Researcher: Bacteria Evenly Distributed Across Horses' Bodies

Residing in horses' skin are microorganisms known as skin flora--generally nonpathogenic bacteria that can cause skin disease, joint infections, and life-threatening illnesses if they enter the bloodstream or a joint, such as through an incision in the skin. According to the results of a recent study performed by researchers at Colorado State University's (CSU) College of Veterinary Medicine and Biomedical Sciences, the different species of skin flora are distributed evenly across a horse's body.

This finding indicates that veterinarians should use the same method of preventive treatment to inhibit infection from skin flora before a horse has undergoes surgery, regardless of the location of the surgical incision. Pre-surgery, veterinarians often use antimicrobial prophylaxis (to prevent the growth or spread of infection) to regulate skin flora before serious infection occurs, and generally focus treatment around the area of the incision.

Mackenzie K. Adams, BA, a fourth-year veterinary student at CSU, under the supervision of Dean A. Hendrickson, DVM, MS, Dipl. ACVS, professor of large animal surgery and director of the James L. Voss Veterinary Teaching Hospital (VTH), wanted to determine if equine practitioners should select different preventive treatments based on incision location, due to the common belief that certain bacteria types concentrate in particular sites on the horse's body.

The researchers examined approximately 200 typical skin samples from the mid-thorax (chest) and the dorsal aspect of nine different joints of 20 client-owned horses admitted to the VTH for routine elective surgery during the summer of 2008. The joint samples taken from each horse included the front and hind coffin joints, fetlocks, knees, elbows, shoulders, hocks, and stifles. These samples were cultured, grown in an aerobic environment (in the presence of oxygen), and the bacteria were grouped into three clinically relevant categories: Gram-positive versus Gram-negative, coliform (fecal bacteria) versus noncoliform, and common septic arthritis pathogens versus uncommon septic arthritis pathogens.

Comparisons between proximal (those closer to the center of the body) and distal sites (farther from the center) revealed that the odds of isolating Gram-positive bacteria were 1.23 times higher at proximal sites; the odds of isolating coliform bacteria were 1.32 times higher at distal sites; and the odds of isolating a common septic arthritis pathogen were 1.16 times higher at distal sites. However, since all distinctions between sites were less than 40%, these results were not considered clinically significant, so the researchers concluded that the different bacteria types were distributed evenly across the horse's body. Antimicrobial prophylaxis, they added, should be consistent regardless of where the treatment area is located on the body.

"It was very surprising that the bacteria seem to be fairly evenly distributed across the horse's body," said Adams. "It was also extremely interesting that no coagulase-positive Staphylococcus bacteria were present on any of the horses, since this is the most common cause of joint infections secondary to joint injections or surgery in the horse."

The absence of coagulase-positive Staphylococcus suggests that iatrogenic infections (inadvertent complications resulting from medical treatment) caused by this organism likely do not occur because of preexisting flora. "Possible sources (of infection) include contaminated instruments, the veterinarian, or contamination of the operative site post-operatively by mucus secretions (protective substances released from the mucous membranes)," Adams explained.

Adams said that multiple factors, including location and time of year, could have affected the study results: "It would be very interesting to know how season, geography, bedding type, proximity to cattle, use of antibiotics, presence of disease, etc., may affect the normal bacterial flora of the horse," she added. "It is possible that horses living in different regions or climates have very different skin bacteria profiles than horses at our hospital."

The study, "The Bacteria Isolated from the Skin of 20 Horses at a Veterinary Teaching Hospital," was published in the December 2010 issue of the Journal of Equine Veterinary Science. The abstract is available online.

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