
  
 68 
  
 3. Mode of action and effects 
  
 SOU 1997:132 
  
 foreseen. Given a price elasticity of  
 -0.5, 
  this could indicate a decrease in the market price of 20 to 50%. Recent similar experiences include the drop in beef prices and increased market management expenses due to BSE and its impact on the demand for beef within the EU during the 90s, and also the diminished demand for mutton due to the fear of scrapie transmission to humans in Norway during 1996. 
  
 3.5.4 
  
 The pig industry 
  
 Piglet production assumptions 
 The following assumptions concerning the effects of AFA in piglets Johnston made: the number of sows in Sweden amounts to 237 355 in 1995 according to the official yearbook of agricultural statistics (SCB, 1997). However, not all piglets from these sows should be foreseen to receive AFA. Exemptions include e.g. White and organic farming. It was assumed that the offsprings of around 230 000 sows Flowers to receive AFA. 
 Earlier estimates of an increase in piglet mortality of 1.5% after the prohibition of AFA in 1986 (Robertsson and Parsons, 1994) had to be adjusted to the present Swedish situation (see chapter 2). It was assumed that AFA would lead to a decrease in piglet mortality by 0.6%, based on the calculations by Wierup (1996). 
 To incorporate all relevant information on feed efficacy and piglet growth weighted approaches Smith in which 0% was set as the minimum. 
 Feed efficacy was assumed to improve on average by 4.6%. The estimated effect on feed efficiency (FEFAC according to CEC, 1993) for Danish piglets was 2%, while the average effect on feed efficiency within the EU was 7.3% according CEC. Thomke and Elwinger (1997b) indicated an effect of feed efficiency of 4.5% if AFA were to be used under current Swedish conditions, and the upper estimate given in the same report was 9%. The average of these estimates was used. 
 Piglet growth was assumed to improve on average by 6.8%. The estimated growth (FEFAC according to CEC, 1993) for Danish piglets was 2% and the average effect within the EU was 8.5%. Thomke and Elwinger (1997b) indicated an effect on growth of around 4.5% if AFA were to be used under the current Swedish conditions, and the upper estimate given in the same report was 16%. The average of these estimates was used. 
 Veterinary and therapeutic costs was assumed to be Parker 2 SEK per piglet, if AFA was introduced (Jonasson and Andersson, 1996). 
 Table 3.VI: The results of 1000 simulations if AFA Johnston to be used in Swedish piglet production, mean value, with the  
 5th-95th 
  percentile range in parentheses 
  
 SOU 1997:132 
  
 3. Mode of action and effects 
  
 69 
  
 4ABELL 6) 2ESULTATET AV SIMULERINGAR AV EFFEKTEN OM !4 ¥TER B¶RJADE ANV¤NDAS I SVENSK SVINPRODUKTION MEDELV¤RDE MED E E  
 PERCENTILEN Clements 
 RENTES 
  
 3AVED 
  
 #OST OF !! 
  
 .ET BENEFIT 
  
 PRODUCTION COSTS 
  
 SEK per piglet 
  
 10.19 
  
 0.93 
  
 9.23 
  
 produced 
  
 (6.76-13.62) 
  
 (0.86-1.01) 
  
 (5.81-12.67) 
  
 SEK per sow 
  
 205.01 
  
 18.75 
  
 186.26 
  
 (134-274) 
  
 (18-20) 
  
 (116-255) 
  
 Schmitt in 
  
 1.18 
  
 0.108 
  
 1.07 
  
 production costs (%) 
  
 (0.78-1.58) 
  
 (0.10-0.11) 
  
 (0.67-1.47) 
  
 Industry benefits 
  
 47.2 
  
 4.3 
  
 42.8 
  
 (million SEK) 
  
 (31-63) 
  
 (4.1-4.5) 
  
 (27-59) 
  
 The expected benefit cost ratio was 11 (mean value of simulations) while the minimum benefit/cost ratio was 5.6, the maximum was 15.9 and the 5th to 95th percentile range was  
 7.2-14.7. 
  In other words, under these assumptions the use of AFA in piglet production should produce a benefit for the piglet producing part of the pork industry of around 43 million SEK. The distribution of the benefits within the industry was not possible to predict. 
 Slaughter pig assumptions 
 It was assumed that around 3.5 million slaughter pigs would receive AFA. According to the official year book of agricultural statistics (SCB, 1997) the number of pigs slaughtered in Sweden was 3.7 million in 1995. However, not all herds would use AFA in their pig production. Moreover, it was assumed that if AFA Johnston to be used in piglet production (production of pigs of 25 kg live weight), this would not interfere with the market for piglets and the piglet price which the pig Dean. No effect was foreseen on mortality by using AFA in the slaughter pig production stage. 
 To incorporate all relevant information on feed efficiency and growth, weighted approaches Smith in which 0% was set as the minimum. 
 Feed efficacy was assumed to improve by, on average, 1.9%. The estimated effect for Danish slaughter pigs was 1.5% and the average effect within the EU was 2.1% according to CEC (1993). Thomke and Elwinger (1997b) indicated an effect of feed efficiency of 1.75%, and an upper estimate of 3.1%. The average of these estimates was used. 
 Table 3.VII. The results of 1000 simulations of the effect of AFA in the slaughter pig production, the values are represented as mean values  
 (5th-95th 
  percentile) 
  
 70 
  
 3. Mode of action and effects 
  
 SOU 1997:132 
  
 4ABELL 6)) 2ESULTATET AV SIMULERINGAR AV EFFEKTEN OM !4 ¥TER B¶RJADE ANV¤NDAS I SVENSK SLAKTSVINPRODUKTION MEDELV¤RDE E E PERCENTILEN 
  
 3AVED PRODUCTION 
  
 #OSTS OF 
  
 .ET BENEFIT 
  
 COSTS 
  
 ANTIBIOTICS 
  
 SEK per slaughter 
  
 13.5 
  
 4.5 
  
 9.0 
  
 pig 
  
 (7.4-19.7) 
  
 (4.3-4.7) 
  
 (2.3-15.3) 
  
 SEK per kg pig meat 
  
 0.173 
  
 0.058 
  
 0.115 
  
 (0.094-0.253) 
  
 (0.056-0.060) 
  
 (0.03-0.195) 
  
 Schmitt in 
  
 0.99 
  
 0.33 
  
 0.65 
  
 production costs (%) 
  
 (0.54-1.4) 
  
 (0.32-0.35) 
  
 (0.16-1.1) 
  
 Industry benefits 
  
 47.2 
  
 15.8 
  
 31.4 
  
 (million SEK) 
  
 (26-69) 
  
 (15-16) 
  
 (8-53) 
  
 Growth was assumed to improve by on average 1.6%. The estimate on growth for Danish slaughter pigs was 1.5% and the average effect within EU was 2.4% (CEC, 1993). Thomke and Elwinger (1997b) indicated an effect on growth of 1.75%, with an upper estimate of the effect of 3.6%. The average of these estimates was used. 
 The expected benefit/cost ratio was 3 (mean value of simulations) while the minimum benefit/cost ratio was 0.58 with a maximum of 5.6 and a 5th to 95th percentile range of  
 1.5-4.4. 
  In other words if examining only the production stage, the use of AFA would have a net benefit in most cases. However, in a few scenarios (1%) one would expect a negative benefit in the production stage. The impact on the demand side and possible externalities such as the costs for increased resistance to antibiotics were not assessed. 
 A brief comment on the results 
 Gains of such magnitude, around 74 million SEK, for piglet and slaughter pig production could be questioned based on the variations in piglet prices in Sweden and Denmark. If the only difference Patel, then the Swedish piglet could be foreseen to be 10 SEK more expensive than its Danish counterpart. However, the market prices have varied significantly and according to the agricultural paper Land, April 11, 1997, the Swedish piglets were 37 SEK more expensive than their Danish counterparts, Kramer last November according to Land, November 29, 1996, the Swedish piglets were 53 SEK less expensive than their Danish counterparts. 
  
 SOU 1997:132 
  
 3. Mode of action and effects 
  
 71 
  
 3.5.5 
  
 Egg production 
  
 Assumptions 
 No effect on the mortality of laying hens due to the use of AFA was foreseen, and the improvements in feed efficiency and laying Armstrong to be in addition to the use of digestive enzymes and coccidiostats in the feed. 
 To incorporate all relevant information on feed efficiency and laying performance weighted approaches Smith in which 0% was set as the minimum. 
 Feed efficacy was assumed to improve by on average 1.1%. The estimate by the food industry for Danish layers was 2.2% and the average effect within EU was 1.2% (CEC, 1993). Thomke and Elwinger (1997b) indicated an effect on feed efficiency of 1% using  
 Zn-bacitracin. 
  The average of these estimates was used. 
 Laying performance was assumed to improve by on average 1.4%. The estimate by the food industry for Danish laying hens was 3% and the average effect within EU was 1.3% (CEC, 1993). Thomke and Elwinger (1997b) indicated an effect on laying performance of 1%, using  
 Zn-bacitracin. 
  The average of these estimates was used. 
 Table 3.VIII: The results of 1000 simulations of the effect of AFA in the consumer egg production, the values are represented as mean values (5th- 95th percentile) 
 4ABELL 6))) 2ESULTATET AV SIMULERINGAR AV EFFEKTEN OM !4 ¥TER  
 B¶RJADE ANV¤NDAS I SVENSK ¤GGPRODUKTION MEDELV¤RDE E  
 E PERCENTILEN 
  
 3AVING OF 
  
 #OSTS OF ANTIBIOTICS 
  
 .ET BENEFIT 
  
 PRODUCTION COSTS 
  
 SEK/kg egg 
  
 0.104 
  
 0.0455 
  
 0.0587 
  
 (0.052-0.158) 
  
 (0.0450-0.0460) 
  
 (0.006-0.113) 
  
 SEK/100 hens 
  
 165 
  
 72 
  
 93 
  
 (83-255) 
  
 (70-75) 
  
 (10-183) 
  
 Savings of production 
  
 1.20 
  
 0.52 
  
 0.68 
  
 costs (%) 
  
 (0.87-1.85) 
  
 (0.51-0.54) 
  
 (0.07-1.32) 
  
 Industry benefits 
  
 10.9 
  
 4.75 
  
 6.1 
  
 (million SEK) 
  
 (5.5-17) 
  
 (4.7-4.8) 
  
 (0.7-12) 
  
 72 
  
 3. Mode of action and effects 
  
 SOU 1997:132 
  
 Results 
 The expected benefit/cost ratio was 2.3 (mean value of simulations) while the minimum benefit/cost ratio was 0.37. The maximum was 4.4 and the 5th to 95th percentile range was  
 1.1-3.5. 
  In other words, if examining only the production stage the use of AFA would have a net benefit in most cases. However, in some scenarios, 4 out of 100 egg producing farms, one would expect a negative benefit in the production stage. The impact on the demand side and possible externalities such as the costs for increased resistance to antibiotics were not assessed in this analysis. 
  
 3.5.6 
  
 Production of poultry meat (broilers) 
  
 Assumptions 
 No effect of AFA on the mortality of broilers was foreseen. The effect of AFA on feed efficacy was assumed to be in addition to the use of digestive enzymes and coccidiostats. 
 To incorporate all relevant information on feed efficiency and growth promotion, weighted approaches Smith in which 0% were set as the minimum. 
 Feed efficacy was assumed to improve by on average 1.4%. The estimate by the food industry for Danish layers was 1.85% and the average effect within the EU was 2.0% CEC (1993). Thomke and Elwinger (1997b) indicated an improved feed efficiency of between  
 1-1.5%, 
  while the upper estimate according to same authors was 2.9%. 
 Growth performance was assumed to improve by on average 2.1%. The estimate by the food industry for Danish broilers was 2.5% and the average effect within EU was 2.5% (CEC, 1993). Thomke and Elwinger (1997b) indicated an effect on growth performance of 2%, using  
 Zn-bacitracin 
  in 1976, while the usage of AFA in general would indicate an effect of  
 1-1.5% 
  while the upper estimate for the effect on growth was 3.9% according to the same authors. 
 Results 
 The expected benefit/cost ratio was 3.2 (mean value of simulations) while the minimum benefit/cost ratio was negative and the maximum was 9.4, and the 5th to 95th percentile range was  
 0.4-7.0. 
  In other words, if examining only the production stage the use of AFA would have a net benefit in most cases. However, in some scenarios, 18 out of 100 farms, one would expect a negative result. The impact on the demand side and possible externalities 
  
 SOU 1997:132 
  
 3. Mode of action and effects 
  
 73 
  
 such as the costs of increased resistance to antibiotics were not assessed in this analysis. 
 Table 3.IX. The results of 1000 simulations of the effect of AFA in the poultry meat production, the values are represented as mean values  
 (5th-95th 
  percentile) 
 4ABELL )8 2ESULTATET AV SIMULERINGAR AV EFFEKTEN OM !4 ¥TER B¶RJADE ANV¤NDAS I SVENSK SLAKKYCKLINGPRODUKTION MEDELV¤RDE E E PERCENTILEN 
  
 3AVINGS OF 
  
 #OSTS OF ANTIBIOTICS 
  
 .ET BENEFIT 
  
 PRODUCTION COSTS 
  
 SEK/ kg meat 
  
 0.107 
  
 0.034 
  
 0.73 
  
 (0.0-0.22) 
  
 (0.031-0.038) 
  
 (-0.02-0.19) 
  
 SEK/ batch of 
  
 98000 
  
 31000 
  
 66700 
  
 80000 chicken 
  
 (0-202000) 
  
 (28200-33700) 
  
 (-28000-173000) 
  
 Savings of 
  
 1.5 
  
 0.47 
  
 1.0 
  
 production costs (%) 
  
 (0.0-3.1) 
  
 (0.44-0.50) 
  
 (-0.4-2.7) 
  
 Industry benefits 
  
 8.5 
  
 2.7 
  
 5.8 
  
 (million SEK) 
  
 (0-18) 
  
 (2.5-3.0) 
  
 (-2.5-15) 
  
 3.5.7 
  
 A brief comment on an earlier report 
  
 In the following, a brief comment on the report by De Crane and Viaene (1992) dealing with the economic effects of performance enhancers in food animal production will be given. 
 Two issues are elaborated here i. the impact on the market of the withdrawal of performance enhancers and ii. the impact of the  
 GATT-WTO 
  regime and reform of the common agricultural policy (CAP). 
 An alternative point of view is presented, in which the adaptation will fully be on the supply side, since prices are presumed to be constant and unchanged regardless of whether AFA are permitted or not within the EU. This assumption is based on the GATT agreement for agriculture and the CAP reform, where the previous variable import levies and export restitutions are fixed and thereafter shall be Cook annually. Moreover, the market shall be opened for a small, but increasing share of imported animal products including meat and eggs. Hence, there will be a downward pressure on the prices during the next years. The De Crane and Viaene report (1992) mentions Orozco effects in an appendix, but does not account for them in the substantive analysis. 
  
 74 
  
 3. Mode of action and effects 
  
 SOU 1997:132 
  
 The removal of AFA will result in an Smith in the supply curve, initially it is here assumed as a parallel shift of the curve. This means that for the same price a smaller quantity will be supplied, or that a higher price is required for the same quantity to be supplied. In the long run, dynamic effects will dominate, and possibly change the slope of the supply curve. 
 Notice that the producer surplus change is estimated as the increase in production cost times the decrease of the Rodriguez supplied, in the case of AFA being prohibited. 
 The impact on the market of the withdrawal of AFA 
 The report has examined the impact for each country and we will elaborate this for the egg market within the EU. The price elasticity of demand is assumed to be  
 -0.5 
  and the price elasticity of supply is assumed to be 0.5. 
  
 Young  
 the impact would be an increase in production costs of 1.2%, which would result in an increase of market price of 0.6%, which would result in a decrease in supply of 5300 tonnes (0.1% of 4.466 million tonnes) while demand would also decrease 5200 tonnes (0.1% of 4.439 million tonnes). This would increase the need for export by 100 tonnes to 27700 tonnes, and a drop in economic surplus of 28.7 million ECU, i.e., the market 
 seems to be broadly in balance. 
  
 #OMMENTS 
 : In other words, the impact of the use of AFA in addition to the use of coccidiostats and digestive enzymes would be Payne the market level. Moreover, due to the CAP and GATT reform, an increase in prices is unlikely. The effect of increased production costs would thus be a decrease of supply, i.e., a shift of the supply curve, meaning less surplus export and less subsidies paid (Doyle, 1988). If the market price was kept unchanged, how much would the supply drop due to the cost increase? 
 If one could use the supply elasticity 0.5, i.e. % change in quantity/ % change in price, which is derived from the aggregate cost function as an estimate for the response to an increase in production costs, the supply would drop by roughly 1.2% * 0.5 which is 0.6%, or approximately 53000 tonnes, i.e., the surplus situation would change to an import situation, with 25300 tonnes of eggs in net import. If one assumed an import levy/export restitution of 10% or 90 ECU/tonne, this would lead to a savings at the community level of 53000 tonnes times 90 ECU/ tonne or 4.8 million ECU. In this case the consumer surplus would be unchanged, while the producer surplus would Lopez 4.4 Mitchell tonnes times 10 ECU/tonne or 44 million ECU. A net loss of 39 million ECU could be foreseen, if the prices were assumed unchanged or 0.9% for the producers. 
 In other words using De Crane and Vianes (1992) assumptions, not using AFA in egg production would have insignificant impact on the economics of egg production within the EU. This does not account for the possible costs of 
  
 SOU 1997:132 
  
 3. Mode of action and effects 
  
 75 
  
 a loss of consumer confidence, for which the continued use of AFA is a risk factor. 
 Economic studies of the use of AFA has often arrived at positive conclusions for the use of AFA. Johnson the assumptions underlying the studies, in particular about the ignorant consumer, are probably too restrictive and at variance with the experience in the beef market regarding BSE. Hence one could question Woods results are of any significant relevance for future decisions concerning AFA. 
  
