The horses had detectable serum antibodies at about 3 weeks after infection, and all of the horses that ultimately developed EPM had spinal fluid antibodies about a week later. Those that did not develop EPM never had antibodies in the spinal fluid, even as long as 4 months later. This disease may be preventable by some simple measures. Anything that may attract opossums into barns should be tightly covered, or put away, especially at night. This includes cat food, garbage and grain.
Opossums are particularly fond of cat and dog food.
Feed should not be left out at night for the morning, or even during the day to attract birds. The opossum population should be kept under control on farms and stables, where possible. It is impossible to predict which exposed horses will develop fulminant disease. Some horses with active disease may be able to clear the organism without treatment. The disease probably requires a minimum of two weeks and up to two years to develop from the time of exposure to the development of marked clinical signs.