MRSA in horses (Proceedings)


Methicillin Resistant Staphylococcus aureus (MRSA) was first described in the human health care setting in the 1960's. Infections were first described in otherwise healthy people in the 1980's. Disease severity in humans ranges from asymptomatic colonization of the upper respiratory tract and skin, to clinical disease with varying severity.

Methicillin Resistant Staphylococcus aureus (MRSA) was first described in the human health care setting in the 1960's. Infections were first described in otherwise healthy people in the 1980's. Disease severity in humans ranges from asymptomatic colonization of the upper respiratory tract and skin, to clinical disease with varying severity. Disease ranges from mild to severe and invasive infections. Examples of mild disease include furunculosis and dermatitis which can progress to soft tissue abscessation. MRSA can cause life-threatening illness, including necrotizing pneumonia, sepsis syndrome, pyomyositis, osteomyelitis, and necrotizing fasciitis. Purpura fulminans is a shock state with cutaneous hemorrhage and necrosis; systemic signs are present, including hypotension and coagulopathies, such as disseminated intravascular coagulation (DIC).

Several veterinary species are susceptible to MRSA, including horses, dogs, cats, cattle, and pigs. MRSA isolates are resistant to all beta-lactam antimicrobials, including penicillins. MRSA isolates are also variably resistant to many other antimicrobial classes including macrolides and azalides, fluoroquinolones, tetracyclines, clindamycin, among others. This resistance is genetically coded with the presence of mecA gene. This gene encodes for a penicillin-binding protein, PBP2a. This gene confers resistance to methicillin and oxacillin, providing the name 'methicillin resistant'.

There are two forms of MRSA affecting humans, hospital- and community-acquired (HA and CA-MRSA). Initially these were discrete, but over time they have become blended. Hospital acquired MRSA is infection that develops ≥ 48 h after hospitalization or within a year of hospitalization, surgery, dialysis, or residence in a long-term care facility. Community acquired-MRSA is infection with onset in the community rather than hospital. Such patients do not have typical risk factors; they are usually young, and are often athletes, military personnel, students or prisoners. Community acquired MRSA is easily spread with physical contact and through abrasions or wounds. Shared equipment, including towels, razors, or sports equipment can serve as a means of spread. The community acquired strains of MRSA possess unique virulence and transmissibility factors, including a Panton-Valentine Leukocidin (PVL) toxin in > 95 % of isolates. Community acquired strains, despite being methicillin resistant, are often susceptible to fluoroquinolones, aminoglycosides, macrolides and clindamycin.

Hospital acquired MRSA (HA-MRSA) are most common in immunocompromised, elderly or premature patients, and affected patients are usually hospital residents. Risk factors for infection with HA-MRSA include indwelling catheters, hemodialysis, and long term antimicrobial administration. Transmission is usually through person-to-person contact, with hands or fomites such as hospital equipment. The HA-MRSA strains have a low prevalence of the PVL gene (< 5 %). These strains also differ from community acquired strains by their resistance MIC patterns- they are not susceptible to other commonly antimicrobials. Initially distinct, now there is considerable overlap in terms of genotype and antimicrobial susceptibility patterns among CA-MRSA and HA-MRSA.

The clinical syndrome in horses is somewhat similar to that in humans. They can be colonized subclinically in the nares and gastrointestinal tract. Infections can vary and range from superficial to deep infections. Superficial infections include dermatitis, wound or incisional infections, joint or tendon sheath infections, and phlebitis. Deep infections include pneumonia, metritis, umbilical abscesses, scirrhous cord, sinusitis, osteomyelitis, mastitis and septicemia. Horses are most commonly colonized or infected with USA 500, which is a minor or uncommon human strain that lacks PVL genes.

Treatment of subclinical colonization is isolation of affected horses until they test negative on cultures of nasal swabs. Otherwise there is no specific recommended treatment for subclinical carriers. Superficial skin infections can be treated topically with silver sulfadiazine, chlorhexidine, povidone iodine, mupirocin or fusidic acid. More serious superficial infections such as joint infections and deep infections should be treated with systemic antimicrobials. Selection of antimicrobials should be based on culture and susceptibility testing, as the susceptibility patterns vary widely among isolates. Chloramphenicol and amikacin appear to be effective in most isolates seen at our hospital. It should be noted that despite a high percentage of enrofloxacin-susceptible isolates, these results must be interpreted with caution as in vivo resistance to fluoroquinolones may develop rapidly.

One study in the equine literature showed that 10 % of equine veterinarians and technicians were colonized asymptomatically. Another study showed that 16 % of large animal ACVIM diplomates were nasally colonized. The majority of these isolates were USA500, which is an uncommon human strain. Hand washing between infectious horses and between farms was protective against being asymptomatically colonized in that study.

It is estimated that subclinical colonization in horses ranges from 0-10 %, with young horses and horses from endemic farms being at highest risk. MRSA infections reported in horses most commonly include surgical incisions, skin and soft tissue infections, septic arthritis, bone or tendon infections, IV catheter site infections, and pneumonia. Less common sites include guttural pouch, sinus, nose, eye, uterus, udder and septicemia. Horses that were subclinical carriers on admission are far more likely to develop infections during hospitalization than those that are not carriers. In one retrospective report of MRSA infections in 115 horses, affected animals had an 83.8 % survival to discharge. In this study, previous hospitalization, prior treatment with gentamicin, and incision infection were factors significantly associated with MRSA infections in these horses. Surgical site infections were most common (58 %) in hospitalized horses, whereas skin and soft tissue infections were most common (29 %) in farm-acquired infections (i.e., "community"-acquired). Factors associated with nonsurvival included IV catheterization, infection with community-acquired MRSA, and dissemination of infection to other body sites (Anderson et al 2009).

In order to minimize the risks of colonization or infection with MRSA on a farm, good hygiene and infection control principles are important. Hand washing and hand sanitizers are the most effective means of preventing transmission of MRSA. Housing horses in small groups, quarantine of new horses, and use of separate feed and water bins are important means of preventing transmission. Screening of new arrivals with nasal swab cultures provides a high level means of maintaining negative farms free of MRSA.

Eradication of MRSA from endemic farms has been successfully performed using testing and isolation protocols. All horses on the premises should be tested with nasal swabs, and then segregated into positive- and negative- groups of the farm. Positive horses should be isolated until they have 2 consecutive negative swabs. Antimicrobials are not necessary and do not appear to significantly alter the course of subclinical colonization.


Anderson MEC, Lefebvre SL, Rankin SC, et al. Retrospective multicentre study of methicillin-resistant Staphylococcus aureus infections in 115 horses. Eq Vet J 2009; 41: 401-405.

Anderson ME, Lefebvre SL, Weese JS. Evaluation of prevalence and risk factors for methicillin-resistant Staphylococcus aureus colonization in veterinary personnel attending an international equine veterinary conference. Vet Microbiol 2008; 129: 410-417.

Hanselman BA, Kruth SA, Rousseau J, et al. Methicillin-resistant Staphylococcus aureus colonization in veterinary personnel. Emerg Infect Dis 2006; 12: 1933-1938.

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