A Look at MRSA in Horses

Methicillin was first put to use in the 1950 for staphylococci bacteria that were resistant to penicillin. It did not take long for a strain to emerge that was also resistant to methicillin. Since then, MRSA has become a more serious problem for medical professionals. While it originated in hospital settings, there are now community-acquired strains and strains that are infecting companion animals like horses and dogs.

However, the role of MRSA in animals is not that well understood. MRSA has been found in horses, cattle, and dogs, as well as among the staff that take care of them.

A Large Animal Clinic in Ontario did a study on the horses that came in for treatment – about 2,000 each year. They also looked at the staff that was in contact with the horses on a regular basis.

The hospital started a new screening program for MRSA in the horses. All horses admitted to the hospital received a nasal swab at admittance, every week during their stay, and when they were discharged. Several horse farms took up similar practices of screening for MRSA in the horses there. At the same time, the clinic initiated nasal cultures on the human staff as well.

In the human staff, samples were taken from the anterior nasal passageways. In the horses, the swap was inserted about 10 cm into one nasal passageway. If a colony or infection was seen in the first 72 hours after admission, the MRSA was considered community acquired.

The swabs were then taken and tested for MRSA and antibiotic resistance. DNA sequence analysis was then performed to identify the different strains. Duration of carriage by foals versus adults was also done.

MRSA was found in 79 horses and 27 people over the course of the three year study. 34% came from horses that had been hospitalized, 52% from one farm, and 14% from the other farms. Only 30% of the horses that tested positive were adults, the remaining 70% were under a year of age.

16% of the horses developed clinical infections. There were several infection types: wound and incision infections, IV catheter infections, bacteremia, pneumonia, surgical implant infection, septic arthritis, omphalophlebitis, gluteal abscess, and osteomyelitis. One foal came down with septic arthritis. Of the clinically infected horses, 85% had previous contact with a colonized person, and about 15% had also been around infected or colonized horses. One horse died because of MRSA.

Nearly all of the isolates came from the same strain of MRSA. 63% of the MRSA infections or colonization was thought to be hospital acquired, and 11% were community acquired. The remaining 26% cases the origin of infection was not clear.

Of the humans tested, 27 were positive for MRSA – 14% from the hospital personnel, 12% from the farm, and one horse owner. Only one person was thought to have community-acquired MRSA, and all put one person had contact with the positive horses.

These result shows that there may be some correlation between horse MRSA infections and their human counterparts. Take special care to protect yourself when working with these animals.

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