Therefore, a primary objective in disease prevention should be to minimize stress so a horse’s immune system can operate at maximal capacity. Witonsky comments, “At this time, we still don’t know why some horses develop disease, although based on studies and on my clinical impression, stress from showing, shipping, training, etc. seems to be a risk factor for increased incidence of disease. As a trainer or owner, it is important to be sensitive to what one’s horse believes is stressful, and try to be observant for subtle changes in behavior and performance which could be due to EPM. If a horse does develop disease, hopefully it will be detected early in the onset of disease. In that way, an infected horse can be started on treatment as early as possible to minimize and prevent horse losses and to improve overall outcome with regard to return to overall health and performance.”
Bill Saville, DVM, PhD, Dipl. ACVIM, professor and chair in the Department of Veterinary Preventive Medicine at The Ohio State University, and his colleagues investigated risk factors for development of EPM. In this study they acknowledged the important role of the immune system in fending off disease.
“When animals are stressed, suppressive proteins produced by the central nervous system are released and lead to suppression of lymphocyte production and function,” said Saville.
This, coupled with elevated cortisol levels related to stress, might increase a horse’s risk of developing EPM.
Control Measures to Reduce Risk
Saville’s comprehensive study (Journal of the American Veterinary Medical Association, 2000) revealed the following findings that tell us how we can more effectively prevent EPM in our horses:
Age The highest risk of infection occurred in horses aged 1-5 years. This could be due to the use of young horses in competitive situations and the associated stress.
Opossums Presence of opossums on a farm poses an increased risk.
Location Horses on farms with previously EPM-infected horses had a higher risk of developing EPM, likely due to the presence of protozoa in the feed or water and increased likelihood of exposure.
Seasonal effects More EPM cases occur in spring, summer, and fall, possibly related to hot weather acting as a stressor, as well as this being a time of increased travel to competitions with accompanying transport stress affecting the immune system.
Stress An association of stressful events (such as injury, accidents, foaling, surgery, transport, and illness) with increased risk might be related to suppression of a horse’s immune system.
Natural water source Presence of water sources (creek or river) on the farm provided a preferred habitat for opossums away from the horse barns, thereby decreasing exposure and risk.
Food storage Securing feed and water sources from opossum fecal contamination is important in limiting exposure and risk.
It is important to limit opossum presence since sporocysts (the infective stage of the protozoon) are able to survive for as much as a year in the environment. Additionally, birds feed on insects and plant material in the feces of opossums, thereby serving as a vehicle to disseminate sporocysts in the environment. David Granstrom, DVM, PhD, one of the pioneer researchers of EPM when he worked at the University of Kentucky, emphasizes how environmental management can go a long way toward limiting infection.
“It looks like the only way to clean barns that is effective and will not destroy the barn is by the heat of steam cleaning.” –Dr. David Granstrom
“It’s most important to protect feed and the local environment from contamination with opossum feces,” states Granstrom. “Protect livestock feeds and hay from opossums. Keep the local area free of anything that attracts opossums, such as pet food, garbage, and carrion.”
Saville says it isn’t easy to kill the parasites in the environment, and sporocysts are resistant to even the most intense disinfectants.
Granstrom adds, “It looks like the only way to clean barns that is effective and will not destroy the barn is by the heat of steam cleaning.”
Because disinfectant foot baths will not impact sporocysts, it is suggested to change boots or use disposable boot covers in areas where there is the potential for barn contamination.