Saville and researchers at The Ohio State University have started to shed some light on this phenomenon of EPM cluster outbreaks.
They discovered there was a 2.5 times greater risk for the development of an EPM case on a particular farm if there had been
a previous case at that location.3
As part of the research study, numerous other risk factors were then identified that made EPM cases more likely. "Most if
not all risk factors necessary for this disease were present at those facilities deemed EPM hot spots," says Saville.
The simplest and most obvious risk factor was the presence of opossums on the farm, with an even higher risk tied to the number
or frequency of opossum sightings.
The presence of woods on the farm was another risk factor associated with increased development of the disease, while a creek
or river on the property decreased the risk for exposure or disease. It's thought that Sarcocystis neurona sporocysts are less likely to be present in high numbers in running water as opposed to ponds or other stagnant water sources
(Photo 2). Interestingly, unlike on the OHA property, California farms adjacent it all have water troughs and no ground water
drinking sources in their pastures, and there's never been an EPM case at those facilities.
Woods located within five miles of the property decreased the rate of EPM occurrence, since opossums would be less likely
to leave this safe location to venture onto farm pastures and come into contact with horses. Safe, secure protection of feed
decreased the rate of disease development, while high numbers of horses on a farm, purchased vs. home-grown grain, use of
wood chips or shavings as bedding, presence of rats or mice and increased human population density all increased the risk
of EPM cases, according to the Ohio State report.
Saville also reported a seasonal effect to EPM cases, with a much higher risk during the summer and fall. Compared with all
horses admitted to The Ohio State's College of Veterinary Medicine, horses admitted in the summer were twice as likely to
have EPM, and horses brought in the fall were three times more likely. Trailering, competition and training stress also increase
the likelihood of developing EPM cases. These risk factors may help explain the increased EPM risk seen in the fall, since
many competitions take place then, requiring increased training programs and more hauling.
The life cycle and migration patterns of the opossum probably also contribute to the seasonality of EPM cases, since opossums
are more active in the summer and fall, and breeding season occurs during these times.
While this was not part of the Ohio State study, human activities such as development or construction, especially involving
the clearing of large pieces of wooded land, also may contribute to increased EPM cases. Removing the natural habitat of the
opossum forces these animals into adjacent areas, and the relatively open and protected pastures of horse farms are preferred
over residential or more developed areas.
The final and perhaps most important risk factor mentioned in the Ohio State study was a health-related event before the development
of EPM such as colic, viral or bacterial infections, foot issues, skin problems, digestive changes and almost any medical
condition. These health events are often quite stressful and may serve to weaken a horse's immune system, making EPM infection
Saville and other EPM researchers concede that prevention of this disease has proved more difficult that anyone imagined.
Sporadic, isolated cases did not help researchers develop prevention strategies. However, EPM cluster outbreaks have helped
to define risk factors and enabled practitioners to provide suggestions to their clients, such as the following:
> Keep all feed materials in secure and protected containers.
> Remove wooded sections in pastures, or provide wooded areas at a site far removed from horse contact.
> Keep all water sources flowing, or use water troughs and remove stagnant water.
> Pick up and remove all dead animals, which may serve as intermediate hosts, from the farm.
> Discourage the presence of opossums if at all possible.
> Since a previous health event was highly correlated to an increased risk of EPM infection, keep horses healthy and as
stress-free as possible. Horses recovering from infections, colic or any other conditions should be kept somewhat isolated
from general pasture access so as to reduce the likelihood of sporocyst ingestion during their weakened state.
Wildlife management and risk-factor manipulation provide the best chances for reducing the risk of cluster outbreaks of EPM.
Although this disease still seems to frustrate and confuse researchers, practitioners and clients alike, the veterinary community
seems to be making some slow, steady progress toward understanding it.
Dr. Marcella is an equine practitioner in Canton, Ga.
1. Proposal to reduce incidence of equine protozoal myeloencephalitis at Orinda Horsemen's Association pasture. Orinda Horsemen's
Association, Nov. 11, 2010.
2. Dubey JP, Lindsay DS, Saville WJ, et al. A review of Sarcocystis neurona and equine protozoal myeloencephalitis (EPM). Vet Parasitol 2001;95:89-131.
3. MacKay RJ, Granstrom DE, Saville WJ, et al. Equine protozoal myeloencephalitis. Vet Clin North Am Equine Pract 2000;16(3);405-425.