Epidemiology

The epidemiology of S. aureus is poorly understood. The anterior nares are the main site of S. aureus colonization in horses, although the gastrointestinal tract can also be involved. Methicillin-resistant S. aureus reside in the nasal passages of approximately 10% of healthy horses. Methicillin-resistant S. aureus appear to be circulating in the horse population in a reservoir of colonized horses. Colonization rates likely vary tremendously among different horse populations and geographic regions, and rates of nasal carriage of 0% to 5% have been reported in different regions, with the prevalence on certain farms approaching 50%. It appears that, although MRSA colonization is usually transient in horses, MRSA is readily transmitted between horses and may be continually circulating in the horse population. Although most colonized horses do not develop clinical MRSA infections, colonization is a risk factor for development of clinical infection in horses that are hospitalized. Colonized horses also pose a risk to other horses and humans.

Methicillin-resistant S. aureus infections can arise sporadically or in outbreaks on farms and in veterinary hospitals. All ages of horses can be infected, including foals as young as 24 hours of age. Age, breed, or sex predispositions have not been reported. Outbreaks appear to be more common on breeding farms, although objective evidence of increased risk is currently lacking. Few proper risk factor analyses have been reported for horses. Previous colonization of the horse, previous identification of colonized horses on the farm, antimicrobial administration within 30 days, admission to the neonatal intensive care unit, and admission to a service other than the surgical service are risk factors for community-associated colonization in horses admitted to a veterinary teaching hospital. Administration of cephalosporins or aminoglycosides during hospitalization is associated with MRSA acquisition in hospitalized horses.

Molecular Epidemiology

In humans a variety of “epidemic clones” account for most MRSA infections. In horses, the clone designated USA500 or Canadian epidemic MRSA-5 (CMRSA-5) appears to predominate (Figure 37-1). This is a human clone, but it is responsible for only a small percentage of human infections and appears to survive preferentially in horses compared with other MRSA clones. This clone, or closely related clones, has been found in horses in North America and various countries in Europe and is likely disseminated widely in the horse population. Infections can be caused by other strains but this is less common. Recently, sequence type 398 (ST398) MRSA has been identified in horses in Europe and Canada and may be an emerging problem.

Figure 37-1 Severe S. aureus skin and soft tissue infection in the forelimb of a horse.

Clinical Presentation

Many manifestations of infection may be encountered, which is not surprising for an opportunistic pathogen such as S. aureus. Joint, incision, and skin or soft tissue infections are most common in community-onset cases, but multiple other types of infections have been reported, including pneumonia, metritis, omphalophlebitis, sinusitis, osteomyelitis, tenosynovitis, and mastitis. Surgical-site infections are most common in hospital settings, but other types of infections, such as joint infections, bacteremia, septicemia, invasive device infections, and other soft tissue infections, have also been reported. The severity of disease can vary ranging from mild and superficial infection to aggressive and life-threatening infection. No clinical findings are specific for the likelihood of MRSA infection compared with infection by methicillin-susceptible S. aureus or other pathogens.

Diagnosis

Diagnosis of MRSA infection involves isolation of S. aureus from an infected site and determination of methicillin resistance. In vitro susceptibility to oxacillin is often used as a marker of methicillin resistance because oxacillin is more stable in vitro. Oxacillin-resistant strains are considered to be methicillin-resistant, but both false-positive and false-negative results can be obtained. False-positive results can occur occasionally from strains that produce high levels of β-lactamase, which can result in low-level oxacillin resistance. Such strains are often identifiable by reviewing the rest of the susceptibility panel, as they may be susceptible to cephalosporins or penicillin–anti-β-lactamase combinations such as amoxicillin-clavulanic acid. Any suspected MRSA isolates that are reported as susceptible to any β-lactam or β-lactam–β-lactamase inhibitor combination should be tested further. False-negative results can be obtained from incomplete expression of mecA, because oxacillin can be a poor inducer of mecA in vitro. Other drugs, such as cefoxitin, are being used more widely for in vitro sensitivity testing because of their greater ability to induce this resistance phenotype. Questionable isolates should be tested for the mecA gene by polymerase chain reaction (PCR) assay or for PBP2a by latex agglutination test. Ideally this would be done routinely. Veterinarians should try to ensure that the laboratory they use tests all S. aureus isolates for methicillin resistance by an accepted method.

In some situations, screening of clinically normal horses for MRSA colonization may be indicated. At present, culture of nasal swabs appears to be the optimal method. Samples should be collected by inserting a nasal swab 10 to 15 cm into the nose and allowing the swab to contact the mucosa upon withdrawal. Several types of culture media are available for selective and differential detection of MRSA. The use of broth enrichment increases the sensitivity of testing. The main disadvantage of using enrichment culture is a further 24-hour delay before results are available. In humans real-time PCR testing of nasal swab specimens is used for rapid identification of MRSA carriers; however, evaluation of a commercial assay in horses revealed a high number of presumed false positives and nondefinitive or unresolved results.

Treatment

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