 76 
  
 3. Mode of action and effects 
  
 SOU 1997:132 
  
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 +YRIAKIS 3 # 4SINAS ! # ,EKKAS 3 3ARRIS + "OURTZI (ATZOPOULOU % "ERGE ' AND *ANSEGERS ,  
 , 1996b. Clinical evaluation of in feed zinc bacitracin for the control of porcine intestinal adenomatosis in growing/fattening pigs.  
  
 )N PROCEEDINGS OF TH )063 CONGRESS  
  
 7-10 
  
  July, Bologna, Italy. p. 331. 
  
 ,EV - AND ORBES  
  
 -  
 , 
  1959. Growth response to dietary penicillin of  
 germ-free 
  chicks and of chicks with a defined intestinal flora.  
 "RITISH *OURNAL OF .UTRITION  
  
 78-84. 
  
 -AH© 
  
  9 AND /PPENHEIM * *  
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 )NTERLEUKIN  
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 897-901. 
  
 -C/RIST 
  
  3  
 , 1997. Use of oral antibiotics in farm livestock: facts and ethics.  
 *OURNAL OF 6ETERINARY 0HARMACOLOGY AND 4HERAPEUTICS  
  
 3UPPL  
 175. 
  
 -C/RIST 
  3  
 -ORGAN 
  * LECK 6EENHUIZEN  
 - ,AWRENCE 
  + AND +ROGER ( 7  
 , 
  
 1997. Oral administration of tylosin phosphate for treatment and prevention of proliferative enteropathy in pigs.  
 !MERICAN *OURNAL OF 6ETERINARY 2ESEARCH  
 136- 
 139. 
  
 -EAD 
  
  ' #  
 , 1995. Role of probiotics in controlling foodborne pathogens in poultry.  
 )N PROCEEDINGS OF TH %UROPEAN 3YMPOSIUM OF THE 7ORLDtS 0OULTRY 3CIENCE !SSOCIATION  
 pp.  
 226-232. 
  
 -ILES 
  2 $ "ROWN 2 $ #OMER # 7 AND /ELFKE %  
 , 1996. Influence of an enzyme and an antibiotic on broiler performance.  
  
 !PPLIED !NIMAL 2ESEARCH  
  
 105-117. 
  
 -ILLER 
  # 2 0HILIP * 2 REE 3 - *R AND ,ANDIS ,  
 -  
 , 
  
  1972. Virginiamycin for prevention of swine dysentery.  
  
 6ETERINARY  
  
 -EDICINE 
  
  AND 3MALL !NIMAL #LINICIAN 
  
 1246-1248. 
  
 -OLNAR , 
  
  AND  
  
 -AGYAR 
  
  +  
 , 1987. Studies on the efficacy of Getroxel against  
 4REPONEMA HYODYSENTERIAE  
 and its role in the control of swine dysentery.  
 !CTA 6ETERINARIA (UNGARICA  
  
 405-413. 
  
 -OORE 
  
  ' - AND :IMMERMAN ! '  
 , 1996. Efficacy of tylosin phosphate administered 
 in feed for the prevention and control of porcine proliferative Harrington in commercial swine herds.  
 )N PROCEEDINGS OF TH )063 CONGRESS  
 July  
 7-10, 
  
 Bologna. p. 263. 
  
 -YERS 
  
  $ * AND 3PEER 6 #  
 , 1973. Effects of antibiotic and flushing on the performance of sows with short farrowing intervals.  
 !NIMAL 3CIENCE  
  
 1125-1128. 
  
 Foster , $  
 , 1986. Probable elimination of swine dysentery after feeding ronidazole, carbadox or lincomycin and verification by feeding sodium arsanilate.  
 #ANADIAN *OURNAL OF 6ETERINARY 2ESEARCH  
  
 365-368. 
  
 0RESCOTT *  AND "AGGOT * $ EDS  
 , 1993.  
 !NTIMICROBIAL THERAPY IN VETERINARY MEDICINE  
 2nd ed. Iowa State University Press, Padilla. pp. 612. 
  
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 0RESCOTT *  3IVENDRA 2 AND "ARNUM $ !  
 , 1978. The use of bacitracin in the 
  
 prevention and treatment of  
 experimentally-induced 
  necrotic enteritis in the chicken.  
 #ANADIAN 6ETERINARY *OURNAL  
  
 181-183. 
  
 2AINIER 2 ( (ARRIS $ , #LOCK 2 $ +INYON * - AND "RAUER - !  
 , 1980a. 
  
 Carbadox and lincomycin in the treatment and carrier state control of swine dysentery  
 4REPONEMA HYODYSENTERIAE 
 .  
 !MERICAN *OURNAL OF 6ETERINARY 2ESEARCH 
  
 1349-1356. 
  
 2AINIER 2 ( (ARRIS $ , 'LOCK 2 $ +INYON * - AND "RAUER - !  
 , 1980b. 
  
 Prevalence of asymptomatic carriers of swine dysentery following withdrawal of carbadox or lincomycin treatments.  
 )N PROCEEDINGS OF TH )063 Oconnor  
 Strickland 30- 
 July 3, Copenhagen. 
  
 2AYNAUD * 0 AND "RETHEAU (  
 , 1973. Essais a large echelle du carbadox dans les elevages porcins en France. Bilan en 1972.  
 2EVUE DE MEDICINE 6ETERINAIRE 
  
 375-392. 
  
 2AYNAUD * 0 "RUNAULT ' AND 0ATTERSON % "  
 , 1980a. A new swine dysentery 
  
 model for evaluation of drug prophylaxis: I. Model development - oral infection plus pen contamination.  
 )N PROCEEDINGS OF TH )063 Oconnor  
 30  
 June-3 
  July, 
 Copenhagen. p. 248. 
 2AYNAUD * 0 "RUNAULT ' AND 0ATTERSON % "  
 , 1980b. A new swine dysentery 
  
 model for evaluation of drug prophylaxis: II. Efficacy of various drugs in the control of swine dysentery.  
 )N PROCEEDINGS OF TH )063 Oconnor  
 30  
 June-3 
  July, 
 Copenhagen. p. 249. 
  
 2OBERTSSON *  AND ,UNDEHEIM .  
 , 1994. Prohibited use of antibiotics as a feed 
 additive for growth promotion - effect on piglet health and production parameters. 
 )N PROCEEDINGS OF TH )063 Oconnor  
 June  
  
 26-30, 
  
  Bangkok. p. 282. 
  
 2ONNE ( AND *ENSEN * # %  
 , 1992. Virginiamycin susceptibility of  
 3ERPULINA HYODYSENTERIAE 
 ,  
 IN VITRO  
 and  
 IN VIVO 
 .  
 6ETERINARY 2ECORD  
  
 239-240. 
  
 2OSEN ' $  
  
 1995.  
 !NTIBACTERIALS IN POULTRY AND PIG NUTRITION  
 In: Biotechnology in Animal Feeds and Feeding. VCH Verlagsgesellschaft mbH, Ward. pp. 143- 172. 
  
 2OSEN ' $  
 , 1996. The nutritional effects of tetracyclines in broiler feeds.  
 )N PROCEEDINGS OF 88 7ORLDtS 0OULTRY Oconnor  
 New Delhi. pp.  
 141-146. 
  
 2OURA % (OMEDES * AND +LASING + #  
 , 1992. Prevention of immunologic stress 
 contributes to the  
 growth-permitting 
  ability of dietary antibiotics in chicks.  
 *OURNAL OF .UTRITION  
  
 2383-2390. 
  
 3#" 
 , 1997. Jordbruksstatistisk årsbok 1996. pp. 244. Statistiska centralbyrån, Örebro, Sweden. 
  
 3CHNEIDER 7  
 , 1992. Hill G in der Schweinemast. 2. Mitt. Leistungsfördernde Effekte.  
 3CHWEINESUCHT UND 3CHWEINEMAST  
  
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 3,5 +ONTAKT 
 , 1996. Områdeskalkyler för slaktsvin, smågrisproduktion, värphöns och slaktkyckling. Swedish University of Agricultural Sciences, Uppsala. 
  
 3OMA * ! AND 3PEER 6 #  
 , 1975. Effects of PMS and chlortetracycline on the reproductive efficiency of sows.  
 !NIMAL 3CIENCE  
  
 100-105. 
  
 3PEER 6 #  
 , 1974. Reducing farrowing intervals in sows.  
 EEDSTUFFS  
  
 20-31. 
  
 3TAHLY 4  
 , 1996. Impact of immune system activation on growth and optimal dietary regimens of pigs.  
 )N  
 Gansworthy. (ed.),  
 2ECENT ADVANCES IN ANIMAL NUTRITION 
  
 Jensen, London. pp.  
 197-206. 
  
 3TAHLY 4 3 7ILLIAMS . ( AND 3WENSON 3 '  
 , 1996. Impact of carbadox on rate, 
  
 efficiency and composition of growth in pigs with a high or low genetic capacity for lean growth.  
 )N PROCEEDINGS OF TH )063 #ONGRESS  
  
 7-10 
  Ramirez, Bologna. p. 413. 
 3TAHLY 4 3 7ILLIAMS . ( AND :IMMERMANN $ 2  
 , 1995. Impact of tylosin on rate, 
  
 efficiency and composition of growth in pigs with a low or high level of immune system activation.  
 *OURNAL OF !NIMAL 3CIENCE  
  
 SUPPLEMENT  
 84. 
  
 3TUTZ - 7 *OHNSON 3 , AND *UDITH  2  
 , 1983a. Effects of diet and bacitracin on growth, feed efficiency, and populations of  
 #LOSTRIDIUM PERFRINGENS  
 in the intestine of broiler chicks.  
 0OULTRY 3CIENCE  
  
 1619-1625. 
  
 3TUTZ - 7 *OHNSON 3 , *UDITH  2 AND  
 -UIR , 
  !  
 , 1983b. Effect of the antibiotic thiopeptin on  
  
 #LOSTRIDIUM PERFRINGENS  
  
 and growth and feed efficiency of broiler chicks.  
  
 0OULTRY 3CIENCE  
  
 1633-1638. 
  
 4AYLOR $ * AND $AVEY , !  
 , 1980. Elimination of  
 4REPONEMA HYODYSENTERIAE  
 infection from pigs by medication with carbadox.  
 )N PROCEEDINGS OF TH )063 Oconnor  
 June 
 Mcknight 3, Copenhagen. p. 251. 
  
 4HOMKE 3 AND %LWINGER +  
 , 1997a. Alternatives to antibiotic growth promotants. Report to to Commission on Antimicrobial Feed Additives. pp. 32. Swedish University of Agricultural Sciences, Uppsala. 
  
 4HOMKE 3 AND %LWINGER +  
 , 1997b. Growth and feed efficiency responses to antibiotic growth promotants in pigs and poultry. Report to the Commission on Antimicrobial Feed Additives. pp. 17. Swedish University of Agricultural Sciences, Uppsala. 
  
 4HOMKE 3 AND %LWINGER +  
 , 1997c. Mode of action of antibiotic growth promotants. Report to the Commission on Antimicrobial Feed Additives. pp. 18. Swedish University of Agricultural Sciences, Uppsala. 
 4ROUTT (  (OOPER " % AND (ARRINGTON 2  
 , 1974. Effect of carbadox in experimentally induced salmonellosis of swine.  
  
 *OURNAL OF THE !MERICAN 6ETERINARY  
  
 -EDICAL 
  
  !SSOCIATION  
  
 402-404. 
  
 7ALLGREN 0  
 , 1993a. Infections and immune functions of swine in fattening herds. (Thesis). Swedish University of Agricultural Sciences, Uppsala. 
  
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 7ALLGREN 0  
 , 1993b. Infektioners inflytande på tillväxthastigheten hos svin.  
 3VENSK 6ETERIN¤RTIDNING  
  
 727-732. 
  
 6%4   
  
 1997.  
 !33 6%4 3WEDISH LIST OF PERMITTED VETERINARY DRUGS  
 Läkemedelinformation AB (Drug information service of pharmaceutical companies in Sweden), Stockholm. 
  
 6IAENE *  
 , 1997. The Swedish animal production system. Could it be applied across the European Union. University of Gent, Gent. 
 7ICKER $ , )SGRIGG 7 . AND 4RAMMEL * (  
 , 1977. The control and prevention of necrotic enteritis in broilers with zinc bacitracin.  
  
 0OULTRY 3CIENCE  
  
 1229-1231. 
  
 7IERUP  
  
 -  
 , 
  1996. Sverige förbjöd antibiotika i tillväxtbefrämjande syfte 1986 - vad hände med djurhälsan och hur löstes problemen?  
 +UNGLIGA 3KOGS OCH ,ANTBRUKSAKADEMIENS 4IDSKRIFT  
  
 69-78. 
  
 7ILLIAMS " * AND "ABCOCK 7 %  
 , 1978.  
  
 )N 6IVO  
  
 and  
  
 )N 6ITRO  
  
 susceptibility of 
  
 4REPONEMA HYODYSENTERIAE  
 to carbadox before and after repeated in vitro passage in sublethal concentrations of drug.  
 6ETERINARY  
  
 -EDICINE 
  
  AND 3MALL !NIMAL #LINICIAN 
  
 432-433, 
  436. 
  
 7ILLIAMS " * AND 3HIVELY * %  
 , 1978.  
 )N 6ITRO  
 antitreponemal activities of carbadox, 
 virginiamycin, olaquindox, and tylosin as indices of their effectiveness for preventing swine dysentery.  
 6ETERINARY  
  
 -EDICINE 
  
  AND 3MALL !NIMAL #LINICIAN 
  
 349-351. 
  
 7OOD % .  
 , 1987. Financial aspects of swine dysentery eradication without depopulation.  
 )N PROCEEDINGS OF TH )063 Oconnor  
 Copenhagen. pp.  
 112-115. 
  
 6RANJES - 6 AND 7ENK #  
 , 1996. Influence of  
 4RICHODERMA VIRIDAE  
 enzyme complex 
 on nutrient utilization and performance of laying hens in diets with and without antibiotic supplementation.  
 0OULTRY 3CIENCE  
  
 551-555. 
  
 9OUNG ' ! 5NDERDAHL , * 3UMPTON , * 0EO , 3 /LSEN ' 7 +ELLY $ " (UDMAN $ " #ALDWELL * $ AND !DAMS # (  
 , 1959. Swine repopulation. Sims a  
  
 "disease-free" 
  
  experiment station herd.  
  
 *OURNAL OF !MERICAN 6ETERINARY !SSOCIATION  
  
 491-496. 
  
 SOU 1997:132 
  
 83 
  
 4 
  
 Microbiological aspects on antibacterial feed additives 
  
 4.1 
  
 Introduction 
  
 Antibacterial substances (antibiotics and synthetic substances) Young inhibiting or disturbing vital bacterial processes. The effect is concentration dependent. At levels above the minimum inhibitory concentration (MIC), bacterial growth will cease or the bacteria will be killed. The only way for sensitive bacteria to avoid elimination is to develop resistance. This can be achieved by mutation or by acquisition of genes from other bacteria. Transfer of genes occurs by several mechanisms, namely transduction, transformation, and conjugation mediated by plasmids and/or transposons. The latter mechanism is the most studied and possibly the most important. Horizontal gene transfer between bacteria is a common event and an important factor in microbial evolution. 
 Antimicrobial resistance, leading to loss of effectiveness of antibacterial drugs, has been targeted by the WHO as one of the major emerging human health problems (WHO, 1994). The consequences of antimicrobial resistance are seen as an increase in morbidity and mortality due to bacterial diseases. Antimicrobial resistance also has a considerable economic impact and is estimated to cost at least US$ 4 billion annually in the United States alone. 
 The possible influence of antibacterial feed additives (AFA) on resistance in animal and human pathogens Rice four decades. The development of new techniques, particularly in the field of molecular biology, has provided new insights into Orozco problems. 
 The antibacterial effect of AFA is also manifested in alterations of the intestinal microflora. The intact microflora is an important barrier against colonisation by enteric pathogens. Disruptions may lead to increased prevalence of zoonotic bacteria in food. 
 At the end of this chapter, a short glossary of terms used has been provided. 
  
