The most common neurological disease of horses is equine protozoal myeloencephalitis, or EPM.  Possums serve as the definitive host for the parasite called sarcocystis neuroma, which cause the disease.  Possums are responsible for shedding the infective parasite.  It has recently been discovered that the cysts of the organism have been identified in the muscles of cats, raccoons, armadillos, sea otters, and skunks. 

A recent study was published in the Journal of Veterinary Parasitology to determine the effect that farm cats might have on the transmission of EPM.  Nine horse farms in Ohio that have had horses diagnosed with EPM had their farm cats examined for the presence of antibodies to the sarcocystis parasite.  As a control group, cats from a local Ohio spay/neuter clinic were also examined for the presence of sarcocystis antibodies.  Results were 40% of the cats from horse farms were positive for sarcocystis antibodies, while only 10% of the cats at the spay/neuter clinic were positive. 

In fact, 80% of the cats were positive on one farm that had a severe problem with EPM in their horses.  This indicates that cats at horse farms where EPM has been diagnosed are more commonly affected with sarcocystic neuroma than cats in the general population.  This information confirms earlier reports that cats are natural intermediate hosts for the parasite. 

Sarcocystis infected tissue must be ingested by the definitive host and possums are a scavenger by nature and prey on small rodents and abandoned carcasses.  One study found domestic cats make up 2.4% of the possums’ diet.  Although cats are not the only intermediate host for the parasite, they are possibly an important host in maintaining the parasite on horse farms.  Control of the parasite still involves preventing exposure of horses to possum feces.