The emergence of methicillin-resistant Staphylococcus aureus (MRSA) as a major human health concern has heightened awareness of the occurrence of this agent in companion animals, including
horses. The purpose of this presentation is to review what basic knowledge equine practitioners should have regarding MRSA
and other methicillin-resistant staphylococcal species in horses.
There are numerous species of Staphylococcus. All are Gram-positive and catalase-positive. Most of the pathogenic species are coagulase-positive, including S. aureus. S. aureus is a commensal organism that colonizes the skin, mucous membranes, gastrointestinal tract, and urogenital tract of approximately
⅓ of humans. In domestic animals, other species of coagulase-positive staphylococci are the predominant commensal organisms.
Infection with staphylococcus outside of their normal commensal niche can result in pathology. From a historical perspective,
it is worth remembering that S. aureus infections were associated with high rates of disease and death.
Bacteria can either be intrinsically resistant to 1 or more antimicrobials, or can acquire resistance via genetic changes.6 Genetic changes can occur through spontaneous mutation or by acquisition of resistance genes from other organisms. Genes
can be acquired by conjugation, transduction by bacteriophages, or transformation (i.e., acquiring and incorporating DNA from
other lysed bacteria). Resistance to penicillin and other β-lactam antibiotics can occur from intrinsic and acquired mechanisms.
Some staphylococci can produce β-lactamases that destroy the β-lactam ring and consequently the ability of penicillin to
bind to a transpeptidase thereby interfering with bacterial cell-wall synthesis. Resistance to methicillin occurs by a different
mechanism. MRSA have acquired a mobile genetic element known as the staphylococcal cassette chromosome mec (SCCmec), which includes a gene known as mecA. The mecA gene encodes for an altered penicillin-binding protein that has lower affinity for β-lactam antibiotics, thereby rendering
mecA-carrying isolates resistant to methicillin and other β-lactam antibiotics. The SCCmec can be transferred horizontally
among staphylococci. Moreover, it also can accept other antimicrobial resistance genes, such that many methicillin-resistant
Staphylococcus species (including MRSA) also are resistant to other organisms.3 Coagulase-positive staphyloccoci (CPS) other than MRSA are rare in horses, unlike dogs where methicillin-resistant Staph. intermedius/pseudintermedius are an important pathogen. Horses are commonly colonized by coagulase-negative staphylococci (CNS). These isolates can
be mecA-positive. The significance of mecA-positive CNS in horses is unknown, but they might cause infection and can serve
as a reservoir of the gene for CPS. Isolates of MRSA are not necessarily more pathogenic than methicillin-susceptible Staph. aureus. The principal concerns with MRSA are the potential for human infection from horses and nosocomial infection.
Staphylococci are not routinely tested for methicillin. Rather, they are tested for susceptibility with either oxacillin
or cefoxitin. The Clinical and Laboratory Standards Institute recommends that staphylococci be tested with the cefoxitin
disk test, the latex agglutination test for the penicillin binding protein 2a (PBP2a, encoded by the mecA gene), or a plate containing 6 µg/ml of oxacillin in modified Mueller-Hinton agar. Some laboratories use a combination of
these screening tests. In addition, a polymerase chain reaction (PCR) test for the mecA gene can be performed. The PCR test is considered most accurate because there are some subpopulations of MRSA that carry
but don't express the mecA gene. It is important, however, that culture as well as PCR be performed because there can be other CPS and CNS that carry
the mecA gene.