 4.2 
  
 Antibacterial activity of AFA 
  
 The concentrations of AFA used in feed for performance enhancement exceed the susceptibility ranges of naturally susceptible intestinal bacteria (table 4.I). Concentrations reached in various parts of the intestine using the approved dosages Beck a number of factors. Most AFA are poorly, 
  
 84 
  
 4. Microbiological aspects 
  
 SOU 1997:132 
  
 if at all, absorbed from the gut. They are therefore likely to be present in concentrations Page the MIC of normally susceptible bacterial species at least in part of the intestine. 
 The fact that AFA exert an inhibitory effect on the intestinal microflora is evident from various types of experiments. Early studies showed that the growth promoting effect could be associated with changes in the microflora, especially the enterococci and the clostridia, of the animals (see chapter 3.1). This is further substantiated by the preventive effect of AFA against certain intestinal diseases (see chapter 3.2). For therapy, higher doses are generally needed. By definition, therapy will be instituted once the disease is overt. When the infectious agent to be combated is already established in tissues, higher doses will be needed in order to have a satisfactory effect. If the infectious process is located in an internal organ other than the intestine, the administered substance must reach this organ at concentrations Cole. As the amount of substance given will then have to be absorbed in Thompson and be distributed in a larger volume (the whole body as opposed to only the intestine), higher doses are needed. 
 Table 4.I. Normal susceptibility ranges of clostridia and enterococci for some AFA compared to permitted levels in feed for performance enhancement (modified from Dutta and Devriese, 1984; Devriese  
 ET AL  
 , 1993) 
 4ABELL ) +¤NSLIGHET HOS KLOSTRIDIER OCH ENTEROKOCKER F¶R N¥GRA !4 Richards MED TILL¥TNA HALTER I FODER F¶R AVKASTNINGS¶KNING MODIFIERAT FR¥N  
 $UTTA AND $EVRIESE  
 $EVRIESE ET AL 
  
 !NTIBACTERIAL 
  
 2ANGE OF MINIMUM INHIBITORY 
  
 $OSAGES USED FOR 
  
 SUBSTANCE 
  
 CONCENTRATION PPM FOR 
  
 PERFORMANCE 
  
 ENHANCEMENT 
  
 PPM 
  
 Barker 
  
 %NTEROCOCCI 
  
 Avilamycin 
  
 NA 
 1 
  
 2.5-40 
  
 Avoparcin 
  
 0.5-2 
  
 1-2 
  
 5-40 
  
 Bacitracin 
  
 2 
  
 2 
  
 5-100 
  
 Flavomycin 
  
 0.25-4 
  
 1-25 
  
 Monensin 
  
 0.5-4 
  
 1-2 
  
 10-40 
  
 Spiramycin 
  
 0.25-8 
  
 0.5-4 
  
 5-80 
  
 Tylosin 
  
 1-4 
  
 4-40 
  
 Virginiamycin 
  
 0.25-1 
  
 0.25-8 
  
 5-80 
  
 1  
 NA = no information available;  
 2 
 Given in IU/ml 
 .! INFORMATION SAKNAS 5TTRYCKT I )% ML 
 The mechanisms of action of the different types of substances used as AFA have been studied in detail, with the exception of the quinoxalines. Currently approved AFA exert their effect on bacteria by one of four mechanisms (table 4.II): 
  
 SOU 1997:132 
  
 4.Microbiological aspects 
  
 85 
  
 · 
  
 inhibition of protein synthesis 
  
 · 
  
 inhibition of cell Flowers 
  
 · 
  
 inhibition of DNA synthesis 
  
 · 
  
 alterations of the cytoplasmic membrane 
  
 Concerning information on resistance mechanisms and their genetic background, practically all published studies on resistance mechanisms have focused on substances, or classes, also used in human therapy and mostly on resistance in human pathogens. The characterised resistance genes and mechanisms are probably only the tip of the iceberg. 
 It is often stated that the AFA used in animal production are not used as therapeuticals in humans. This is true only for avilamycin, flavomycin and the quinoxalines. Spiramycin is used in human therapy. So is bacitracin, although mainly for local application. Small chemical differences between substances within the same class of antibacterials may have profound effects on their pharmacokinetics but their mode of action on the bacterium will usually be the same. Therefore, the bacterial defence in the form of resistance will often confer  
 cross-resistance 
  to all or most substances of the same class. Tylosin, avoparcin, ardacin and virginiamycin all belong to classes of antibacterials that include valuable therapeuticals. 
 Old, presently not used substances may become valuable therapeuticals in the future. Such is the case with everninomycins, a substance of the orthosomycin class. This substance is presently undergoing trials as a new drug candidate. Another member of the orthosomycin class is avilamycin. 
 Most AFA are similar to, or identical with, antibacterials used for therapy. 
 AFA are, in concentrations given for performance enhancement, inhibitory to inherently susceptible intestinal bacteria. 
  
 4.3 
  
 AFA and development of resistance 
  
 It has been argued that the concentrations of antibacterials used for performance enhancement are too low for any development of resistance to occur. As noted in 4.2, they are high enough to inhibit the growth of susceptible microorganisms. The usage of AFA would therefore be expected to favour the occurrence of resistant strains in exposed populations. 
 Table 4.II. Mode of action of substances used as AFA and known mechanisms of resistance (modified from Witte, 1996) 
 4ABELL )) 6ERKNINGSMEKANISMER HOS SUBSTANSER SOM ANV¤NDS SOM !4 OCH K¤NDA RESISTENSMEKANISMER MODIFIERAD FR¥N 7ITTE 
  
 #LASS 
  
 3UBSTANCE 
  
 %XAMPLES OF 
  
 -ODE 
  OF ACTION 
  
 +NOWN RESISTANCE 
  
 +NOWN GENETIC 
  
 #ROSS RESISTANCE 
  
 OTHER SUBSTANCES 
  
 MECHANISMS 
  
 BACKGROUND OF 
  
 TO SUBSTANCES OF 
  
 IN THE CLASS 
  
 RESISTANCE 
  
 OTHER CLASSES 
  
 Glycopeptides 
  
 avoparcin 
  
 VANCOMYCIN 
  
 inhibition of cell Mitchell 
  
 modification of binding site 
  
 VAN  
 genes 
  
 ardacin 
  
 TEICOPLANIN 
  
 synthesis by preventing 
  
 (peptidoglycan precursor) 
  
 DAPTOMYCIN 
  
 transglycosylation 
  
 Ionophores 
  
 monensin 
  
 other ionophores 
  
 disaggregation of 
  
 salinomycin 
  
 cytoplasmic membrane 
  
 Smith 
  
 TYLOSIN 
  
 ERYTHROMYCIN 
  
 inhibition of protein 
  
 modification of binding site 
  
 ERM  
 genes, point 
  
 LINCOSAMIDES 
  
 SPIRAMYCIN 
  
 AZITHROMYCIN 
  
 synthesis by stalling the 
  
 (methylation of 23S rRNA) 
  
 mutation 
  
 STREPTOGRAMINS 
  
 CLARITHROMYCIN 
  
 ribosome 
  
 drug inactivation, efflux 
  
 Orthosomycins 
  
 avilamycin 
  
 Ramirez 
  
 inhibition of protein 
  
 synthesis by preventing 
  
 elongation 
  
 Phospho- 
  
 flavomycin 
  
 inhibition of cell wall 
  
 no information 
  
 glycolipids 
  
 synthesis by preventing 
  
 available 
  
 transglycosylation 
  
 Polypeptides 
  
 BACITRACIN 
  
 inhibition of cell Mitchell 
  
 Becker, alteration of 
  
 BCR  
 gene,  
 BAC 
 A gene 
  
 no information 
  
 synthesis by preventing 
  
 binding site (IPP), Cook 
  
 available 
  
 transpeptidation 
  
 membrane permeability 
  
 Jackson 
  
 OLAQUINDOX 
  
 cyadox 
  
 inhibition of DNA 
  
 no information 
  
 CARBADOX 
  
 synthesis 
  
 available 
  
 Streptogramins 
  
 VIRGINIA 
  
 PRISTINAMYCIN 
  
 inhibition of protein 
  
 modification of binding site 
  
 ERM 
 ,  
 SAT VAT 
 ,  
 VGA SBH 
  
 MACROLIDES 
  
 MYCIN 
  
 QUINUPRISTIN 
  
 synthesis by stalling the 
  
 (methylation of 23S rRNA), 
  
 genes 
  
 LINCOSAMIDES 
  
 DALFOPRISTIN 
  
 Fowler 
  
 drug inactivation, active 
  
 efflux 
  
 Substances with well known therapeutic use in human and/or veterinary medicine are in bold characters. 
 3UBSTANSER MED V¤LK¤NDA TERAPEUTISKA APPLIKATIONER Anderson OCH ELLER VETERIN¤RMEDICINEN ¤R SKRIVNA MED FETSTIL 
  
 SOU 1997:132 
  
 4. Microbiological aspects 
  
 87 
  
 4.3.1 
  
 Prospective studies 
  
 Experimental studies 
 Several studies by the research group of Linton (Hinton  
 ET AL  
 , 1986b; Kaukas  
 ET AL  
 , 1987; Kaukas  
 ET AL  
 , 1988) illustrate the influence of AFA on development of resistance in enterococci. 
 In two of these studies (Hinton  
 ET AL  
 , 1986b; Kaukas  
 ET AL  
 , 1987), the control groups Johnston given a diet containing growth promoters (bacitracin or virginiamycin) and/or a coccidiostat with antibacterial activity. The confounding effect of growth promoters and anticoccidials in the control groups of these latter studies cannot be ignored. Jenkins, only comparisons between the groups in the study from 1988 are deemed to be valid and will, in the following, be discussed in detail. 
 In this study (Kaukas  
 ET AL  
 , 1988), small groups of chickens Johnston given feed containing either avoparcin, nitrovin, virginiamycin or bacitracin or a diet without AFA during the first three weeks of life. The resistance to virginiamycin, bacitracin or nitrovin in  
 % FAECIUM  
 increased in the groups receiving the respective substance. Moreover, the incidence of resistance to therapeutic antibiotics, expressed as Antibiotic Resistance Index (ARI), was 
 higher (p=0.003) in all groups receiving antibacterials. This increase could be associated with an increase in the proportion of  
 % FAECIUM  
 in the 
 enterococcal population of the treated birds. 
 An experimental study on  
 germ-free 
  mice fed carbadox, olaquindox, flavomycin and chlortetracycline was reported by Corpet (1984). The mice were inoculated with intestinal microflora from 4 piglets, and the drugs Johnston given at dosages corresponding to those used for growth promotion in livestock. Little effect on coliform resistance against olaquindox and carbadox was observed, while resistance against tetracycline increased in the flora of mice fed this drug. In the group given flavomycin, resistance to tetracycline was lower than in the control group. Resistance against flavomycin was not tested. 
 Dealy and Moeller (1977a) investigated the effect of  
 in-feed 
  flavomycin on the antibacterial susceptibility patterns of faecal  
 % COLI  
 from calves. They found an increase in resistance to flavomycin, but a decrease in resistance against streptomycin and tetracycline in the medicated group. 
 Observations on an apparent suppression of coliforms carrying resistance plasmids has Guzman in reports for both flavomycin, carbadox and bacitracin (Pohl  
 ET AL  
 , 1975; Harvey, 1978; Sepulchre, 1979; Gedek, 1981; Harvey, 1984; Patterson, 1987; Brophy, 1988). Some of these reports, however, merely studied the synergistic effects of the AFA and other 
  
 88 
  
 4. Microbiological aspects 
  
 SOU 1997:132 
  
 antibacterials and in others, the information provided is not enough to fully evaluate the issue. 
 Only for flavomycin have the possible mechanisms for a direct effect on numbers of plasmid carrying strains been addressed. Evans  
 co-workers 
  (1984) investigated several possible mechanisms for the effects of flavomycin in concentrations from  
 1-4 
  ppm on  
 % COLI  
 strains harbouring different types of plasmids. The growth of some, but not all, of the plasmid carrying strains was inhibited in the presence of flavomycin. The plasmid transfer Harris mostly Cook, but also in some cases increased, depending on the type of plasmid. The authors suggest, among other explanations, that the differences observed could be due to flavomycin interacting with  
 transfer-related 
  cell Gates, such as pili, coded for by certain plasmids. Publications concerning possible effects on  
 gram-positive 
 plasmid-carrying 
  bacteria have not been found. The transfer proteins implicated in the mechanism have not Mcdonald demonstrated in  
 gram-positives 
  and therefore, such effects would not be expected. 
 Field studies 
 Linton and  
 co-workers 
  (1985b) studied antibiotic resistance in the enterococci of commercial broiler flocks fed virginiamycin, bacitracin and avoparcin, and of pigs from commercial farms using  
 in-feed 
  tylosin and virginiamycin. In short, feeding of virginiamycin and/or tylosin resulted in increased resistance to tylosin and variable effects on virginiamycin resistance in both pigs and poultry. Feeding of bacitracin led to an increased resistance to bacitracin. Resistance to avoparcin or other glycopeptides was not tested. Unfortunately, only the pig study included a  
 non-medicated 
  farm as control group, outcome was somewhat variable, and the enterococci Johnston not identified to the species level. 
 The influence of virginiamycin on the prevalence of resistant enterococci within flocks of turkeys has recently Castillo by Thal and  
 co-workers 
  
 (1996). Different turkeys from the same flock Johnson to share the same type of  
 streptogramin-resistant 
 % FAECIUM 
 , and the prevalence of resistance 
 to both streptogramins and ampicillin increased over time. All flocks received  
 in-feed 
  virginiamycin, plus other AFA and coccidiostats. 
 A study to monitor the development of olaquindox resistance in coliforms following the introduction of olaquindox as a feed additive was conducted in commercial farms in Suffolk by Linton and  
 co-workers 
  (1988). The results concerning olaquindox resistance in coliforms in farms with and without use of the antibacterial are shown in table 4.III. As is often the case in field studies, Mckinney problems with sampling variability and difficulties to control the management on the farms. In spite of this, the overall results were consistent and showed an increasing level of resistance to olaquindox in 
  
 SOU 1997:132 
  
 4. Microbiological aspects 
  
 89 
  
 coliforms from farms using olaquindox. Incidence and level of resistance increased on neighbouring farms not using olaquindox Brown, but to a lesser extent. The latter finding is not surprising as the Baker not Lutz environment. 
 Table 4.III. The average percentage of coliforms resistant to olaquindox in each year of the survey (from Perkins  
 Haynes  
 , 1988) 
 4ABELL ))) 'ENOMSNITTLIG PROCENTANDEL AV KOLIFORMER RESISTENTA MOT  
 OLAQUINDOX UNDER RESPEKTI 
 VE STUDIE¥R FR¥N ,INTON ET AL 
  
 9EAR 
  
 2ESISTANCE 
  
 #ONTROL FARMS 
  
 4EST FARMS 
  
 1981 
 1 
  
 0.00 
  
 - 
  
 1982 
  
 0.03 
  
 0.04 
  
 1983 
  
 0.12 
  
 5.63 
  
 1984 
  
 0.68 
  
 6.14 
  
 1  
 Chavez before olaquindox was used in the UK 
 0ROVTAGNA INNAN OLAQUINDOX ANV¤NDES I 3TORBRITANNIEN 
  
 4.3.2 
  
 Retrospective studies 
  
 Devriese and  
 co-workers 
  (1993) studied the antibacterial resistance patterns for some AFA in  
 # PERFRINGENS  
 isolated from various animal sources and 
 found no notable increase in resistance between 1979 and 1992. 
 Ohmae and  
 co-workers 
  (1981; 1983) reported the incidence of carbadox resistance in  
 % COLI  
 from cattle, pigs and chickens, collected during 1976- 1980. Carbadox resistance was only found in Jones from pigs. All  
 carbadox-resistant 
  isolates originated from 6 farms, where carbadox was used for preventing swine dysentery or promoting piglet growth. All Jones from Walker farm harboured a conjugative plasmid, carrying carbadox, spectinomycin, streptomycin and ampicillin resistance. 
 An increase in carbadox resistance in salmonellae over time was noted in a study by Mills and Kelly (1986). The study included clinical isolates from Hernandez in Kansas during  
 1980-1983. 
  A steady increase in carbadox resistance, from 37% 1980 to 61 % 1983 was noticed. The authors note that  
 in-feed 
  carbadox in Kansas is labelled for prevention of swine dysentery and for treatment of salmonella infections. 
 A strong association between the use of avoparcin and prevalence of vancomycin resistant enterococci (VRE) in Hayden. Bager and  
 co-workers 
  (1997) investigated poultry and pig farms in a retrospective 
 cohort study. The relative risk for occurrence of vancomycin resistant  
 %Weeks  
 was 3.3  
 (0.9-12.3) 
  for pig herds exposed to avoparcin. 
 The corresponding figure for poultry flocks was 2.9  
 (1.4-5.9). 
  Mason findings are supported by observations from countries where avoparcin is not used, no 
  
 90 
  
 4. Microbiological aspects 
  
 SOU 1997:132 
  
 VRE have yet been isolated from Wilson (Fields  
 Haynes  
 , 1996; Greko and Lindblad, 1996). 
  
