Antimicrobial susceptibility of bacterial isolates from 233 horses with musculoskeletal infection during 1979–1989
Authors: MOORE R. M. , SCHNEIDER R. K. , KOWALSKI J. , BRAMLAGE L. R. , MECKLENBURG LINDA M. , KOHN CATHERINE W. 
Published: April 2010

Bacterial culture and susceptibility results were analysed from 233 horses with septic arthritis/tenosynovitis or osteomyelitis that developed after fracture repair. Antibiotics were deemed highly effective, effective or ineffective if ≤85%, 70–84.9% or <70% of the isolates were susceptible respectively. In total, 424 bacterial types were isolated; 386 were aerobic or facultative and 38 were anaerobic. Enterobacteriaceae (28.8%) were the most common bacterial group isolated, followed by non‐beta‐haemolytic streptococci (13.0%), coagulase‐positive staphylococci (11.8%), beta‐haemolytic streptococci (9.4%), and coagulase‐negative staphylococci (73%). The remainder of the organisms were other Gram‐negative (15.8%), other Gram‐positive (2.3%) and miscellaneous (2.6%) bacteria. Penicillin and ampicillin were highly effective against beta‐haemolytic streptococci, but were ineffective against other bacteria. Ampicillin was no more effective than penicillin against most bacteria. Amikacin was the most effective antibiotic against the wide range of bacteria isolated in this study. Amikacin was highly effective against coagulase‐positive staphylococci, Enterobacteriaceae andPseudomonasand was effective against coagulase‐negative staphylococci andActinobacillus. Gentamycin was not highly effective against any bacterial group; but was effective against coagulase‐positive and negative staphylococci,Pseudomonas, SalmonellaandActinobacillus. Kanamycin was ineffective against all bacteria with the exception ofActinobacillus. Cephalothin was highly effective against beta‐haemolytic streptococci, coagulase‐positive staphylococci andActinobacillusand was effective against coagulase‐negative staphylococci. Cephalothin is a very useful antibiotic for Gram‐positive organisms, especially staphylococci, which are an important cause of iatrogenic infections. Trimethroprim‐sulphonamides were highly effective againstActinobacillusand were effective against coagulase‐positive staphylococci, beta‐haemolytic streptococci, non‐beta‐haemolytic streptococci andRhodococcus equi. Due to the narrow spectrum of activity of trimethoprim‐sulphonamides against the common pathogenic organisms, these drugs should be used only after culture and susceptibility results are known. Erythromycin was highly effective against coagulase‐positive staphylococci, beta‐haemolytic streptococci andRhodococcus equiand was effective against coagulase‐negative staphylococci. Chloramphenicol was effective against a wide range of organisms, but it cannot be recommended because of the human health hazard associated with handling this drug. Other less commonly used antibiotics were effective against some types of bacteria.

The combination of a cephalosporin and amikacin provides the best coverage against the bacteria isolated in this study. These are the antibiotics that should be considered when treating horses with musculoskeletal infection before culture and susceptibility results are known. However, these drugs are not recommended for prophylaxis in routine elective orthopaedic surgery. Many factors affect the selection of an antibiotic, including costs, ease of administration, the source of the infection, and the location of the infection. These factors, considered with the results of this study, should assist clinicians in making rational decisions regarding antibiotic therapy for horses with musculoskeletal sepsis.