sou · GLB3132d1 · 1998-01-01 00:00:00 — Antibiotic-resistant gram- positive bacteria have also been isolated from various foods (Roosen ET AL , 1989; AL , 1994; Perreten and Teuber, 1995; DANMAP, 1997; Perreten ET AL , 1997; , 1997). transferable resistance genes, including VANA-genes in enterococci and ERM- genes in ,ISTERIA MONOCYTOGENES, have been found in gram-positive bacteria isolated from food (, 1994; Aarestrup, 1995; , 1995a; Perreten and Teuber, 1995; , 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 106-109 (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 been demonstrated in water, soil, on kitchen towels, on cutting boards, and on the surface of food (and Sørum, 1994). 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 animals 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. 4.5.4Likelihood of resistance being transferred from animals to man It has been proposed that both types of transfer discussed above are extremely rare events (Knothe, 1977; Lacey, 1980; Lacey, 1984; Lacey, 1988; , 1993). The basis for this opinion is the rarity of isolation of the same bacteria from animals and man (Lacey, 1988; , 1993), the rarity of gene transfer IN 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 (Knothe, 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 indeed rather host specific, and transfer frequencies between staphylococci are, compared to other genera, rather low irrespective of host origin. Nevertheless, it is doubtful whether the finding of a low number of animal isolates capable of transferring resistance to human isolates, 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 amount of bacterial 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. isolated from humans 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 isolates 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 in pigs. As the main animal host of # JEJUNI is poultry, prevalence of macrolide resistance in this bacterial species provides information about the effects of macrolide use in pigs. In # COLI, which is mainly found in pigs, a high prevalence of macrolide resistance has been reported (, 1984; Burridge ET AL , 1986; Jacobs-Reitma ET AL , 1994; DANMAP, 1997). This has also been shown in # COLI isolated from humans (, 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. 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. 4.6Epidemiology of antibacterial resistance The causal relationship between use of antibacterials and resistance has been shown both for human and animal bacteria (among others , 1972; , 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 will also 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 and commerce, microbial adaptation and change and breakdown of public health measures (, 1996). Corresponding factors are known to influence epidemiology of contagious diseases in animals. Apart from the two latter, these factors all influence the degree of contact between individuals, either by affecting the population density or by providing new contact routes. This emphasises the similarity between spread of resistance and spread of infectious diseases. 4.6.1Reservoirs - 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 (, 1996). This 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 reservoir) will primarily depend on the selective 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. 4.6.2Risk 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 (, 1996). 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. Further, 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 will exert 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, as well as the number of subpopulations exposed, is of importance for the spread of resistance between the microflora of individuals. When antibacterials are used as , 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). 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. The degree of contact between individuals and subpopulations, population density, is also crucial (, 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 (, 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. Transfer of resistance and/ or colonisation with resistant strains appears to occur more readily in the intestine of young animals as compared to adults (Langlois, 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 ratio. 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 (, 1990; Seppälä ET AL , 1997). Certain genera, such as enterococci, seem to have a high capacity for both acquisition and transfer of resistance (, 1990). Field observations indicate that the relation between antibacterial use and resistance is not as clear-cut as it may seem (Leak ET AL , 1986; Langlois, 1988). Whether resistance would regress in the absence of antibacterials depends, among other things, on the availability of susceptible strains to replace the resistant ones. The importance of the environmental reservoir is illustrated by some experiments by (1976) in which chickens excreting multiply resistant % COLI were studied. Despite repeated cleaning of the 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 selective pressure can lead to bacterial adaptation (see 4.4.5). Another factor to be considered is co-selection (see 4.4.3). to this phenomenon, exposure to factors other than the substance in question might help sustain antibacterial resistance within 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 isolated from others and transmission of resistance genes can easily occur between groups. This is illustrated in the findings of (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, were 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 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 resistance within 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 will also affect the emergence of resistance. Microbiota of animals serve as resistance gene reservoirs for human bacteria and vice versa. 4.7Effects 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 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; McOrist ET AL , 1996; McOrist ET AL , 1997). Tylosin resistance in 3 HYODYSENTERIAE is already widespread, leaving tiamulin as the only available alternative (, 1991; Buller and Hampson, 1994; Molnar, 1996). In table 3.III (chapter 3) some indications for treatment and prophylaxis with substances also used as 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 ). these 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 (Coranaglia ET AL , 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 (, 1992). Some potential alternatives have not yet been introduced into human medicine. , underexploited antibiotic 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; Chopra 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, as these agents are widely used as , their lifespan as therapeuticals may be shortened by rapidly emerging resistance problems. Although safe and effective new antibacterial substances may still be found, the efficacy of the older drugs is imperative in fatal infections at bay. Resistance becomes a problem when bacteria causing a disease withstand antibiotic therapy. Some substances used as are also used for therapy in animals and humans. Increased resistance to may therefore cause clinical problems Other substances used as may serve as templates for future drugs. An increase in resistance may shorten the life-span of the drugs. 4.8Other 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. Concern over potential salmonella contamination of human food has led to the instigation of various measures for its control. Reports in the literature that antibiotics may prolong the course of salmonella colonisation has led to the scrutiny of the effects of 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 , apart from quinoxalines and perhaps flavomycin, are not 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. There 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 (Lillard, 1990). This 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). Thus, the epidemiological unit of concern is the flock or herd. Theoretical effects of 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 animals or a prolonged duration of shedding by colonised animals. 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 will contribute to the spread of the infection within the flock. importantly, animals shedding 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, as well as the proportion of animals shedding salmonella at slaughter. A substantial amount of studies have been performed on the effects of 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. Further comments on the different studies can be found in annexes A-F. In table 4.VIII an attempt has been made to summarise the results of some studies in this area. One must remember, though, that do not allow for comparisons. Results from various studies on the effects of on No. of animals shedding salmonella. Resultat från olika studies avseende effekten av antibakteriella fodertillsatser på antal djur som utsöndrar sal monella No statistical difference between No statistical difference between salmonella in medicated group Reduced duration and prevalence of Infecting organism susceptible to salmonella shedding by medicated Reduced duration and prevalence of Infecting organism susceptible to salmonella shedding by medicated et al, 1982 No statistical difference between et al, 1981 No statistical difference between All birds also received monensin et al, 1983 No statistical difference between Also tested effect of clean versus used 4ABELL 6))) &ORTS¤TTNING Hinton et al, 1986 All birds also received monensin No statistical difference between et al, 1991 No significant effect of alone, interaction with CE treatment All birds shed salmonellae, comparisons based on amount of organisms in faeces Jacks et al, 1988 No statistical difference between Both quantity, duration and prevalence of shedding evaluated