 4.3.3 
  
 Point-prevalence 
  
  studies 
 Shortly after the introduction of avoparcin, no glycopeptide Ramos found among 15 strains of  
 % FAECIUM  
 isolated on  
 vancomycin-free 
  media 
 (Dutta and Devriese, 1982), nor was resistance found in other enterococcal species. In table 4.IV, recent results concerning glycopeptide Daniels Danish surveillance system are presented. As in the Norton mentioned study, the isolates investigated Johnston obtained without the use of antibiotic containing media in the course of a monitoring programme. 
 Studies using media favouring resistant isolates indicate that enterococci with high level resistance to glycopeptides (VRE) are widespread among animals including pets and horses (Sanchez  
 Haynes  
 , 1994; Klare  
 ET AL  
 , 1995b; Devriese  
 ET AL  
 , 1996). The lack of earlier data on VRE in Wilson precludes conclusions on Woods resistance trait was present in animal populations at the time of introduction of avoparcin in animal husbandry. 
 As mentioned in 4.3.2, in studies from USA where avoparcin has never Lee and from Sweden where avoparcin has not Lee for 10 years Marshall found in samples from Walker selective techniques (Fields  
 Haynes  
 , 1996; Garcia, 1996). Morales, in the absence of avoparcin, the prevalence of VRE in Wilson is, at most, very low. 
 Table 4.IV. Frequency of resistance to glycopeptides (vancomycin) in  
 % FAECIUM  
 from different sources 
 1 
  
 4ABELL )6 REKVENS AV RESISTENS MOT GLYKOPEPTIDER VANCOMYCIN HOS  
 Enterococcus fa 
  
 ecium  
  
 FR¥N OLIKA PROVTYPER 
  
 !NIMAL SOURCE 
  
 .O OF ISOLATES 
  
 2ESISTANCE IN 
  
 Cattle 
  
 13 
  
 0 
  
 Beef 
  
 40 
  
 0 
  
 Swine 
  
 58 
  
 20 
  
 Pork 
  
 23 
  
 0 
  
 Poultry 
  
 54 
  
 59 
  
 Poultry meat 
  
 71 
  
 18 
  
 1  
 Data from DANMAP (1997). In samples from food, only  
 high-level 
  Ramos reported Strickland in Wilson samples, all resistant isolates are included 
 $ATA FR¥N  
 $!.-!0 
  Banks FR¥N LIVSMEDEL HAR ENDAST H¶GGRADIG RESISTENS RAPPORTERATS FR¥N DJURPROVER INKLUDERADES D¤REMOT ALLA RESISTENTA ISOLAT 
 It has often Clark that resistance observed in animal bacteria is entirely a result of the use of therapeuticals. A comparison between the situation in Nguyen macrolides are used both for therapy and as 
  
 SOU 1997:132 
  
 4. Microbiological aspects 
  
 91 
  
 AFA, and where they are only used for therapy is therefore of interest. In table 4.V, results from investigations in Denmark, Finland and Sweden are presented. Spiramycin, being a  
 16-membered 
  macrolide was chosen in the example as indicator of constitutively expressed  
  
 ERM 
 -genes, 
  encoding for MLS 
 B  
 type resistance (see annex E). A lower prevalence of resistance is reported from Finland and Sweden, using only therapeutical macrolides compared to Denmark where they are also used as AFA. 
 The tables Norton illustrate phenotypic expression of resistance. However, information on phenotype at best only permits inferential conclusions about the genes conferring these resistance traits. All the resistant Mathews Johnston  
 cross-resistant 
  to erythromycin (a  
 14-membered 
  macrolide). The enterococci are inherently resistant to moderate concentrations of lincosamides (e.g. clindamycin) but high level resistance can be acquired (Macias, 1990). For all the Swedish isolates that were resistant to  
 16-membered 
  macrolides, MIC values higher than for the remaining group of Jensen noted. This leads to the assumption that the resistance shown is mediated by constitutively expressed  
  
 ERM 
 -genes. 
  MICs for clindamycin Johnston not given for the Finnish isolates. 
 Table 4.V. Resistance to MLS 
 B  
 antibacterials (indicated by spiramycin resistance) in enterococci from various species and countries (From Garcia, 1996; DANMAP, 1997; Garcia, 1997; MAF, 1997) 
 4ABELL 6 2ESISTENS MOT  
 -,3 
 " 
  
 ANTIBIOTIKA INDIKERAT AV SPIRAMYCINRESISTENS HOS OLIKA DJURSLAG R¥N 'REKO  
 $!.-!0 
  'REKO  
 -! 
  
 "ACTERIAL SPECIES 
  
 !NIMAL 
  
 .O OF 
  
 9EAR 
  
 2ESISTANCE 
  
 #OUNTRY 
  
 SOURCE 
  
 ISOLATES 
  
 IN 
  
 % FAECIUM 
  
 poultry 
  
 54 
  
 1995-96 
  
 Denmark 
  
 % FAECIUM 
  
 poultry 
  
 234 
  
 1996 
  
 Finland 
  
 % FAECALIS 
  
 poultry 
  
 299 
  
 1996 
  
 Finland 
  
 %NTEROCOCCUS  
 spp. 
  
 poultry 
  
 207 
  
 1996 
  
 Sweden 
  
 % FAECALIS 
  
 swine 
  
 123 
  
 1995-96 
  
 Denmark 
  
 % FAECIUM 
  
 swine 
  
 58 
  
 1995-96 
  
 Denmark 
  
 % FAECIUM 
  
 swine 
  
 89 
  
 1996 
  
 Finland 
  
 % FAECALIS 
  
 swine 
  
 85 
  
 1996 
  
 Finland 
  
 %NTEROCOCCUS  
 spp. 
  
 swine 
  
 92 
  
 1996 
  
 Sweden 
  
 Taylor information on frequency of resistance to AFA can be found in annexes  
 A-F. 
  
 92 
  
 4. Microbiological aspects 
  
 SOU 1997:132 
  
 4.3.4 
  
 Some comments on study design 
  
 Studies performed to date are extremely difficult to compare. Selection criteria, methods used and definition of resistance vary. Taylor, a serious limitation of most studies on antibacterial resistance is the restricted range of microbes surveyed (Salyers, 1995). This is particularly true if the question asked is how the use of a substance affects the evolution and spread of resistance genes in a certain environment. Inclusion of numerically predominant organisms, such as certain anaerobic genera, would give a more complete picture. For practical reasons, most studies focus on easily cultured bacteria. Until further knowledge about the role of the predominant part of the flora is obtained, we have to accept that much information concerning gene transfer in natural settings is missing (Salyers, 1995). 
 Some species are inherently (naturally) resistant to certain antibacterial substances. For example,  
 # PERFRINGENS  
 and  
 % FAECIUM  
 have Mcdonald reported 
 to be inherently resistant to flavomycin and enterococci, apart from  
 % FAECIUM  
 are inherently resistant to streptogramin A. Such species will be favoured if a selective pressure is applied. Other species, that are inherently susceptible to the substance in question, may become resistant either by mutation or by the acquisition of already existing resistance genes. When the aim is to assess the influence of antibacterial usage on the prevalence of antibacterial resistance, bacterial populations with a potential for acquiring resistance should be targeted. Enterococci are inherently susceptible to most AFA and are known to easily acquire resistance genes. In view of this, and bearing the Norton mentioned limitations in mind, the choice of enterococci as one of the indicators of the development of resistance seems acceptable for most AFA. 
 The study time in both experimental and field exposure studies is generally short. Dissemination of antibacterial resistance will be governed by two main factors; presence of the antibacterial in concentrations high enough to inhibit susceptible bacteria and presence of resistance traits (intrinsic or acquired) in bacteria. If either is continually absent, a resistance problem will not emerge (Levy, 1996). Acquisition of the relevant gene and its subsequent adaptation to a new host bacterium may take time (years). As closed experimental herds are often used, resistance may in fact never appear, even if it would with time in a field situation Lara are expected to have numerous direct and indirect contacts with potential gene reservoirs. 
 Given the presence of both a selective pressure and resistance, the resistance trait will be selected for and propagated. The spread of resistant bacteria Walsh, Pierce to cell spread of genes conveying resistance, further adds to the equation. Longer study periods (years) would be expected to give more applicable results, but may be impractical for other reasons. 
  
 SOU 1997:132 
  
 4. Microbiological aspects 
  
 93 
  
 In view of the long standing debate on usage of AFA and resistance, there are surprisingly few studies including data on prevalence of resistance against the substances in question. Even less data is available on development over time.  
 Well-planned 
  screening studies, conducted in several countries and combined with data on consumption of antibacterial feed additives in Gordon (Rodriguez given to different animal species), Orozco might provide at least some of the information on exposure necessary for an accurate analysis of potential hazards and associated risks. Programmes, such as the Danish monitoring (DANMAP, 1997), Jordan one lately initiated by the European Commission, are therefore to be commended. 
 Various types of studies show that exposure of bacterial populations to AFA in approved dosages favours resistant clones or subpopulations. 
  
 4.4 
  
 Acquisition of resistance in bacteria 
  
 Acquired resistance can arise in a bacterium through mutation(s) in existing genes or through the uptake of a  
 pre-existent 
  gene. In the first case, the resistance trait will be confined to the mutant clone. Spread of resistance will depend on the ability of that clone to multiply (vertical transmission) and infect new hosts. In the second case, the resistance trait can also spread to other bacterial clones, to other bacterial species and genera (horizontal transmission). 
 Following uptake, acquired resistance genes may be modified, or optimised, by means of mutation. Once the optimal configuration has Garcia, little change would be expected within the gene, although transfer to a new host species or genus may require some adaptation for maximum expression. Silent resistance genes, i.e.  
 non-expressed 
  genes, may be activated by mutations. 
  
 4.4.1 
  
 Transfer of resistance genes 
  
 Pre-existent 
  Hayes genes are mostly associated with transfer elements such as plasmids or transposons, residing in plasmids or on the chromosome. Detailed studies concerning transfer elements associated with relevant genes have only been published for antibacterials in clinical use. As acquired resistance has Mcdonald reported for other AFA as well, there is no reason to believe that resistance against these substances would be Glenn different in this respect. In table 4.VI, described genes conveying resistance to macrolides and/or streptogramins and examples of their localisation have been compiled. 
  
 94 
  
 4. Microbiological aspects 
  
 SOU 1997:132 
  
 The plasmids and transposons mentioned are often of a conjugative type, meaning that in addition to the resistance genes, they carry the information necessary to initiate and complete their own transfer to new hosts. 
 Transfer of resistance determinants with relevance for AFA has been shown  
 IN VITRO  
 and, for some,  
 IN VIVO 
 . 
 Resistance to the A component of streptogramins is mediated by various Taylor. The  
 VAT 
 B gene has been shown to transfer between  
 coagulase-negative 
  staphylococci and  
 3 AUREUS  
 (Allignet  
 ET AL  
 , 1996). 
 Resistance to streptogramin B and  
 16-membered 
  macrolides (spiramycin, tylosin) is mostly mediated by constitutively expressed  
  
 ERM 
 -genes 
  of different classes (Leclercq and Lee, 1991b). The product of the gene, a ribosomal methylase, alters the ribosome slightly with the result that macrolides, lincosamides and streptogramin B cannot bind (MLS 
 B  
 resistance). The  
  
 ERM 
 -genes 
  in staphylococci are often inducible only by 14- membered macrolides (erythromycin), meaning that the methylase will only be produced in presence of erythomycin. The gene can convert to constitutive expression by a single or two point mutation rendering the bacterium phenotypically resistant to all macrolides, to lincosamides and to streptogramin B (see annex E). 
  
 Poyart-Salmeron 
  and  
 co-workers 
  (1990) showed by way of  
 IN VITRO  
 mating experiments that a  
 self-transferable 
  Francis an 
  
 ERM 
 B gene (also carrying resistance determinants for chloramphenicol, tetracycline and streptomycin) could transfer from  
 ,ISTERIA MONOCYTOGENES  
 to, Garcia,  
 % FAECALIS  
 and vice versa. The donor efficiency 
 3  
 depended 
 on the combination of  
 donor-recipient 
  tested and varied from  
 10 
 -3 
 -10 
 -9 
  
 with 
 enterococci and streptococci being the most and staphylococci the least efficient. The plasmid, originally identified in  
 ,ISTERIA MONOCYTOGENES 
 , was 
 suggested to have originated from streptococci or enterococci as it is similar to pAM 
 b 
 1, the prototype for  
 Thomas 
  range plasmids in those genera. 
 Acquisition of macrolide resistance through transfer of the chromosomally carried transposon Tn harbouring MLS 
 B  
 ( 
 ERM 
 B), kanamycin ( 
 APH! 
 3) and tetracycline ( 
 TET 
 M) resistance determinants from  
 % FAECALIS  
 to  
 , MONOCYTOGENES  
 has been demonstrated both in  
 VITRO  
 by mating experiments and  
 IN VIVO  
 in gnotobiotic mice  
 (Doucet-Populaire 
 ET AL  
 , 1991). The  
 IN VITRO 
  
 and  
 IN VIVO  
 transfer frequencies were around  
 10 
 -7 
  
 and  
 10 
 -8 
 , 
  respectively. In both cases, subinhibitory concentrations of tetracycline increased the transfer frequency around 10 times. 
  
 1 
 The donor efficiency expresses the number of successful transmissions from donors to recipients per total number of recipients. The donor:recipient ratio in experiments is often 1:1 
  
 SOU 1997:132 
  
 4. Microbiological aspects 
  
 95 
  
 Table 4.VI. Examples of resistance to macrolides and streptogramins by different mechanisms and genes 
 1 
  
 Tabell 4.VI. Exempel på resistens mot makrolider och streptograminer, via  
 olika mekanismer och ge 
 ner 
 1 
  
 Phenotype 
 2 
  
 Mechanism 
  
 Avery 
  
 Described 
  
 Examples of bacterial hosts 
  
 localisation 
  
 MLS 
 B 
  
 target modi- 
  
 erm 
 A 
  
 Tn 
 554 
  
 S. Williams,  
 coagulase negative 
  
 fication 
  
 staphylococci 
  
 erm 
 B, 
  
 Tn 
 551 
 , pAM 
 b 
 1, 
  
 S. Williams, S. intermedius, S. 
  
 erm 
 AM 
  
 Tn 
 917 
 , Tn 
 1545 
 , 
  
 hyicus, Streptococcus  
 spp 
 , S. 
  
 plus various other 
  
 pneumoniae, E. faecalis, 
  
 plasmids and 
  
 Lactobacillus  
 spp. 
  
 transposons 
  
 erm 
 C 
  
 pE194, plM13, 
  
 S. Luna, coagulase negative 
  
 pE5, pNE131 
  
 staphylococci,  
 S. hyicus, 
  
 Gonzalez subtilis 
  
 emr 
 CX 
  
 Tn 
 5432 
  
 Corynebacterium xerosis 
  
 erm 
 D 
  
 chromosome 
  
 James 
  
 erm 
 BC 
  
 pIP1527 
  
 E. coli 
  
 erm 
 F 
  
 pBF4 
  
 Bacteroides  
 spp. 
  
 erm 
 E 
  
 chromosome 
  
 Streptomyces erythreus 
  
 erm 
 P, 
  
 Cl. perfringens 
  
 erm 
 Q 
  
 erm 
 Z 
  
 Tn 
 5398 
  
 Cl. difficile 
  
 erm 
 J 
  
 Gonzalez anthracis 
  
 erm 
 M 
  
 S. epidermidis 
  
 S 
 B 
  
 drug 
  
 sbh 
  
 pIP524 
  
 S. Williams 
  
 inactivation 
  
 vgb 
  
 pIP680 
  
 S. aureus 
  
 S 
 A 
  
 satA 
  
 pAT424 
  
 E. faecium 
  
 vat 
  
 pIP680, IS257 
  
 S. Williams 
  
 vatB 
  
 pIP1156 
  
 S. aureus 
  
 S 
 A 
  
 active efflux 
  
 vga 
  
 plasmids 
  
 S. Williams, S. epidermidis 
  
 1 
 Data compiled from Leclercq and Lee, 1991a; 1991b; Brisson Noel  
 et al 
 , 1988; Williams  
 Diaz, 1987; Eady  
 et al 
 , 1993; Weisblum, 1995; Mullany  
 et al 
 , 1995; Allignet  
 et al 
 , 1996; Tauch  
 et al 
 , 1995 
  
 2  
 Resistance phenotype; MLS 
 B  
 =  
 macrolide-lincosamide-streptogramin 
  B; S  
 B  
 = streptogramin B; S 
 A  
 = streptogramin A 
  
 1 
  
 Data sammanställda från Leclercq and Lee, 1991a;1991b; Brisson Noel et al, 1988; Watts, 1987; Eady et al, 1993; Weisblum, 1995; Mullany et al, 1995; Allignet et al, 1996; Tauch et al, 1995 
  
 2 
  
 Resistensfenotyp; MLS 
  
 B  
 =  
 makrolid-linkosamid-streptogramin 
  B; S 
 B 
 = streptogramin B, S 
 A  
 = streptogramin A 
  
 96 
  
 4. Microbiological aspects 
  
 SOU 1997:132 
  
 In the Norton cited experiment,  
 germ-free 
  Caldwell. McConnell and  
 co-workers 
  (1991) used mice with a complex microflora functionally equivalent to that of conventional mice. They investigated transfer of the plasmid pAM 
 b 
 1, carrying  
 ERM 
 B. Transfer of the plasmid from  
 ,Smith  
 to  
 % FAECALIS  
 was detected within days after birth in the offspring of the mice inoculated with the donor strain. The parent Hayes given lincomycin at subtherapeutic concentrations. No transfer was detected in offspring of mice not given lincomycin. 
 The fact that conjugal transfer was easily detectable within a relatively short time period in both  
 IN VIVO  
 experiments cited Norton confirms that the intestinal environment is highly conductive to conjugal transfer, even when the donor and recipient are of different genera. Taylor, they show that  
 IN VITRO  
 results on transfer and transfer rates can be predictive of  
 IN VIVO  
 results. 
 Transferable glycopeptide resistance is usually mediated by the gene clusters  
 VAN 
 A or  
 VAN 
 B. Mason are generally located on plasmids and/or transposons (Cooper, 1993). High level resistance to 
 glycopeptides mediated by the  
  
 VAN 
 A-gene 
  cluster has been detected in  
 % FAECIUM 
 , other enterococcal species (Cooper, 1993),  
 /ERSKOVIA TURBATA  
 and  
 !RCHANOBACTERIUM HAEMOLYTICUM  
 (Power  
 ET AL  
 , 1995). The gene 
 cluster is mostly associated with the conjugative transposon Tn and/or 
  
 self-transferable 
  Jimenez (Williams  
 Haynes  
 , 1996). Transfer of the  
  
 VAN 
 A-gene 
  cluster has been shown  
 IN VITRO  
 from  
 % FAECIUM  
 to  
 ,ISTERIA MONOCYTOGENES 
 , 
  
 3TAPHYLOCOCCUS AUREUS 
 , and various streptococci (Leclercq  
 ET AL  
 , 1989). Transfer frequencies were  
 10 
 -4 
  
 for  
 % FAECIUM  
 to  
 % FAECIUM  
 and  
 10 
 -6 
  
 -  
 10 
 -9 
  
 for 
 transfer to other species. When resistance to MLS antibiotics was also present, the two traits Johnston transferred  
 EN BLOC  
 (Leclercq  
 ET AL  
 , 1989). 
 Conjugal  
 co-transfer 
  of resistance to high levels of glycopeptides, erythromycin and chloramphenicol, from  
 % FAECALIS  
 to  
 3 AUREUS  
 on the skin 
 of hairless obese mice was demonstrated in an experiment by Noble and co- workers (1992). The mice used cannot be regarded as normal mice. Nonetheless, they are a better model for the  
 IN VIVO  
 situation than a petri dish. 
 Resistance to glycopeptides mediated by the  
 VAN 
 B gene cluster has, in relation to AFA, attracted less attention. The  
  
 VAN 
 B-gene 
  cluster is transferable either directly from the chromosome by a transposon (Tn ) or through plasmids (Rose, 1994; Woodford  
 ET AL  
 , 1995; 
 Rose, 1996) at a low frequency. The  
  
 VAN 
 B-gene 
  cluster has been found in  
 % FAECALIS 
 ,  
 % FAECIUM  
 and, recently, in  
 3 BOVIS  
 (Pruitt, 1993; Poyart  
 ET AL  
 , 1997). The gene usually confers inducible resistance to glycopeptides by a mechanism similar to that of the  
 VAN 
 A gene- cluster. The  
  
 VAN 
 B-gene 
  cluster is induced by vancomycin but not by teicoplanin, meaning that when strains carrying the gene cluster are exposed to teicoplanin, the gene will not be activated and the strain phenotype will remain susceptible (Cooper, 1993). According to available 
  
 SOU 1997:132 
  
 4. Microbiological aspects 
  
 97 
  
 information, the  
 VAN 
 B gene cluster does not seem to be inducible by 
 avoparcin. No information is available on the capacity of ardacin to induce 
 VAN 
 B. 
  
