Prevalence of colonization and antimicrobial resistance among coagulase positive staphylococci in dogs, and the relatedness of canine and human Staphylococcus aureus

Abstract

Coagulase positive staphylococci, Staphylococcus aureus and Staphylococcus pseudintermedius, are important causes of infection in human beings and dogs respectively. The rapid increase in the incidence of methicillin resistant S. aureus (MRSA) in people and its emergence in dogs has raised the profile of this organism in the veterinary community. Similarly, human S. pseudintermedius infections have also been recognized as the awareness of bidirectional human-dog transmission increases. Antimicrobial resistance has been complicating the treatment of S. aureus infections since the first penicillin resistance was observed in the 1940s. Methicillin resistance (resistance to the majority of â-lactams), is particularly troublesome as the â-lactams are a safe and effective class of antimicrobials for treating susceptible staphylococcal infections in both human beings and dogs. Additionally, resistance to other antimicrobial classes such as the macrolides, tetracyclines, sulfonamides and chloramphenicol, further complicates the treatment of staphylococcal infections. Particularly in small animal private practice, infections are often treated empirically, requiring knowledge of locally prevalent susceptibility patterns. The emergence of resistance to commonly used drugs necessitates surveillance to monitor the dissemination of resistance, and to guide antimicrobial therapy. In the last decade there have been many studies attempting to address gaps in our knowledge of the ecology of S. aureus and S. pseudintermedius in dogs. In particular, the prevalence of colonization with methicillin resistant staphylococci has been documented in different dog populations. However, failing to sample all relevant sites of colonization, may have decreased the sensitivity of these studies. The sites where coagulase positive staphylococci colonize dogs have not been systematically evaluated. The clinical and infection control implications of S. aureus infections, or colonization in the case of MRSA, requires timely laboratory identification. The tube coagulase test is arguably the most important tool used for identifying of staphylococcal species. Studies dating from the 1970s and 1980s suggested that the use of rabbit plasma, which is the current standard, may not be the ideal media for all situations and that different plasmas may need to be considered in different diagnostic situations. In this thesis, the ecology of coagulase positive staphylococci in dogs was studied from start to finish including sample collection, bacterial identification, antimicrobial susceptibility testing and molecular epidemiological investigations. This thesis will serve as a template to be used for follow up studies or by investigators setting up a surveillance program in their region. We found that multiple sites of colonization (nares, pharynx and rectum), are involved in both S. aureus and S. pseudintermedius carriage in dogs. Single site colonized dogs were identified, suggesting that maximal screening sensitivity requires sampling multiple body sites. When canine and rabbit plasma were compared, the time until clot formation was found to be significantly shorter with canine plasma. Although, the availability of canine plasma may limit its use in the diagnostic laboratory, investigators should be aware that rabbit plasma may not be ideal for all applications of the tube coagulase test. Antimicrobial susceptibility testing of canine S. aureus and S. pseudintermedius and human S. aureus isolates was done. Consistent with previous reports from Saskatoon, the S. pseudintermedius isolates were found to be overwhelmingly susceptible: pan-susceptibility was the most common phenotype identified. Antimicrobial resistance was more common among S. aureus than S. pseudintermedius including resistance to drugs which all S. pseudintermedius were susceptible to. No resistance to vancomycin, linezolid, daptomycin or quinupristin/dalfopristin was found. All isolates remained susceptible to at least one of tetracycline, clindamycin, chloramphenicol or trimethoprim/sulfamethoxazole which are often used for treating infections caused by multidrug resistant staphylococci. Finally, DNA fingerprinting revealed that the canine and human S. aureus isolates tested did not belong to mutually exclusive populations. Using AFLP, IS-typing and spa typing, many human and canine isolates were indistinguishable suggesting a common population, supporting the hypothesis that interspecies transmission occurs. The complex and under-characterized ecology of S. aureus and S. pseudintermedius requires more study so that risk factors for infection can be defined and effective infection control measures implemented. Because multiple species are involved, collaboration between veterinarians and human health professionals is imperative, and will no doubt yield the most success in our efforts to understand these potential pathogens.