Update on MRSA in Horses

MRSA (methicillin-resistant Staphylococcus aureus) is a strain of bacteria that has been identified with increasing frequency in recent years. Humans, dogs, cats, horses, cattle, pigs, poultry, and possibly other animals can be affected by MRSA. Approximately 25% of adults and children and around 5% of horses carry MRSA bacteria on their skin or in their nasal passages. Carrying the bacteria does not necessarily cause infection or illness. At the recent International Conference on Equine Infectious Diseases, speakers from several countries reported on MRSA status among horses.

Microbiologists from the Centre Hospitalier Memorial de Saint-Lo in France reported that certain strains of S. aureus bacteria become resistant to methicillin when they acquire the mecA gene that encodes a protein having a very low affinity for this class of antibiotics. The strains are often resistant to a wide range of other types of antibiotics.

An outbreak of MRSA infections occurred in Sweden, where the strain is uncommon. Ten horses were diagnosed with MRSA infections and were treated by veterinarians. Beginning with the day the infection was diagnosed, skin testing was carried out monthly on each horse to look for MRSA bacteria. MRSA were detected in only 16 of 333 samples tests, and the probability of finding positive MRSA samples significantly declined over time. It was concluded that MRSA populations naturally disappear or drop to undetectable levels by about 200 days after the first detection.

A report from Canada indicated that of 2,235 admissions to the Ontario Veterinary College equine clinic during a two-year period, only 3% of horses were found to be positive for MRSA based on routine nasal swabs. The nasal passage is the most common site to find MRSA in horses.

MRSA has been found to affect horses with skin infections, wounds, surgical or catheter sites, septic arthritis, and pneumonia.

The bacteria can be spread from humans to horses and also from horses to humans. For veterinarians and other people handling horses that are being treated for MRSA, precautions against infection involve washing hands with soap and water, wearing protective clothing such as gloves, using an alcohol-based hand sanitizer, keeping wounds and scrapes clean and covered, and avoiding direct contact with affected wounds and dressings.