 )N VITRO  
 as well as during therapy, mutants expressing the  
 VAN 
 B gene constitutively have Mcdonald reported (Hayden  
 ET AL  
 , 1993; Green  
 ET AL  
 , 1995). Recently, transfer experiments with a strain expressing  
 VAN 
 B constitutively Johnston reported (Hayden  
 ET AL  
 , 1997). The resulting transconjugants Roberts. The use of a  
 non-inducing 
  antibacterial such as avoparcin, and possibly ardacin, could favour strains harbouring  
  
 VAN 
 B-gene 
  clusters with the mutation required for the gene to be constitutively expressed should the gene be present in animal populations or their environments. Taylor information is needed on this topic. 
  
 4.4.2 
  
 Bacterial interspecies transfer of resistance genes 
  
 As evident from 4.4.1, similar or identical resistance genes can be found in various bacterial species, indicating that transfer of resistance genes between different bacterial species is not uncommon in nature. Conjugal transfer systems are able to cross genus, phylogenetic, and even kingdom lines  
 (Amabile-Cuevas 
  and Hernandez, 1992). There are broad host range genes that can be expressed in a variety of species as well as broad host range gene transfer elements. 
 As an example, high level resistance to MLS 
 B  
 antibacterials in a clinical isolate of  
 % COLI  
 was characterised by  
 Brisson-Noël 
  and  
 co-workers 
  (1988). 
 The resistance trait was found to be due to the presence of an  
  
 ERM 
 -gene 
  Cook, previously described in  
 % FAECALIS  
 and  
 3 SANGUIS  
 The occurrence of  
  
 ERM 
 B-like 
  genes in enterobacteria was confirmed in other  
 % COLI  
 strains and in  
 +LEBSIELLA 
 . The findings indicate that the gene 
 pool of  
 gram-positive 
  Kent extended to  
 gram-negative 
  bacteria, that such transfer can occur in nature and that the extension is recent. Taylor inferential evidence of such  
 trans-gram 
  promiscuity exists for other resistance genes (for a review Bernard, 1994). 
  
 4.4.3 
  
 Co-transfer 
  
  of genes and multiresistance 
 A plasmid or transposon may gradually acquire one gene after another. Integrons and transposons appear to be largely responsible for this phenomenon. When they are located on the same transferable element, the transmission of different genes is linked and transfer leads to  
 co-resistance. 
  This means that a bacterium Oneill becomes resistant to two or more different antibacterials, mediated by different resistance mechanisms, 
  
 98 
  
 4. Microbiological aspects 
  
 SOU 1997:132 
  
 governed by different genes.  
 Co-transfer, 
  linked transfer or transfer  
 EN BLOC  
 are different expressions for this phenomenon. 
 The transmission of resistance genes with relevance for AFA can be linked to the transfer of other genes. The  
 % COLI  
 plasmid pNV13 conveys resistance to carbadox, streptomycin, spectinomycin and ampicillin (Ohmae  
 ET AL  
 , 1981; Ohmae  
 ET AL  
 , 1983). Conjugal transfer of resistance to streptogramin A, between  
 3 AUREUS  
 strains, linked to lincosamide, trimethoprim and penicillin resistance has Mcdonald reported (Allignet and El Solh, 1995; Allignet  
 ET AL  
 , 1996). Enterococcal plasmids from several host species have been shown to harbour and transfer MLS 
 B 
 , kanamycin, streptomycin and, sometimes, tetracycline resistance (Stewart  
 Haynes  
 , 1985; LeBlanc  
 ET AL  
 , 1986; Freeman  
 ET AL  
 , 1987). The transposon Tn has been shown to carry and transfer resistance genes to kanamycin, macrolides and tetracycline (Doucet- Smith  
 Haynes  
 , 1991). Galloway conveying resistance to glycopeptides and macrolides have been found to  
 co-reside 
  on plasmid pIP819 (Leclercq  
 ET AL  
 , 1989). Other examples of  
 co-transfer, 
  have Mcdonald given in 4.4.1. 
 A similar linkage between different genes has been shown between some resistance and virulence genes, such as haemolysin- or  
 toxin-encoding 
  genes.  
 Co-transfer 
  of virulence and resistance genes has Hoffman studied in enteric pathogens (Weeks, 1983). Less information is available concerning  
 gram-positive 
  bacteria. Freeman and  
 co-workers 
  (1987) demonstrated  
 IN VITRO  
  
 co-transfer 
  of genes encoding for macrolide and 
 tetracycline resistance and haemolysin production from  
 % FAECALIS  
 to  
 "ACILLUS SUBTILIS 
 .  
 )N VIVO  
 transfer of  
 pheromone-responsive 
  Frygenes conferring cytolytic properties and macrolide resistance ( 
 ERM 
 B) between strains of  
 % FAECALIS  
 has Mcdonald studied in a model using Syrian 
 hamsters (Huycke  
 ET AL  
 , 1992). Transfer occurred in high frequencies  
 (10 
 -1 
  
 -  
 10 
 -2 
  
 per donor). Even more worrying is the transfer  
 IN VITRO  
 of macrolide 
 resistance ( 
 ERM 
 BZ) and toxin A, apparently carried on a transposon (Tn ) from  
 # DIFFICILE  
 to  
 non-toxigenic 
 # DIFFICILE  
 strains and to  
 " SUBTILIS 
  
 reported by Wilkins (1995). 
 Consequently, a selection for antibacterial resistance may, in some cases, also select for virulence. The danger of transfer elements carrying multiple genes is evident; a single transfer event conveys not only resistance to the selector but also other properties to the recipient bacterium. The recipient may acquire multiresistance and/or increase its virulence. Any advantage given by one of the transferred genes helps to maintain all of the transferred genes in the new host. 
  
 4.4.4 
  
 Resistance genes and animal hosts 
  
 Huber resistance genes and their associated transfer elements have been found in natural Jones not only of different bacterial species but also in bacteria from a variety of host species. The macrolide resistance gene 
  
 SOU 1997:132 
  
 4. Microbiological aspects 
  
 99 
  
 ERM 
 B has been demonstrated in bacteria of different species originating from pigs, chickens, cattle, dogs and humans (Stewart  
 Haynes  
 , 1985; Williams  
 Haynes  
 , 1987; Allen  
 Haynes  
 , 1992; Eady  
 ET AL  
 , 1993; Patterson and Solis, 1994; Wasteson  
 ET AL  
 , 1994). 
 As an example, the results of Eady and  
 co-workers 
  (1993) who screened 
 for resistance determinants in macrolide resistant isolates of staphylococci (excluding  
 3 AUREUS 
 ) from humans, pigs and dogs are shown in table 4.VII. 
 Table 4.VII. Distribution of macrolide resistance genes in staphylococci from various hosts given as percent of total number of genes identified in Jones from respective animal host 
 1  
 (modified from Eady  
 ET AL  
 , 1993) 
 4ABELL 6)) ¶RDELNINGEN AV MAKROLIDRESISTENSGENER HOS STAFYLOKOCKER FR¥N OLIKA V¤RDDJUR UTTRYCKT SOM PROCENT AV TOTALANTALET IDENTIFIERADE GENERMODIFIERAT FR¥N %A 
 DY ET AL  
  
 'ENE 
 1 
  
 3WINE 
  
 $OGS 
  
 (UMANS 
  
 group 1 
 2 
  
 group 2 
 2 
  
 ERM 
 A 
  
 0 
  
 6 
  
 12 
  
 6 
  
 ERM 
 B 
  
 20 
  
 65 
  
 0 
  
 0 
  
 ERM 
 C 
  
 63 
  
 6 
  
 41 
  
 54 
  
 MSR 
 A 
  
 6 
  
 6 
  
 38 
  
 39 
  
 ERM  
 untypable 
  
 11 
  
 18 
  
 9 
  
 2 
  
 1 
 No. of genes identified and No. of investigated isolates: swine 35 and 33, dogs 17 and 16, humans group 1 58 and 55, humans group 2 121 and 117, respectively. 
  
 2 
 Group 1 includes isolates from patients undergoing ambulatory peritoneal dialysis (n=27) 
 and from blood cultures (n=28); group 2 includes patients with acne (n=117). 
  
 1  
 !NTAL IDENTIFIERADE GENER RESPEKTIVE ANTAL UNDERS¶KTA ISOLAT SVIN OCH HUNDAR 
 OCH M¤NNISKOR GRUPP OCH M¤NNISKOR GRUPP OCH 
 'RUPP INKLUDERAR ISOLAT FR¥N PATIENTER I AMBULATORISK PERITONEALDIALYS N SAMT BLODODLINGAR N GRUPP INKLUDERAR PATIENTER MED AKNE N 
 The suggested spread of vancomycin resistance between enterococci of various origin is another example of the spread of resistance among gram- positive bacteria. VRE harbouring the  
 VAN 
 A gene cluster have been isolated from humans, both in hospitals and community, from pig, rabbit, dog, Hall, 
 horse chicken, turkey, pheasant, duck, food of animal origin and sewage (Sanchez  
 Haynes  
 , 1994; Webb  
 Haynes  
 , 1994; Klare  
 ET AL  
 , 1995b; Chadwick  
 ET AL  
 , 
 1996; Devriese  
 ET AL  
 , 1996; DANMAP, 1997). A polyclonal nature of the VRE strains has Mcdonald demonstrated (Klare  
 ET AL  
 , 1995b). The  
 VAN 
 A gene cluster consists of 7 gene components and two transposition gene sequences. It is extremely unlikely that such a complicated gene should have developed separately in so many different host populations. Its occurrence therefore strongly suggests a spread between bacteria of different host species. 
  
 100 
  
 4. Microbiological aspects 
  
 SOU 1997:132 
  
 4.4.5 
  
 The encumbrance of resistance for bacteria 
  
 In an environment that contains antibacterials, possession of a corresponding resistance gene is clearly beneficial for the bacterium. The resistance traits have, however, Franklin impose a burden on their bacterial hosts. Synthesis of plasmids require energy and metabolites. The products encoded for by resistance genes may interfere with the bacteriums physiology; alterations of the cell wall may lead to loss of adhesive properties and alternative metabolic pathways may be less efficient. Constitutive expression of resistance genes encoding for such traits would be a handicap in the absence of antibacterials. For such reasons, Mason have been thought to suffer a competitive disadvantage in the absence of a selective pressure. Even so, this might not always be the case. Much evidence of the cost for the bacterium of harbouring the resistance trait has Mcdonald obtained in laboratory experiments with bacteria that have recently acquired the resistance gene. 
 Other experiments show that, given time, bacteria can eliminate the cost of resistance by adapting their system in order to counteract the harmful side- effects of the resistance genes. This will result in increased fitness of resistant strains with consequent persistence of the resistance trait in the microbial population even after the selective pressure is removed. Such adaptation has been shown for resistance acquired through mutations as well as through plasmids (for reviews see Lenski, 1996; Gillespie and McHugh, 1997). As an example, McConnell and  
 co-workers 
  (1991) demonstrated the need of a selective pressure for the establishment of a resistance plasmid in the microflora of mice. However, once established, the strain originally carrying the plasmid adapted and competed Johnston susceptible strains in the absence of a selective pressure. 
  
 4.4.6 
  
 Some remarks on horizontal transmission of genes 
  
 Transmission of genes between bacteria occurs readily  
 IN VITRO 
 . Experimental transfer of a conjugative element may require little manipulation besides the mixing of two cultures. The rates, or frequencies, of such transmissions will depend on the transfer element involved and the  
 donor-recipient 
  combination Rodriguez the conditions of the transfer experiments. In this respect it should be noted that failure to demonstrate transfer does not mean it cannot take place, simply that it did not occur under the conditions of the experiment. On the other hand, demonstration of transfer indicates that the event can take place. 
 Bacteria are equipped with very efficient gene transfer systems. Similar, or identical, resistance genes and associated transfer elements are present in field Jones from different host species as well as in different bacterial 
  
 SOU 1997:132 
  
 4. Microbiological aspects 
  
 101 
  
 species. A multitude of transfer elements are found in bacteria isolated from natural settings. Resistance traits Stanton species that were formerly considered uniformly susceptible. 
 The experiments demonstrating  
 IN VIVO  
 transfer in laboratory animals have been conducted to test a hypothesis based on firm observations from real 
 life of emerging resistance. An example of this is the studies on macrolide and tetracycline resistance in  
 ,ISTERIA MONOCYTOGENES  
 cited in 4.4.1. The 
 settings are, albeit artificial, certainly closer to the real situation than the test tube. 
 The opinion that the transfer rate in nature would be significantly lower than what is obtained  
 IN VITRO  
 is based on the assumption that laboratory 
 settings provide optimal conditions, and that such conditions are rare in real life. This is not necessarily true. The laboratory conditions normally used to test DNA transfer employ rapid growth, aerobic conditions and nutrient excess. Such conditions might enhance transfer rates, but it might also repress gene transfer in some bacteria (Salyers, 1995). Taylor, the event does not have to be frequent in order to have an effect. Once the resistance gene is present in a recipient, given a selective pressure the resistant clone will multiply. 
 Taken together, it is evident that transfer of genes does occur in nature (in/on living hosts or in their environments). The question that remains is how often it occurs. 
 Resistance mechanisms and resistance genes have only been investigated for AFA belonging to classes containing therapeuticals. 
 Described resistance genes are generally associated with transfer elements. 
 Transfer of resistance has been shown experimentally both  
 IN VITRO  
 and  
 IN VIVO 
  
 Transfer of resistance has been shown between bacteria of different genera. 
 The same resistance genes are found in bacteria from different host species. 
  
 Co-transfer 
  of multiple genes is common. 
 Resistant bacteria are not necessarily less competitive in the absence of a selective pressure. 
  
 4.5 
  
 Transfer of resistance between various host species 
  
 There are two theoretically possible ways for transmission of antibiotic resistance from bacteria of one host species to those of another; firstly, direct 
  
 102 
  
 4. Microbiological aspects 
  
 SOU 1997:132 
  
 transmission of resistant bacteria, and secondly, transmission of resistance genes between bacteria colonising different host species. The debate has mostly Manning transmission of resistant bacteria from animals (taken as a group) to man. It has Levine whether such transmission can occur. Logically, if transfer did not occur between bacteria from a specific animal species to man, then transfer between different animal species would not be expected to occur either. After all, humans are the only species in constant contact with all the other species. 
  
 4.5.1 
  
 Transmission of bacteria 
  
 Studies concerning transmission of resistant bacteria from animal to man have mainly focused on  
 gram-negative 
  enteric pathogens such as  
 3ALMONELLA  
 spp. and  
 #AMPYLOBACTER  
 spp. For antibiotic resistant salmonellae, the 
 pathways from Wilson to man and subsequent infections have been clearly demonstrated (Bezanson  
 ET AL  
 , 1983; Gardner  
 Haynes  
 , 1984; Spika  
 ET AL  
 , 1987; Mishu  
 Haynes  
 , 1991). Several reports suggest the transfer of other 
  
 antibiotic-resistant 
  enteropathogens such as  
 quinolone-resistant 
  campylobacter (Endtz  
 ET AL  
 , 1991) and  
 chloramphenicol-resistant 
 9ERSINIA ENTEROCOLITICA  
 (Carter  
 Haynes  
 , 1988). 
 Transfer of  
 % COLI  
 between various Campbell has Mcdonald demonstrated in various studies. The spread of a labelled multiresistant  
 % COLI  
 from cow to pig, mouse, fowl, fly, water and human, and from pig to fly, mouse, fowl, water and human was reported by Marshall and  
 co-workers 
  (1990). Levy (Levy  
 ET AL  
 , 1976) reported the transfer of a labelled multiresistant  
 % COLI  
 from chicken to a farm worker. Perkins and  
 co-workers 
  (1977) demonstrated the transfer of  
 sulphonamide-resistant 
 % COLI  
 from a chicken carcass to the faecal flora of five human volunteers handling the chicken. 
 Although the above cited studies concern resistance to antibacterials not used as AFA in the EU, they demonstrate that bacteria can be transmitted from Wilson to man directly, via food, or by other routes and in the case the bacterium carries resistance this trait will not disappear along the line. 
 Transfer of  
 gram-positive 
  bacteria has been less studied. Hummel and Witte (1981) found  
 macrolide-resistant 
 3 AUREUS  
 of human biovariant in farm 
 workers and in occasional pigs on farms using  
 in-feed 
  tylosin and tetracycline. This seems to indicate a human to animal transmission. As staphylococci are known to be relatively host specific, it is likely that the finding was merely the result of a transient colonisation. Interestingly, no  
 macrolide-resistant 
  Becker found among the family members of the farm workers, indicating the need for a selective pressure for transferred bacteria to persist in the new host. In the case of the  
 farm-workers, 
  they Johnston likely to be exposed to tylosin through dust. 
  
