Methicillin-Resistant Staphylococcus Aureus (MRSA)

Filed Under: Dogs, Diseases

Does your pet have a chronic skin, ear, or nail bed infection that just will not clear up regardless of the antibiotic used? Then a new strain of Staphylococcus bacteria could be the cause of the problem.

Staphylococcus aureus is a bacterium that is a normal inhabitant of the nasopharynx (area between the nose and the mouth at the back of the throat) and the skin of most animals and people. Approximately 30% of all people have S. aureus in their nose at any point in time. Most of the time this bacterium is a normal inhabitant and does not cause any problems. Unfortunately, strains of Staphylococcus are evolving, exchanging genetic information and becoming opportunistic pathogens that are becoming more and more virulent (able to cause disease). It is no longer uncommon to find the presence of the mecA gene in this particular type of bacteria. The mecA gene encodes for a special protein that confers resistance to all penicillin and cephalosporin antibiotics thereby giving this bacterium a competative advantage. Bacteria obtaining the mecA gene are methicillin-resistant (type of penicillin) and are therefore termed methicillin-resistant Staphylococcus aureus or (MRSA). MRSA is commonly found in hospital environments due to the constant use of antibiotics and disinfectants whose use actively selects for resistant strains of the bacteria. Most cases of MRSA infections are actually nosocomial (hospital acquired).

Hospital personnel may actually contribute to the spread of the bacteria through the use of the common neck-tie. A neck-tie is worn just below the nasopharynx where breathing and sneezing may lead to contamination of the tie with bacteria previously located in the owner’s nasal area. Once contaminated, the ties are infrequently laundered. Hands are also a major source of bacterial transmission. Today physicians, veterinarians, and other hospital personnel are being urged to trade in their neck-tie and lab coat for a scrub top, which can be easily laundered, and all personnel are urged to wash hands frequently.

Major morbidity and mortality is associated when MRSA infects a hospitalized, immunocompromised, or otherwise debilitated individual. It is now being seen as a cause of extensive skin infections leading to suppurative spreading lesions. Lesions may appear as boils or abscesses and care must be taken in the drainage of the area. In the dog other commonly infected areas include the eye, ear, urinary tract, and nail bed.

There are three mechanisms of resistance with MRSA against antibiotics. All penicillins and cephalosporins (ß-lactams) require binding to a bacterial cell wall protein which disrupts cell wall synthesis. MRSA bacteria containing the mecA gene produce an effective low-affinity penicillin-binding protein. With no place for the antibiotic to bind, the mutation infers resistance to penicillins and cephalosporins.

MRSA bacteria also possess an unusually thick cell wall that makes penetration by many antibiotics difficult. These resistant bacteria have also acquired an efflux pump. This is a protein pump involved in the active removal of antibiotics from the bacterial cell thereby limiting antibiotics that work against internal cellular structures.

In addition to antibiotic resistance these bacteria carry genes for a host of nasty molecules which help the invading bacteria adhere to cells and actually destroy defensive cells sent out from the body to deal with the developing infection.

All MRSA strains of bacteria may be traced back to a single clone first found in Europe in the mid-1960’s. MRSA was first reported in animal infections in the mid-1990’s. Until recently MRSA was considered a transient pathogen acquired from a human. Today MRSA in dogs is frequently cultured from canine wounds, abscesses, and chronic pyodermas. According to Antech Diagnostics, S. aureus accounts for 3 to 5% of all bacterial culture results reported at their labs. The percentage of infections attributed to S. aureus has not changed since 2004, but what has increased is the percentage of these S. aureus cases that are methicillin-resistant. In 2005 only 19% of the S. aureus cultured was methicillin-resistant. Today that number has jumped significantly to 42%. This is a clear case where antibiotic resistance in humans is now affecting pets.

Staphylococcus intermedius is considered to be the commensal staph (a normal inhabitant of dogs which does not cause disease) of dogs and cats, and is now deemed the primary bacteria involved in pyodermas (skin infections). In addition, S. intermedius is identified in 25 to 30% of all wounds cultured at Antech laboratories. In 2004, <0.6 % of this strain of Staphylococcus was found to be methicillin-resistant. By 2007 that number had climbed to 10.2%.

Staphylococcus schleiferi, subspecies schleiferi, appears to be more commonly detected than Staphylococcus with methicillin-resistance, and is commonly reported in recurrent canine pyodermas and ear cultures.

Chloramphenicol (CAP) is an excellent drug, and for many is the drug of choice in methicillin-resistant staphylococcal infections in animals. Reportedly, CAP is effective in more than 95% of methicillin-resistant Staphylococcus and is relatively inexpensive. Dogs should be dosed at 33 mg/kg three times daily. CAP is not typically used in humans due to the threat of aplastic anemia (an anemia characterized by a lack of blood cell production) which occurs in one of every 10,000 people. Due to the threat of depressing hematopoietic activity, gloves should be worn when administering chloramphenicol as an antibiotic to animals.

Sulfonamides have a reported 55% sensitivity to MRSA but should not be used in breeds of dogs known to have sulfonamide hypersensitivities, including the Weimaraner, Doberman pinscher, Rottweiler, Samoyed, miniature Schnauzer, Lab, and other white-coated breeds. Clindamycin is effective in less than 50% of MRSA cases while that figure with Fluoroquinolones is less than 25% sensitive. Vancomycin is effective against the vast majority of methicillin-resistant Staphylococcus but has several disadvantages including its nephropoisoning and four times daily IV administration. Linezolid is effective against even vancomycin-resistant enterococcus and is very expensive and should be used only if there is no other drug available.

According to the CDC, factors involved in MRSA transmission include close skin-to-skin contact with an infected individual, especially when cuts and abrasions are present, exposure to contaminated items and surfaces, or crowded living conditions and associated poor hygiene.


Aucoin, David. “National Incidence of Methicillin-Resistant Staphylococcal Infections: Understanding, Treatment and Prevention.” Bayer Dermatology Symposium Proceedings, a Supplement of Compendium: Continuing Education for Veterinarians. Vol. 30, No. 1(A). January 2008. Pp. 3-7.

Antech Diagnostics News. “Methicillin-Resistant Staphylococcus Aureus (MRSA) Revisited. April 2008. P. 1-2.

MacDonald, John. “Dermatology Notes on Pyoderma.” Great Smokies Veterinary Conference. Oct. 28 - 29 2007.

Topics: MRSA, staphylococcus

Symptoms: infection, skin lesions

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