Omnivorous opossums creep quietly through the night, in search of a meal of just about anything. Many benefits come from having opossums for neighbors, including insect, tick and rodent control. If you are a horse owner, opossums can be your worst enemy.
Leading a scavenging lifestyle lends itself to picking up a few parasites here and there. Most animals are susceptible to Sarcocystis, a genus of protozoan. Opossums are carrion feeders and can become carriers after eating infected tissues. Infection can occur from ingesting oocysts in the environment. Horses seem to be particularly susceptible to severe clinical disease.
Sarcocystis spp have an indirect lifecycle, requiring both an intermediate and a definitive host. With opossums (definitive host), sarcocysts are ingested from the muscle of an infected prey animal (intermediate host). The sarcocysts release hundreds of bradyzoites in the intestine of the opossum. The bradyzoites enter the wall of the intestine and begin to release oocysts after 7-14 days. Fecal shedding of oocysts contaminates the environment. An intermediate host will consume the oocysts, containing two sporocysts. After hatching in the intestine, sporozoites cross into circulation, and make their way to muscle or nervous tissues. Once there, they encyst and begin forming new sarcocysts.
Clinical signs rarely occur in the definitive host. In horses, S. neurona sporozoites can make their way into the central nervous system (CNS). This leads to a debilitating condition known as Equine Protozoal Myeloencephalitis (EPM) or “opossum disease.” The incubation period ranges from as short as 10-14 days to as long as 5 years. Clinical signs can be as mild as depression and weakness, to more severe including seizures, ataxia, and death. Other reported signs include spontaneous sweating, loss of reflexes, and muscle wasting, often asymmetric.
Diagnosis and Treatment
Diagnosis is made by finding antibodies to S. neurona in the CSF. Serum antibodies do not correlate to current infection. Definitive diagnosis is often made at necropsy, by finding sporozoites in the CNS.
Supportive care is critical in treating EPM. This can include intensive hospitalization with IV fluids and slings to help the horse stand. Treatment options include ponazuril, sulfonamides, and anti-inflammatories. Most horses improve with treatment, with approximately 30% returning to normal function.
The Best Defense
As always, the best defense is a good offense. Good biosecurity practices play a big role in preventing contamination of feed. Store feeds in rodent-proof containers, and hay can be kept behind opossum-occluding fencing. Clean up any spilled grain, pet food, or other waste that could attract opossums. A vaccine is available for horses that are at greater risk.