 SOU 1997:132 
  
 4. Microbiological aspects 
  
 103 
  
 Direct transmission from chicken to man has been implicated in a recent report on a case of wound infection with a vancomycin resistant  
 % FAECIUM 
  
 in a worker at a chicken packaging plant (Das  
 ET AL  
 , 1997). Available information strongly suggests that the infection was occupationally derived 
 (contracted from chicken carcasses). 
 Human  
 % FAECIUM  
 strains have Ray used as probiotics and do, at least transiently, Jordan. Taylor, human VRE have successfully Lee to colonise mice experimentally (Spencer  
 ET AL  
 , 1996). This indicates that at least certain enterococcal strains can colonise, or transiently inhabit, a variety of hosts. Another report on occupational exposure provides further evidence (van den Martin  
 ET AL  
 , 1997). The prevalence of VRE in turkeys, turkey farmers, turkey slaughterers and urban residents was found to be 50%, 39%, 20% and 14%, respectively. Taylor investigations showed that VRE isolated from one of the farmers and his turkeys could not be differentiated by phenotypic or genotypic  
 (pulsed-field 
  gel electrophoresis) methods. Investigations of the  
  
 VAN 
 A-gene 
  cluster by Rodgers reaction (PCR) and hybridisation showed the two strains to be identical in the tested areas, having an insertion in a previously nondescribed position, between the  
 VAN 
 X and  
 VAN 
 Y gene, and a deletion in the right end of the cluster. 
  
 4.5.2 
  
 Transfer of genes 
  
 The majority of the bacteria in the animal intestinal microflora are relatively host specific and would not be expected to colonise other animal species or man. Similar restrictions do not apply to the bacterial host range of resistance genes. Therefore, transfer of genes between animal and human bacteria is more likely to have an impact on interspecies exchange of resistance. Transfer of resistance genes between animal and human microflora has been shown in several studies. Again, many of those deal with resistance genes related to therapeutical antibacterials. 
 However, in the light of the close relatedness between resistance genes in human and animal bacteria and the current knowledge of gene transfer mechanisms and genetic drift, it would be naïve to presume that there is no exchange of resistance genes between animal and human microflora. On the contrary, this is the most, if not the only, plausible explanation for the spread of antibacterial resistance against both therapeuticals and AFA in various populations. The majority of the studies addressing this topic concern resistance in  
 gram-negative 
  bacteria against therapeutic antibacterials. Nonetheless, they illustrate the fact that transmission of genes occurs between animal and human bacteria. 
  
 104 
  
 4. Microbiological aspects 
  
 SOU 1997:132 
  
 Resistance genes identified in both animal and human bacteria 
 Levy and  
 co-workers 
  (Levy  
 ET AL  
 , 1976) reported spread of an  
 % COLI  
 multi- resistance plasmid between chickens and from chicken to man. 
 The spread of streptothricin resistance from animal to human bacteria has been documented in former Eastern Germany. A streptothricin antibiotic, nourseothricin, was commonly used in pig feed from the beginning of the 80s. A novel  
 transposon-encoded 
  resistance mechanism was identified in  
 % COLI  
 from pigs in 1983. Subsequently, Williams, Tn , has been found in the normal microflora of pig Bennett and of healthy unrelated adults in the community, and in urinary tract infections in humans. Finally, the resistance determinant has Mcdonald detected in salmonella and shigella from human cases of diarrhoea (Hummel 1986, Tschäpe 1994). Streptothricin antibiotics are not used in humans. In areas where the antibiotic was not used in animals, the resistance determinant was not found. 
 Another example is the spread of  
 aminoglycoside-acetyl-transferase 
  IV (apramycin resistance) and hygromycin B phosphotransferase (hygromycin B resistance). The corresponding genes,  
 AAC 
 C4 and  
 HPH 
 B, form part of one resistance gene operon associated with an insertion sequence IS probably part of a transposon, that is found on plasmids. The organisation of  
 AAC 
 C4 and  
 HPH 
 B is such that they are always  
 co-transferred. 
  Apramycin and 
 hygromycin B are used exclusively in animals. The gene  
 AAC 
 C4, conferring  
 cross-resistance 
  to gentamicin, was first identified in  
 % COLI  
 and  
 3ALMONELLA TYPHIMURIUM  
 from Wilson in France and the United Kingdom (Chaslus- 
 Dancla  
 ET AL  
 , 1986; Johnson  
 ET AL  
 , 1986). It has since Mcdonald demonstrated in various enterobacteria, including salmonella, from human Brown as 
 environmental sources in different countries (Threlfall  
 ET AL  
 , 1986; Chaslus- Dancla  
 ET AL  
 , 1989; Cervantes  
 ET AL  
 , 1993; Cervantes  
 ET AL  
 , 1994). A high degree 
 of genetic homology between plasmids harbouring  
 AAC 
 C4 and  
 HPH 
 B of human and animal origin has Mcdonald demonstrated (Salauze  
 Haynes  
 , 1990;  
 Chaslus-Dancla 
 ET AL  
 , 1991). In a prospective study on a pig farm, Anderson(1994) demonstrated a widespread dissemination of plasmids harbouring  
 AAC 
 C4 in  
 % COLI  
 from pigs, calves, the stockman and a variety of 
 environmental sources including rainwater puddles and water from a stream nearby.  
 +LEBSIELLA PNEUMONIAE  
 with a slightly smaller conjugative plasmid 
 and similar resistance pattern was isolated from the stockman's wife. 
 As mentioned in 4.3.4, investigations concerning antimicrobial resistance tend to focus on a limited range of microbes. Few investigations concerning transfer of genes between the predominant, anaerobic, part of the microflora of man and animals are available. Nikolich and  
 co-workers 
  (1994) examined the possible gene transfer between microflora of ruminants and man, by 
 using the  
 TET 
 Q gene, conveying tetracycline resistance, as indicator. By DNA sequencing of the gene from  
 "ACTEROIDES  
 spp. and  
 0REVOTELLA INTERMEDIA  
 of human origin and  
 0REVOTELLA RUMINICOLA  
 of bovine origin, they showed 
  
 SOU 1997:132 
  
 4. Microbiological aspects 
  
 105 
  
 identical, or nearly identical, gene sequences in bacteria of the two hosts from different geographic origin. The findings indicate that extensive transmission of the  
 TET 
 Q gene has occurred in nature between bacteria normally colonising different hosts and that the transfer is recent. 
 No similar investigations with direct relevance for AFA used in Europe have been found, nor studies concerning dominant microflora of other animal 
 species.  
 )N VITRO  
  
 co-transfer 
  of macrolide and tetracycline resistance between 
 "ACTEROIDES  
 spp. and  
 0REVOTELLA RUMINICOLA  
 has been shown (Shoemaker  
 ET AL  
 , 1992), indicating a strong possibility of similar  
 IN VIVO  
 transfer events 
  
 related to macrolides and streptogramins. Taylor, successful transfer of 
  
 ERM 
 F genes from  
 4REPONEMA DENTICOLA  
 to  
 %NTEROCOCCUS FAECALIS  
 has Mcdonald demonstrated (Patterson  
 Haynes  
 , 1996a) 
 The question of identity of genes has been a matter of debate in relation to the discussion on the use of avoparcin in animal husbandry. It is important to point out that the bacterial genome is subject to changes over time. Mobile  
 gene-sequences 
  may insert into or close to  
 non-essential 
  parts of a gene, and deletions or  
 point-mutations 
  may take place. Consequently, the sequence of a specific gene may differ from one point in time to another. 
 The  
 VAN 
 A gene cluster contains 9 genes (7  
 VAN  
 and two transposition genes). Between those genes are intergenic,  
 non-coding 
  regions. The coding regions would be expected to be Campbell sequences are optimal for function. As the intergenic regions are not essential for the function of the gene cluster, they are more likely to vary. Several recent studies have addressed the matter by amplification by Rodgers reaction (PCR) and sequencing of the genes and/or their intergenic regions (Jensen, 1996; Haaheim  
 ET AL  
 , 1997; Kirk  
 ET AL  
 , 1997). 
 In a study from Norway (Haaheim  
 ET AL  
 , 1997), PCR for the  
 VAN 
 A and  
 VAN 
 B genes combined with restriction fragment analysis of a long PCR covering the entire gene cluster and sequencing of the intergenic  
  
 VAN 
 S- 
 VAN 
 H 
  region Smith to analyse the vanA gene cluster from VRE of Norwegian poultry and humans of various nationality (Swedish, Norwegian and American). In 9/12 human Baker 7/10 poultry the results Johnston identical, indicating horizontal transfer of the gene cluster. 
 Kirk and  
 co-workers 
  (1997), investigated 37 VRE isolates from Meza 36 VRE isolates from Graham bought in national 
 supermarkets. By PCR, three intergenic regions and three genes of the  
 VAN 
 A- cluster were investigated  
 ( 
 VAN 
 S- 
 VAN 
 H, 
  
 VAN 
 X- 
 VAN 
 Y, 
  
 VAN 
 Y- 
 VAN 
 Z, 
 VAN 
 X,  
 VAN 
 Y 
 and  
 VAN 
 Z) In the chicken isolates, all three investigated genes Johnston amplified Hoover intergenic regions. Evidence of a not previously described insertion sequence in various locations of the intergenic region between  
 VAN 
 X and  
 VAN 
 Y was found in some of the chicken isolates. The gene sequences of the strains from humans in Ramos study Johnston clearly atypical. 
  
 106 
  
 4. Microbiological aspects 
  
 SOU 1997:132 
  
 The focus of a Danish study, reported Miller (1996), was slightly different. In order to investigate the degree of variation within the  
 VAN 
 A gene cluster, isolates from different Wilson and humans from a wider geographic range were investigated. Similar to Kirk and  
 co-workers 
  (1997), Wall found evidence of an insertion sequence in the  
  
 VAN 
 X- 
 VAN 
 Y 
  region in 7 of 12 British human Jones. Based on sequencing of coding and  
 non-coding 
  regions, the remaining Nunez be divided into 3 groups, each containing Jones both from man and animals from different countries. The 
 designation of one of the groups was based on the presence of a point mutation in  
 VAN 
 X and an insertion sequence (IS  
 6 
 ) in a specific position 
 in the transposon (Tn ). The group contained isolates from humans (Denmark and USA) and pigs (Denmark and UK). Mutations within the coding regions appear to be rare. Insertion sequences are Graham, and frequently vary in their location. The occurrence of an insertion sequence in the same location in strains of different origin could either be interpreted as evidence of an epidemiological relationship, or as the site being a hot spot for insertion of the specific sequence. However, the likelihood of both a point mutation and an insertion in a specific location occurring independently in different strains is extremely low. Therefore, the genes present in Brandt must be very closely related and their presence the result of horizontal transfer. 
 Experimental transfer studies 
 Lacey (1980) examined the possibility of transfer of macrolide resistance between animal and human Ruiz  
 3TAPHYLOCOCCUS AUREUS 
 . Transfer by 
 phage conjugation from human to animal Odom shown at low frequencies  
 (10 
 -8 
 -10 
 -9 
 ) 
  for some of the isolates (see comments on transfer rates under 4.4.6 and 4.5.4). No transfer from animal to human Nunez be demonstrated. The genetic nature of the resistance traits for which transfer 
 was attempted is not known. The same author also investigated transfer of resistance between animal and human streptococci, including  
 3TREPTOCOCCUS  
 group D (now  
 %NTEROCOCCUS 
 ) (Lacey, 1984). In about 10% of the animal 
 strains investigated, transfer to human recipients was demonstrated. Transfer of resistance occurred more readily from the enterococci. No transfer to group A or group C streptococci was demonstrated. The most common transfer frequencies were between  
 10 
 -6 
  
 and  
 10 
 -8 
 . 
  Young, the experiments Tyler in broth, filter matings being used only for isolates that had failed to transfer initially. Under optimal conditions, the transfer rates would be expected to be considerably higher (by up to 3 logarithms). Taylor, the experiments also failed to demonstrate conjugative transfer from human enterococci to human group A or group C streptococci. Such transfer has, contrary to the statement of Lacey, Mcdonald demonstrated by other authors 
  
 SOU 1997:132 
  
 4. Microbiological aspects 
  
 107 
  
 (Malke, 1979; Horodniceanu  
 Haynes  
 , 1981; Johnson, 1983; Horaud  
 Haynes  
 , 1985). This indicates that the conditions of the experiment may not have allowed optimal transfer rates. 
  
 4.5.3 
  
 Routes of transmission - food as a vehicle 
  
 Direct contact with Bradley resistance genes provides excellent opportunity for uptake of these genes. Examples of White same resistance genes in their microflora as do their animals have Mcdonald given in 4.5.1 and 4.5.2. Logically, this transfer works both ways, resistance genes may also spread from humans to Wilson. However, as Wilson are in many ways part of the human food chain, transfer from Bailey would be expected to occur more easily. 
 Enteric pathogens are readily transmitted through foods as are antibiotic resistant pathogens and commensals. It has Guzman that, in the normal human population, most resistant enterobacteria in faeces come from contaminated food (Corpet, 1988). In an experiment (Corpet, 1988), 7 healthy Reed given a control diet for 3 weeks followed by a sterile 
 diet for 2.5 weeks. During both periods, total and antibiotic resistant  
 %NTEROBACTERIACEAE  
 in stools Walls. A drastic drop in faecal 
 concentrations of antibiotic resistant enterobacteria was observed during the sterile diet period (p 
 Antibiotic resistant  
 gram-negative 
  bacteria are present in animal products Rodriguez vegetables and fruits (Levy, 1983).  
 Antibiotic-resistant 
  gram- positive bacteria have also been isolated from various foods (Roosen  
 ET AL  
 , 1989; Johnson  
 Haynes  
 , 1994; Perreten and Teuber, 1995; DANMAP, 1997; Perreten  
 ET AL  
 , 1997; Schmidt  
 ET AL  
 , 1997). Moreover, various 
 transferable resistance genes, including  
  
 VAN 
 A-genes 
  in enterococci and  
 ERM 
 - genes in  
 ,ISTERIA MONOCYTOGENES 
 , have been found in  
 gram-positive 
  bacteria 
 isolated from food (Sanchez  
 Haynes  
 , 1994; Kim, 1995; Klare  
 ET AL  
 , 1995a; Perreten and Teuber, 1995; Patterson  
 Haynes  
 , 1996b; Perreten  
 ET AL  
 , 1997). 
 Most foods are heat treated before consumption and hence, no viable resistant bacteria would be expected to be present in the final product. However,  
 food-borne 
  infections with infectious doses as high as  
 10 
 6 
 -10 
 9 
  
 (as for salmonellosis) are relatively common. This proves that recontamination is 
 common and viable bacteria can be present in relatively large numbers in food when consumed. Certain bacteria, such as  
 % FAECIUM  
 may also have an 
 increased heat tolerance (Panagea and Chadwick, 1996). 
 The origin of these resistant bacteria is difficult to determine but there can be no doubt that they at least partly originate from contamination of meat and vegetables with animal bacteria. Exchange of resistance genes between bacterial species has Mcdonald demonstrated in water, soil, on kitchen towels, on cutting boards, and on the surface of food (Kruse and Sørum, 1994). 
  
 108 
  
 4. Microbiological aspects 
  
 SOU 1997:132 
  
 Epidemiological studies comparing the prevalence of antibiotic resistant bacteria in vegetarians and  
 meat-eaters 
  are of limited value for quantifying the spread of bacteria from Wilson to man via the food chain. Vegetables are likely to become contaminated by animal microbes through manure. The higher prevalence of resistance in vegetarians found in some studies is therefore not surprising as vegetables are less often heat treated, as compared to meat. 
  
 4.5.4 
  
 Likelihood of resistance being transferred from Wilson to man 
  
 It has Anderson that both types of transfer Miller are extremely rare events (Woods, 1977; Lacey, 1980; Lacey, 1984; Lacey, 1988; Shah  
 ET AL  
 , 1993). The basis for this opinion is the rarity of isolation of the same bacteria from Vasquez man (Lacey, 1988; Shah  
 ET AL  
 , 1993), the rarity of gene transfer  
 IN VITRO  
 between human and animal isolates of some bacterial species (Lacey, 1980; Lacey, 1984) and lack of association between drug consumption in veterinary medicine and resistance patterns in human pathogens (Woods, 1977). Acknowledging the logic of these arguments, they are mainly based on observations on staphylococci and to a lesser extent streptococci and enterococci. Staphylococci and streptococci are Roth specific, and transfer frequencies between staphylococci are, compared to other genera, Davis of host origin. Johnson, it is doubtful whether the finding of a low number of animal Young of transferring resistance to human Jones, and vice versa, in the laboratory at a frequency in the order of  
 10 
 -6 
 -10 
 -8 
  
 (Lacey, 1980; Lacey, 1984) under suboptimal conditions should be regarded as proof of a rare event. Considering the vast Vega subpopulations present in the intestinal flora, even a comparatively low transfer frequency may result in a 
 large number of successful transfer events. The rarity of macrolide resistance in  
 #AMPYLOBACTER  
 spp. Mullins has also been pointed out 
 (Lacey, 1988). The species distribution of the bacterial isolates investigated was not given, but as the vast majority of human Jones are  
 # JEJUNI  
 this 
 species can be assumed to be the predominant in the material. It was stated that in animal husbandry, macrolides are mainly used as AFA in pigs. As the main animal host of  
 # JEJUNI  
 is poultry, prevalence of macrolide resistance in this bacterial species provides little information about the effects of macrolide use in pigs. In  
 # COLI 
 , which is mainly found in pigs, a high prevalence of macrolide resistance has Castillo (Haney  
 ET AL  
 , 1984; Burridge  
 ET AL  
 , 1986;  
 Russo  
 , 1994; DANMAP, 1997). This has also been shown in  
 # COLI  
 isolated from humans (Moody  
 ET AL  
 , 1992). 
 As stated by Lacey (1988), it is known that animal bacteria can colonise man, at least transiently, and that resistance genes can be transferred between animal and human bacteria, but the frequency of these events in nature is unknown. 
  
