The opossum is the definitive host for the protozoa S. neurona, which causes most cases of EPM. The opossum is infected by feeding on a carcass of an infected intermediate host such as the raccoon, armadillo, skunk, birds, or domestic cat. Once the opossum consumes the immature stage of the protozoa from a carcass, it takes about two to four weeks for the protozoa to mature and be shed in the opossum feces.
The protozoa must cycle between the opossum and
an intermediate host to complete its life cycle.
Most researchers consider the horse to be an aberrant host –
one that can become infected, but not act as the intermediate host
to complete the life cycle.
Infected horses do not shed the protozoa in manure, so HORSES
CANNOT INFECT OTHER HORSES.
EPM life-cycle diagram developed by the USDA
Opossums are nocturnal and live in wooded, and edge of forest habitats. An omnivore, the opossum will eat a variety of items including carrion, berries, cat food, sweet feed, and human snacks left in the trash. It has a very good sense of smell. They need a source of water, and are drawn to the riparian buffers along streams. Opossums are a necessary part of the forested ecosystem, cleaning up carrion, and proving food for animals higher in the food chain.
Horses are infected by ingesting feed or water that has been contaminated with opossum feces. Keep hay and grain in secure locations where wildlife cannot come into contact with it. Feed tubs located on the ground are easier for opossums to enter, and might increase the risk of ingesting the protozoa. Opossums defecating in open water sources including streams can produce another source of infection. Horses may ingest S. neurona in pastures located near wooded areas or fence rows with thick brush.
Keep opossums out of the barn if possible. They are good climbers and may enter the barn over a stall door. Pick up cat food every night, and store it in a raccoon/opossum proof container. Sweep up spilled grain, and empty the trash can often. Keeping food items locked up or out of the barn will make it less tempting for opossums to enter.
Several authorities recommend trapping and removing the opossums from the area. This involves either euthanizing or relocating the tapped animal. Euthanizing the opossum is not fun, and may not be legal in your location. A wildlife control professional could be hired to perform this for you. Relocating the trapped animal may give you peace of mind, but you have transported the animal outside of its territory, and it may not survive long in a new location. If you transport it to an area near other farms, you may create headaches for other equine owners.
Keep the elimination of opossums at your location in perspective. If you take your horse to any other location where it has the ability to graze, he is at risk of ingesting the protozoa. It is just as important to practice prevention strategies for the horse, such as good nutrition and limiting of stressful situations.
Left-fore & Left hind opossum tracks. CC Wikipedia