Although methicillin-resistant Staphylococcus aureus (MRSA) is primarily a nosocomial pathogen, its foothold in the environment is expanding. MRSA is a multidrug-resistant opportunistic pathogen that has become a serious problem in human medicine. Other than causing outbreaks in veterinary settings, it has infrequently been described as an animal pathogen.

However, recent canine cases have been confirmed in North America and overseas. Infections have involved eye, ear, urinary tract, nail bed, abscess, and skin. In most cases, pet owners had been an inpatient, outpatient or frequent visitor to institutional settings. Dogs can become colonized with MRSA from humans, and can transmit it to other dogs and people. The frequency of this is unclear, although the presence of colonized/infected humans in a household does not necessarily mean that the dogs are either likely to be affected or to be sources of infection.

Contributors to the increasing importance of this pathogen:

• pattern of resistance to a wide range of antibiotics besides penicillins and cephalosporins (ß-lactams)
  
• opportunistic nature of the pathogen
  
• mechanism of resistance

There is the potential for zoonotic transmission. Pet owners should be made aware that the dog may be carriers of MRSA. Personal hygiene consisting of washing hands after contact minimizes the spread of MRSA. However, while the risk of overt disease from MRSA is low in healthy people, it may be of significant concern in immunocompromised individuals, or for those taking antimicrobials.

MRSA infections have been reported in animals since the mid-1990s but were considered uncommon in domestic pets and rarely caused disease. However, since 2002, there has been a rapidly growing body of data showing an alarming acceleration of MRSA cultures from both dogs and cats from almost all body sites routinely tested.

Studies to date indicate that MRSA is transmitted from human to pet because methicillin-sensitive strains of S. aureus are not a part of the normal colonizing flora of dogs and cats.

MRSA was isolated from the tracheostomy tube of a young dog in the intensive care unit (ICU) of a veterinary teaching hospital. While the dog had no clinical or radiographic signs of pneumonia, the finding prompted an investigation of MRSA colonization in other animals in this ICU. MRSA was isolated from nasal swabs from 6 of 26 (23%) animals, 4 dogs and 2 cats obtained from 1-78 days later. No clinical signs of MRSA infections developed, and all isolates were identical. No colonized animals were identified during subsequent periodic surveillance. These findings suggest that ICUs may be at particular risk for periodic outbreaks of MRSA colonization and disease.

All animals that were identified as colonized with MRSA remained colonized at the time of discharge from the ICU. Active eradication of MRSA was not recommended because there is no evidence of efficacy of treatment, and dogs and cats almost invariably eliminate MRSA colonization if re-infection is prevented. Owners were counseled on household infection-control practices that might reduce the risk of MRSA transmission, such as hand hygiene and limited contact with the colonized animal’s nose and rectum.

MRSA is most frequently cultured from canine wounds, abscesses, and chronic pyododermas. While the percentage of S. aureus bacteria cultured from these sites has remained unchanged since 2004 (3-5%), the percentage of methicillin-resistant strains has increased exponentially from 19% in 2005 to 42% in 2007. As this infection comes from humans, the rise is not due to an increasing antibiotic resistance in animals, but is from an increase in community acquired MRSA in humans.

Although resistance to methicillin is not itself of major concern, methicillin resistance is a marker for the presence of the mecA gene. This gene encodes synthesis of an altered penicillin-binding protein that has a low affinity for ß-lactam antimicrobials, which confers resistance to all penicillins and cephalosporins in MRSA strains. Treatment of MRSA is further complicated by its resistance to other antimicrobial classes, and there may be few treatment options in some instances. The impact of MRSA among hospitalized humans is difficult to quantify; however, from 1999-2000, MRSA accounted for an estimated 125,000 + hospitalizations, including 31,000 cases of septicemia. A recent major concern is the emergence of MRSA as a community-associated pathogen in humans.

In some areas, MRSA is now the most common cause of skin and soft tissue infections among people who attend emergency departments. Additionally, serious and potentially life-threatening conditions such as necrotizing pneumonia and necrotizing fasciitis, have been reported, even among humans who were previously thought to be at low risk for MRSA infection.

New epidemiologic issues that deserve consideration include the ability of MRSA to increase undetected in a population. Transmission of MRSA most likely occurred via the hands of health-care personnel, as this is a common route of transmission in human health-care settings. Transmission via colonized staff is a possibility because nasal colonization of veterinary medical personnel can develop. The same MRSA strains affect animals within veterinary hospitals and the community.

Nasal and perineal swabs can be used for detection of MRSA carriers, although ideal screening sites are not yet known.

Infection-control measures including active surveillance of all animals in the infected facility, barrier precautions, and hand hygiene are used to control apparent outbreaks. Treatment of infected animals should be based on culture/sensitivity results, while remembering that MRSA is resistant to all penicillins and cephalosporins. Fluoroquinolones are no longer very effective, and trimethoprim-sulfas, while often useful, cannot safely be given to dog breeds known to have sulfonamide hypersensitivity (e.g. Weimaraners, Doberman pinschers, Rottweilers, Samoyeds and other white-coated breeds, and miniature Schnauzers.)

Because of their resistance, MRSA infections should not be treated with ?-lactam antibiotics (penicillins and cephalosporins), even if in vitro testing shows sensitivity to these drugs.

Antibiotics to consider include:

• choramphenicol [33mg/kg TID in dogs; 50 mg BID per cat] (~95% sensitive)
  
• potentiated sulfonamides, although these antibiotics are associated with sensitivities which can produce significant adverse effects (65-75% sensitive)
  
• clindamycin (< 40% sensitive)
  
• fluoroquinolones (25% sensitive)
  
• vancomycin [15 mg/kg q 6 hr IV with fluids]
  
• linezolid (Zyvox®) , a new family of antibiotics [20-30 mg/kg q 24 hr in dogs] – low toxicity, very expensive

MRSA is an emerging veterinary and zoonotic pathogen, and veterinary clinic personnel must be prepared to identify infected animals. The present case study highlights the potential for clinically inapparent transmission of MRSA within a facility. Objective investigation of infection-control precautions is required to determine the most appropriate and effective responses to MRSA outbreaks. Further, principles of due diligence dictate that veterinary clinics should establish a plan to address MRSA, whether it be colonization or infection of animals, in an attempt to control the spread of this significant pathogen.

References: Aucoin, Comp Cont Edu Vet; Supple 30,#1, 3-7, 2008; Weese et al, JAVMA, 231:1361-1364, 2007; Willey et al, Abstract, 2007; Hanselman et al, Emerg Infect Dis 12:1933-38, 2006;Kania et al, Am J Vet Res 65:1265-1268, 2004, Trepanier, J Vet Int Med 17:647-652, 2003.