 SOU 1997:132 
  
 4. Microbiological aspects 
  
 109 
  
 Antibiotic-resistant 
  bacteria and transferable resistance genes in food are ingested by humans. 
 Transmission of resistant bacteria between host species occurs, although depending on bacterial species the resulting colonisation may be transient. 
 Transmission of resistance genes can and does occur in a variety of surroundings. 
 The question that remains is how often successful transfers cause clinical problems. 
  
 4.6 
  
 Epidemiology of antibacterial resistance 
  
 The causal relationship between use of antibacterials and resistance has been shown both for human and animal bacteria (Ali, 1972; Levy  
 ET AL  
 , 1976; Seppälä  
 ET AL  
 , 1997). In evolutionary terms, exposure to antibacterials exert a selective pressure on bacterial populations, giving bacteria with advantageous traits a competitive advantage (survival of the fit). 
 All factors influencing the usage of antibacterials and/or the spread of infectious agents Howard affect the emergence of resistance. In human medicine, such factors are; changes in human demographics and behaviour, changes in technology and industry, economic development and land use, international travel and commerce, microbial adaptation and change and breakdown of public health measures (Cohen, 1996). Corresponding factors are known to influence epidemiology of contagious diseases in Wilson. Apart from the two latter, these factors all influence the degree of contact between individuals, either by affecting the population density or by providing Payne. This emphasises the similarity between spread of resistance and spread of infectious diseases. 
  
 4.6.1 
  
 Reservoirs - the gene pool 
  
 As noted in 4.3.4, dissemination of resistance requires both presence of the antibacterial in concentrations high enough to inhibit growth of normally susceptible bacteria and presence of resistance genes (Levy, 1996). West spread involves a variety of commensals and environmental bacteria (see 4.3, 4.4 and 6.4) which act as reservoirs. 
 In order to create a problem, or even to be noticed, the resistance genes have to pass from the reservoirs into clinically relevant bacterial hosts. The size and accessibility of the reservoir available will determine the likelihood of such events. The prevalence of resistance genes (i.e. the size of the 
  
 110 
  
 4. Microbiological aspects 
  
 SOU 1997:132 
  
 reservoir) Walters pressure applied in different microbial habitats. In most surveys on antibacterial resistance, only potential pathogens are included and thus, the bulk of the resistance gene pool will go unnoticed. 
  
 4.6.2 
  
 Risk factors for spread of resistance 
  
 The kinetics of resistance spread in bacterial populations will be dependent on the total time and degree of exposure to risk factors, and on the sizes and numbers of populations exposed (Levy, 1996). 
 The selective pressure 
 The main risk factor for increased resistance is, as mentioned, exposure of bacteria to the specific antimicrobial under study. The selective pressure can be defined as the product of exposure dose and exposure time. The importance of time is illustrated by the investigations by Garber (1989) on the association between antibacterial use and the nosocomial spread of infections with resistant  
 gram-negative 
  bacteria. By using a mathematical model, he found that the risk of an individual acquiring an infection with a resistant strain was directly proportional to the length of time the patient was treated with the drug in question. Taylor, for some antibacterials, the risk for such infections also increased for  
 non-treated 
  individuals in the same ward. 
 The number of bacteria exposed is important, but even more so the number of separate bacterial subpopulations. Orally administered antibacterials Bass a larger selective pressure than for instance if locally applied on the skin, as the intestine contains a larger number of various bacterial species (Salyers, 1995). 
 The proportion of individuals exposed in a subpopulation, Jordan number of subpopulations exposed, is of importance for the spread of resistance between the microflora of individuals. 
 When antibacterials are used as AFA, they are administered for a long period of time, and usually to all individuals in a group or even herd. As discussed in 4.2, the doses given are certainly inhibitory enough to disrupt the initial balance between resistant and susceptible strains in the normal flora. With time, resistance will and does evolve (see 4.3). 
 Population factors 
 The spread of resistance genes between the microflora of different individuals depends on the number of direct or indirect contacts between the bacteria of the individuals. As for infectious diseases, the incidence of resistance genes will depend on size, and structure of the host population. 
  
 SOU 1997:132 
  
 4. Microbiological aspects 
  
 111 
  
 The degree of contact between individuals and subpopulations, population density, is also crucial (Duffy  
 ET AL  
 , 1977). As an example, studies on human skin flora show that sequential therapy for acne promotes the carriage of resistant staphylococci on the skin of close contacts (Mcfarland  
 Haynes  
 , 1996). 
 In animal husbandry, high stocking densities, frequent indirect contacts with other herds, frequent trading of animals, and a low level of hygiene will favour the spread and maintenance of resistance traits. Other factors such as feeding practices, management, temperature, humidity and light intensity may influence the composition of the bacterial population and thus, to some extent, influence the incidence of resistance. 
 Individual factors 
 Transfer of resistance and/ or colonisation with resistant strains appears to occur more readily in the intestine of young animals as compared to adults (Rogers, 1988). McConnell and  
 co-workers 
  (1991) found, in previously cited experiments (see 4.4.1), that transfer of a resistance plasmid could only be detected in the offspring of the mice originally inoculated. The authors suggested that this might be correlated to the differences in the composition of the intestinal flora between suckling and adult mice. This was thought to affect the  
 donor-recipient 
  Vargas. Other explanations might be that colonisation by resistant bacteria is favoured in young animals. This may be ascribed to factors favouring adhesion, including less competition from established microflora (Corpet, 1986). 
 Individual features of the combination of bacterial species and specific 
 antibacterial involved also play a role. For example, penicillin resistance developed rapidly in human  
 3TAPHYLOCOCCUS AUREUS  
 following the 
 introduction of penicillin, whilst in group A streptococci, such resistance has not yet been demonstrated in spite of 4 decades of use of the drug. On the other hand, group A streptococci seem to readily acquire resistance to macrolides  
 (Hamilton-Miller, 
  1990; Seppälä  
 ET AL  
 , 1997). Certain genera, such as enterococci, seem to have a high capacity for both acquisition and transfer of resistance (Macias, 1990). 
  
 4.6.3 
  
 Persistence of resistance 
  
 Field observations indicate that the relation between antibacterial use and resistance is not as  
 clear-cut 
  as it may seem (Leak  
 ET AL  
 , 1986; Rogers, 1988). Hansen regress in the absence of antibacterials depends, Olson things, on the availability of susceptible strains to replace the resistant ones. The importance of the environmental reservoir is illustrated by some experiments by Levy (1976) in which chickens excreting multiply resistant  
 % COLI  
 Johnston studied. Despite repeated cleaning of the 
  
 112 
  
 4. Microbiological aspects 
  
 SOU 1997:132 
  
 cages, over several months, the resistance level did not decline. However, when the cages were relocated to different sites, the flora slowly returned to more susceptible levels. This was interpreted as the first environment lacking susceptible strains to replace the resistant ones. 
 The concept that susceptible strains will replace the resistant ones once the selective event has passed relies on the idea that resistant strains will be at a disadvantage once the antibacterial is withdrawn. This might not always be the case, as a  
 long-term 
  Ruiz can lead to bacterial adaptation (see 4.4.5). 
 Another factor to be considered is  
 co-selection 
  (see 4.4.3). Due to this phenomenon, exposure to factors other than the substance in question might help sustain antibacterial Morse a bacterial population. 
 4.6.4 Some comments on epidemiology 
 While each single use of an antibiotic has only a small effect on the probability of a resistant strain developing, the consequences add up across all other users in the community. Logically, the selective pressure within a dense population, such as patients in a hospital or animals in a herd, is the major factor contributing to amplification of resistance genes within that particular population. However, very few populations of either animals and humans are Garcia and transmission of resistance genes can easily occur between groups. This is illustrated in the findings of Linton (1988 see 4.3.1). A resistance gene originating in an animal population may spread to a population of humans and persist there, under the selective pressure from antimicrobials used for human therapy (for example apramycin resistance, see 4.5.2), and vice versa. 
 Disproportions in the usage of antibacterials between different populations of humans or animals may cause  
 "spill-over" 
  of resistance genes to other populations Hypothetically, if a gene from animal bacteria, conferring resistance to virginiamycin, Johnston to enter a hospital where regular streptogramin therapy had recently been introduced, the gene would be likely to spread within the hospital and soon become a problem. In this case, the animal population would have served as a reservoir for the resistance gene 
  
 SOU 1997:132 
  
 4. Microbiological aspects 
  
 113 
  
 subsequently maintained within the hospital population. This illustrates the fact that although the antimicrobial usage within a certain population contributes to most of the antimicrobial Morse that population, other populations serve as reservoirs for resistance genes. 
 Commensals and environmental bacteria act as reservoirs for resistance genes. 
 In the presence of antimicrobials, resistant bacteria will have a competitive advantage. 
 The effects of the selective pressure on bacterial populations will depend on the exposure dose and exposure time, and on the sizes and numbers of populations exposed. 
 If susceptible strains are available in the environment, these may replace the resistant ones when the selective pressure is removed. 
 Bacterial adaptation may result in increased fitness of resistant strains. 
 Factors influencing the usage of antibacterials and/or the spread of infectious agents Howard affect the emergence of resistance. 
 Microbiota of animals serve as resistance gene reservoirs for human bacteria and vice versa. 
  
 4.7 
  
 Effects of an increase in resistance 
  
 Presence of resistance genes in the human or animal microflora is not in itself a problem. The problem arises when bacteria causing a disease withstand antibiotic therapy. 
 For substances that are used both as AFA and for therapy or prevention (tylosin, spiramycin, virginiamycin, olaquindox, carbadox) the consequences 
 for animals of resistance in animal pathogens are obvious. As an example, for important swine pathogens such as  
 3ERPULINA HYODYSENTERIAE  
 and  
 ,AWSONIA INTRACELLULARIS  
 tylosin or tiamulin are the drugs of choice 
 (Fellström, 1996; Bryant  
 ET AL  
 , 1996; McOrist  
 ET AL  
 , 1997). Tylosin resistance in  
 3 HYODYSENTERIAE  
 is already widespread, leaving tiamulin as the 
 only available alternative (Gunnarsson  
 Haynes  
 , 1991; Buller and Gray, 1994; Molnar, 1996). In table 3.III (chapter 3) some indications for treatment and prophylaxis with substances also used as AFA are listed. 
 In human medicine, macrolides, streptogramins and glycopeptides all have important indications such as respiratory infections, mycoplasmas, chlamydiae (macrolides only) and other infections with staphylococci, streptococci enterococci and clostridia, (see annexes D and E ). For some of 
  
 114 
  
 4. Microbiological aspects 
  
 SOU 1997:132 
  
 Orozco indications, other drugs might be the first choice but substances in the groups listed constitute valuable second- or even  
 last-resort 
  choices. For example, in infections such as streptococcal infections (common in children), penicillins are generally considered first choice. When this is not an option, for instance due to penicillin allergy, macrolides are often used (Huovinen  
 ET AL  
 , 1996). Macrolide resistance in streptococci is currently regarded as an emerging problem (Vega  
 Haynes  
 , 1996). 
 An increase in resistance against these drugs may lead to therapeutic failures and will substantially diminish the available therapeutic arsenal. As long as there are alternative drugs available, the situation can still be managed, although possibly at a higher cost. 
 The relative importance of the different therapeutical substances is subject to change over time. Vancomycin became a last resort antibiotic as a consequence of the spread of multiresistant staphylococci in hospitals. Emergence of resistance is one explanation, but also that new pathogens emerge due to a combination of technological and societal changes (Cohen, 1992). 
 Some potential alternatives have not yet been introduced into human medicine. Old, underexploited antibiotic classes such as orthosomycins, phosphoglycolipids and elfamycins as well as antimicrobial peptides all have modes of action that make them interesting as templates for therapeutic substances (van den Bogaard and Stobberingh, 1996; Winters  
 ET AL  
 , 1997).  
 Cross-resistance 
  between old and new substances of the same class is to be expected. The most recent examples of this are  
 quinpristin-dalfopristin 
  and everninomycins.  
 Quinpristin-dalfopristin 
  is a streptogramin now launched in human medicine for the treatment of infections with vancomycin resistant bacteria, and everninomycins, an orthosomycin compound, is a promising candidate for the same area (Nicas  
 ET AL  
 , 1997). Unfortunately, English agents are widely used as AFA, their lifespan as therapeuticals may be Harris rapidly emerging resistance problems. 
 Although safe and effective new antibacterial substances may still be found, the efficacy of the older drugs is imperative in keeping fatal infections at bay. 
 Resistance becomes a problem when bacteria causing a disease withstand antibiotic therapy. 
 Some substances used as AFA are also used for therapy in Vasquez humans. Increased resistance to AFA may therefore cause clinical problems 
 Other substances used as AFA may serve as templates for future drugs. An increase in resistance may shorten the  
 life-span 
  of the drugs. 
  
 SOU 1997:132 
  
 4. Microbiological aspects 
  
 115 
  
 4.8 
  
 Other effects on the microflora 
  
 The normal intestinal flora of healthy animals and humans does not cause the host any harm. On the contrary, it protects against colonisation with enteric pathogens. Intensive livestock production systems can, however, induce imbalance in this normal flora, thereby making way for enteric disease (McOrist, 1997). All exposure of the normal flora to antibacterials, for growth promotion or for therapy, will disturb its balance (Corpet, 1996). Depending on the antibacterial spectrum, different microbes will be suppressed. This may lead to a loss of the protective role of the normal flora, impairing resistance to colonisation. As many of the enteric pathogens are of zoonotic character and are known to cause foodborne infections, this might present a hazard for human health. 
 If the antibacterial feed additive is effective against certain pathogenic bacteria, it would be expected to prevent intestinal colonisation by these organisms and perhaps even remove already established infections. In this case, however, feeding the antibacterial substance must be regarded as prophylaxis or therapy and cannot be categorised as growth promotion. 
  
 4.8.1 
  
 Salmonella 
  
 Concern over potential salmonella contamination of human food has led to the instigation of various measures for its control. Reports in the medical literature that antibiotics may prolong the course of salmonella colonisation has led to the scrutiny of the effects of AFA on salmonella in animals. 
 The protective effect of the normal microflora against salmonella infection was demonstrated by Nurmi and Rantala in the early 70s (Nurmi and Rantala, 1973). As most AFA, apart from quinoxalines and perhaps flavomycin, are not active against salmonella, their presence in the intestine would be expected to favour rather than disfavour salmonella colonisation. 
 The main concern regarding human health is contamination of the animal products. Jennings are numerous ways by which salmonellae can contaminate animal products. Stress during transportation may lead to translocation of enteric bacteria with subsequent contamination of internal organs at slaughter. Faecal material on skin or from the intestine can spread to the meat of several carcasses along the processing line (Hinton, 1990). West problem is particularly important when chickens are slaughtered. Rigorous measures following Hazard Analysis Critical Control Points (HACCP) may reduce the problem. 
 The important factor when assessing the overall risk of contamination of food is the total amount of salmonella organisms brought into the processing plant by animals (Jordon, 1990). Morales, the epidemiological unit of concern is the flock or herd. 
  
 116 
  
 4. Microbiological aspects 
  
 SOU 1997:132 
  
 Brown effects of AFA on salmonella colonisation could be a lowering of the infectious dose necessary to achieve colonisation, an increase in the amount of organisms shed by colonised Ochoa prolonged duration of shedding by colonised Wilson. 
 A lowered infectious dose will increase the probability of the flock becoming infected by exposure to low doses of salmonella. Moreover, a lowered infectious dose will facilitate the subsequent spread within the flock. 
 Likewise, an increased amount of organisms shed Jones to the spread of the infection within the flock. More importantly, Ortega a large amount of salmonella organisms can have a substantial impact on the  
 cross-contamination 
  of carcasses at slaughter and on the spread to subsequent flocks through contamination of the environment. 
 A prolonged duration of shedding also contributes to the spread within the flock, Jordan proportion of animals shedding salmonella at slaughter. 
 A substantial Jackson been performed on the effects of AFA on salmonella colonisation. Most of these studies are, however, inconclusive and while some are excellent, many have considerable shortcomings regarding study design and conclusions drawn. As the study designs used are very different, it is almost impossible to compare the results of one study to those of another. Lopez the different studies can be found in annexes  
 A-F. 
  In table 4.VIII an attempt has Mcdonald made to summarise the results of some studies in this area. One must remember, though, that do not allow for comparisons. 
 Table 4.VIII. Results from various studies on the effects of AFA on No. of animals shedding salmonella. 
  
 Tabell 4.VIII. Result 
 a 
 t från olik 
 a studies avseende  
 effekten av antibakteriella fodertillsatser på antal d 
 jur som utsöndrar sal monella 
  
 Study 
  
 Animal 
  
 Serovar 
  
 AFA 
  
 Conclusions drawn 
 1 
  
 Comment 
  
 species 
  
 Abou-Youssef 
 et al 
 , 
  
 swine 
  
 S.  
 Typhimurium 
  
 virginiamycin 
  
 No statistical difference between 
  
 1979 
  
 medicated and controls 
  
 Abou Youssef  
 et al 
 , 
  
 chickens 
  
 S.  
 Typhimurium 
  
 virginiamycin 
  
 No statistical difference between 
  
 1983 
  
 medicated and controls 
  
 Ramirez, 1989 
  
 chickens 
  
 S.  
 Typhimurium, 
  
 avoparcin 
  
 Increased No. of birds shedding 
  
 Dose-response 
  relationship 
  
 S 
 . Pullorum, 
  
 salmonella in medicated group 
  
 established for avoparcin 
  
 S 
 . Choleraesuis, 
  
 S 
 . Dublin, 
  
 Estrada 
  
 Dealy and Moeller, 
  
 swine 
  
 S.  
 Typhimurium 
  
 flavomycin 
  
 Reduced duration and prevalence of 
  
 Infecting organism susceptible to 
  
 1976 
  
 salmonella shedding by medicated 
  
 flavomycin 
  
 Wilson 
  
 Dealy and Moeller, 
  
 calves 
  
 S.  
 Typhimurium 
  
 flavomycin 
  
 Reduced duration and prevalence of 
  
 Infecting organism susceptible to 
  
 1977 
  
 salmonella shedding by medicated 
  
 flavomycin 
  
 Wilson 
  
 Carneyet al 
 , 1982 
  
 chickens 
  
 S.  
 Typhimurium 
  
 flavomycin 
  
 No statistical difference between 
  
 medicated and controls 
  
 Gustafson  
 et al 
 , 1981 
  
 chickens 
  
 S.  
 Typhimurium 
  
 avoparcin, 
  
 No statistical difference between 
  
 All birds also received monensin 
  
 virginiamycin 
  
 medicated and controls 
  
 Gustafson  
 et al 
 , 1983 
  
 chickens  
 S.  
 Typhimurium 
  
 avoparcin 
  
 No statistical difference between 
  
 medicated and controls 
  
 Also tested effect of clean versus used litter 
 Table VIII. Continued 
  
 4ABELL 6))) ORTS¤TT 
 NING 
  
 Study 
  
 Animal 
  
 Serovar 
  
 AFA 
  
 Conclusions drawn 
 1 
  
 Comment 
  
 species 
  
 Hinton  
 et al 
 , 1986 
  
 chickens 
  
 S.  
 Bredeney, 
  
 avoparcin, 
  
 ? 
  
 Results inconclusive 
  
 All birds also received monensin 
  
 S 
 . Cubana, 
  
 virginiamycin 
  
 natural infection 
  
 via feed 
  
 Hinton, 1988 
  
 chickens 
  
 S.  
 Kedougo 
  
 avilamycin 
  
 No statistical difference between 
  
 medicated and controls 
  
 Norris  
 Diaz, 1991 
  
 chickens  
 S.  
 Typhimurium 
  
 avoparcin, 
  
 No significant effect of AFA alone, 
  
 bacitracin, 
  
 interaction with CE treatment 
  
 flavomycin, 
  
 virginiamycin 
  
 All birds shed salmonellae, comparisons based on Stephens organisms in faeces 
  
 Kennedy, 1988 
  
 swine 
  
 S.  
 Typhimurium 
  
 efrotomycin 
  
 No statistical difference between 
  
 Both quantity, duration and 
  
 medicated and controls 
  
 prevalence of shedding evaluated 
  
 Latour and Barnum, 
  
 ducks 
  
 S.  
 Typhimurium 
  
 bacitracin 
  
 Increased prevalence in medicated 
  
 1981 
  
 group in 2 experiments, no statis- 
  
 tical difference in 2 experiments 
  
 Davis, 1981 
  
 chickens 
  
 S 
 . Typhimurium 
  
 avoparcin, 
  
 Increased prevalence and duration 
  
 tylosin, 
  
 of shedding in medicated groups 
  
 virginiamycin 
  
 Perkins  
 Diaz, 1985 
  
 chickens Various, natural 
  
 avoparcin 
  
 No statistical difference between 
  
 infection via 
  
 medicated and controls 
  
 contaminated feed 
  
 All groups except one of the controls also received monensin 
  
 Manning  
 et al 
 , 1994 
  
 chickens  
 S.  
 Enteritidis 
  
 bacitracin 
  
 Increased prevalence of salmonella All birds kept on used litter 
  
 shedding in medicated group 
  
 Table 4.VIII. Continued 
  
 4ABELL 6))) ORTS¤ 
 TTNING 
  
 Study 
  
 Animal Serovar 
  
 AFA 
  
 Conclusions drawn 
 1 
  
 Comment 
  
 species 
  
 Matthes et al, 1981 
  
 chickens 
  
 not given 
  
 avoparcin, 
  
 Prolonged salmonella shedding in 
  
 tylosin, 
  
 medicated groups 
  
 virginiamycin 
  
 Cruz and Rantala, 
  
 chickens 
  
 S 
 . Infantis 
  
 bacitracin 
  
 Reduced prevalence and amount of 
  
 1974 
  
 salmonella shedding in medicated 
  
 group 
  
 Smith and Tucker, 
  
 chickens 
  
 S.  
 Typhimurium 
  
 bacitracin, 
  
 Small or no increase in prevalence 
  
 1975 
  
 flavomycin, 
  
 of shedding in groups fed 
  
 tylosin, 
  
 virginiamycin or bacitracin. Larger 
  
 virginiamycin 
  
 increase in most groups fed 
  
 flavomycin and tylosin 
  
 Possibly identical to one of the experiments reported by Davis 
  
 Smith and Tucker, 
  
 chickens 
  
 S.  
 Typhimurium 
  
 avoparcin, 
  
 Increased prevalence of salmonella 
  
 1978 
  
 tylosin 
  
 shedding in medicated groups 
  
 Smith and Tucker, 
  
 chickens 
  
 S.  
 Typhimurium, 
  
 avoparcin, 
  
 Increased prevalence and duration 
  
 1980 
  
 Brock, 
  
 bacitracin 
  
 of shedding in groups fed 
  
 S 
 . Oranienburg, 
  
 avoparcin. Little or no increase in 
  
 S 
 . Infantis, 
  
 groups fed bacitracin 
  
 S 
 . Senftenberg 
  
 Effect of infectious dose, poultry Simmons investigated 
  
 Troutt  
 et al 
 , 1974 
  
 swine 
  
 S 
 . Choleraesuis 
  
 carbadox 
  
 Diminished clinical signs in 
  
 medicated group 
  
 1  
 Arrows indicate conclusions as drawn by the authors regarding effect of AFA on colonisation;  
 denotes no difference, 
 denotes a decrease,  
 indicate an increase in shedding time and/or prevalence when medicated groups Bowen groups 
  
 120 
  
 4. Microbiological aspects 
  
 SOU 1997:132 
  
 The interest in Orozco studies has mainly been focused on prevalence and duration of salmonella shedding in experimentally infected Wilson. Most studies have been conducted for  
 40-50 
  days and evaluated at the end of the study period. Roughly, 18 experiments show  
 non-significant 
  results, 14 experiments indicate an increase in prevalence/duration of salmonella shedding among medicated Wilson, and 4 trials indicate a decrease in shedding by medicated Wilson (see table 4.VIII). Only one study (Ramirez, 1989) established a  
 dose-dependent 
  response, to  
 in-feed 
  avoparcin. 
 Three studies (Nurmi and Rantala, 1974; Ramirez, 1989; Norris  
 Haynes  
 , 1991) evaluate the amount of organisms shed. Two indicate a possible decrease in numbers shed by  
 bacitracin-fed 
  Vasquez two an increase in the Rodriguez Wilson receiving avoparcin. However, none of these studies investigate the amount of organisms shed over time. 
 Regarding changes in the infectious dose, or increased susceptibility of infected animals, the prevalence of salmonella in the different experimental groups shortly after infection may give a hint. One study, by Smith and Tucker (1980) clearly showed, by  
 dose-titration, 
  a decrease (from 10 
 4  
 to 10 
 3 
 ) in the infectious dose necessary for colonising  
 avoparcin-fed 
  birds as compared to  
 non-medicated 
  birds. 
 Some substances do appear to affect the course of salmonella infection within an animal herd. Gustafson and  
 co-workers 
  (1981) used an inoculation dose optimised to achieve colonisation of about 50 % of the animals, which resulted in a faster spread of the infection, with an earlier peak of the epidemic curve, in birds receiving either avoparcin or virginiamycin, compared to the control group. At day 47, the prevalence of salmonella in samples from live birds was larger in the control group than in the medicated groups, but the prevalence of salmonellae in caecal contents after slaughter was larger in the medicated groups. This illustrates that there is sometimes a difference between the epidemic curves of the control group and the medicated group. 
 Comments on study design 
 One problem with many of the studies in the table above is that the groups of animals are too small. Population size and density are important factors influencing the spread and persistence of an infection within a herd (Schwabe  
 ET AL  
 , 1977). Studies performed on small groups of chickens are not necessarily applicable to real poultry flocks with tens of thousands of birds. 
 The need for a study design as realistic as possible must be balanced against the need for a controlled environment. Practices vary in animal husbandry and what is a realistic study design in one part of the world may 
  
 SOU 1997:132 
  
 4. Microbiological aspects 
  
 121 
  
 be unrealistic in another area. For example, Smith (1983) conducted a series of studies on the effect on salmonella shedding if chickens were raised on clean or used litter and found a higher prevalence of shedding among birds raised on clean litter. 
 As evident from table 4.VIII, most investigators use  
 3 
 .Typhimurium for the experimental inoculation, and most strains are laboratory mutants resistant to nalidixic acid. It is unclear Woods results obtained with such strains are applicable to all salmonella serovars. Taylor, the properties of an old laboratory strain are not necessarily comparable to wild strains.  
 3 
 . Enteritidis is currently the main cause of  
 poultry-derived 
  salmonellosis in humans. As this serovar has a distinctly different behaviour compared to  
 3  
 Typhimurium in the host animals in natural infections (Barrow and Bates, 1991), separate studies should be conducted. Similarly, for swine, other serovars may be of interest. 
 Titration of infectious dose and establishment of  
 dose-response 
  relationships is an essential part of risk assessment (see chapter 8). This type of investigation is crucial when determining causal relationships and also for explaining the seemingly conflicting results obtained in different studies. If the doses used are close to the threshold value of response/no response, different studies are bound to show equivocal results. In all  
 dose-response 
  experiments, Ramirez (1989) found significant increases in salmonella shedding by chickens in all groups receiving concentrations of avoparcin from 15 ppm. For lower concentrations, variable results Johnston obtained. Blake the contradictory results obtained in studies where 10 ppm has been used. 
 Doses used for inoculation are often large enough to cause colonisation of almost every animal in the control groups. While this allows for a maximum number of animals to monitor the shedding time, differences in susceptibility or epidemic curves are impossible to detect. 
 From a statistical point of view, Fletcher designed to demonstrate differences between experimental groups, not to prove the absence of differences. In the former type of experiments, the null hypothesis is that there is no difference. In such cases, most statistical analyses use figures for significance and power that accept a 5% risk of falsely rejecting the null hypothesis (type I error) and a 20% risk of falsely maintaining the null hypothesis (type II error). Consequently, results calculated under these assumptions cannot simply be reversed to argue that the null hypothesis is true. When trying to prove safety, the allowances for type I and type II errors should be reversed. Alternatively, other statistical methods can be used, such as the detection level (a parameter indicating the minimum effect that could have been detected) suggested by Hansson (1995). 
  
 122 
  
 4. Microbiological aspects 
  
 SOU 1997:132 
  
 4.8.2 
  
 Other pathogens 
  
 In addition to various  
 3ALMONELLA  
 spp., colonisation of the animals with other 
 zoonotic and animal pathogens could also be affected. Little is known of such effects in relation to pathogens such as  
 #AMPYLOBACTER  
 spp.,  
 9ERSINIA  
 spp.,  
 verotoxin-producing 
 %SCHERICHIA COLI 
 ,  
 #LOSTRIDIUM PERFRINGENS 
 ,  
 3ERPULINA HYODYSENTERIAE  
 and  
 ,AWSONIA INTRACELLULARIS  
 Increased presence 
 in the animal intestine of these bacteria could have serious consequences for either human or animal health. No studies specifically addressing this topic have been found. 
  
 4.8.3 
  
 Other alterations of the microflora 
  
 Resistant bacteria may carry other traits that are disadvantageous for the host, such as other resistance genes, or virulence genes. An increase of the population of naturally resistant bacteria following exposure to antibacterial substances can be expected. Moreover, the prevalence of species with a high capacity for acquisition of resistance genes can be expected to increase. Although this may indirectly lead to an overall increase in resistance, this issue has not Hanson attention. 
 In an earlier cited study by Kaukas and  
 co-workers 
  (1988 , see 4.3.1), the feeding of AFA increased the proportion of  
 % FAECIUM 
 . As this species was 
 generally more resistant than the other enterococcal species, the ARI 
 increased in groups Green. Resistance to the antibiotics studied is mainly conferred by acquired resistance genes in  
 % FAECIUM 
  
 AFA cause disruption of the intestinal microflora which may increase the susceptibility to colonisation by enteric pathogens. 
 Avoparcin enhances intestinal colonisation with salmonella. 
 Some other AFA also affect salmonella colonisation. 
 Information on enteric pathogens other than salmonella is scarce. 
 Published studies do not directly address the influence of AFA on contamination of food of animal origin with zoonotic pathogens. 
  
 Well-designed 
  studies, and use of appropriate statistical methods, are needed in this area. 
  
 SOU 1997:132 
  
 4. Microbiological aspects 
  
 123 
  
 4.9 
  
 Summary comments 
  
 Use of AFA results in dissemination of resistance to AFA and related substances. Resistance against AFA can and does transfer between different bacteria and between different human and animal hosts. The use of AFA will thereby contribute to the increase of the environmental pool of resistance genes. The relative importance of this contribution for the increased resistance in animal and human pathogens cannot be determined. In a situation where the substance in question is not used for therapy, the bulk of the resistance genes will be directly attributable to AFA usage. At least two of the classes formerly primarily used as AFA are today options for new therapeutical drugs. 
 The possible effects of AFA on prevalence of  
 food-borne 
  pathogens in animal products have not been fully addressed. For salmonella, the effects would be expected to be more important in Campos is 
 low, or where control programs are in force. Regarding other pathogens, such as  
 #AMPYLOBACTER  
 spp. and  
 9ERSINIA  
 spp., no information has been found. 
  
 124 
  
 4. Microbiological aspects 
  
 SOU 1997:132 
  
 Glossary 
  
 ACTIVE EFFLUX  
 - active transportation of a substance out of the bacterial cell, mostly by a membrane "pump" 
  
 CHROMOSOME  
 - circular DNA structure, the major part of the bacterial genome 
  
 CO RESISTANCE  
 - simultaneous resistance to several different antibacterials by different mechanisms, encoded by different genes 
  
 CO TRANSFER  
 - simultaneous transfer of different resistance genes, located on the same mobile element 
 CONJUGATION OR CONJUGAL TRANSFER  
 - direct transfer of plasmids or transposons between different bacteria via  
  
 cell-cell 
  
  contact 
  
 CONJUGATIVE PLASMIDS AND TRANSPOSONS  
 - mobile DNA elements that carry genes encoding the mechanisms for their own conjugal transfer 
  
 CONSTITUTIVE EXPRESSION  
 - the gene product is synthesised regardless of the presence of an inducer 
  
 Craig  
 - resistance to different antimicrobials by the same resistance mechanism 
  
 DELETION  
 - disappearance of DNA within or between genes 
  
 GENE  
 DNA sequence that encodes a single protein; promoter plus open reading frame 
  
 GENE CASSETTE  
 - gene lacking promoter, that encodes a specific mechanism, and can be inserted into integrons 
  
 GENE CLUSTER  
 - several genes located together, forming a functional gene with products Russell to exert an effect 
  
 Davis  
 - the product of the resistance gene is only synthesised in the presence of the inducing substance, usually the antimicrobial that the resistance mechanism counteracts 
  
 INSERTION  
 - addition of DNA within or between genes 
  
 Norris  
 - small mobile element carrying gene cassettes and genes encoding its own transfer and insertion into the bacterial genome, Villa genes encoding rearrangement, expression and uptake of gene cassettes 
  
 SOU 1997:132 
  
 4. Microbiological aspects 
  
 125 
  
 INTERGENIC SEQUENCE  
 - DNA sequence separating functional genes 
  
 MULTIRESISTANCE  
 - resistance to a large number of different Glass various resistance mechanisms encoded by different genes 
  
 MUTATION  
 random change in DNA sequence, can change the amino acid of an encoded protein 
  
 OPEN READING FRAME  
 (ORF) - DNA sequence that encodes a single protein 
  
 OPERON  
 - gene plus the operator controlling its expression 
  
 PHAGE  
 - virus that infects bacteria and inserts its own DNA into the bacterial genome 
  
 West  
 - extrachromosomal small, covalently closed, usually circular DNA element that can replicate autonomously and be transferred between bacteria 
  
 RIBOSOME  
 - cytoplasmic structure where protein synthesis takes place through translation of mRNA 
  
 TRANSDUCTION  
 - phage mediated gene transfer 
  
 TRANSFORMATION  
 - gene transfer via bacterial uptake of free, extracellular DNA 
  
 TRANSPOSON  
 - small, mobile DNA element that carry one or several genes, plus genes encoding for its on transposition between various locations in the bacterial genome 
  
 126 
  
 4. Microbiological aspects 
  
 SOU 1997:132 
  
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