‘Road-kill hot line’ helps UF solve puzzle of horse disease

August 29, 2001

GAINESVILLE, Fla. — University of Florida veterinary researchers have found two critical missing pieces in a mysterious food chain puzzle that results in devastating neurological disease for tens of thousands of horses each year.

The finding – that the striped skunk and nine-banded armadillo play key roles in hosting the parasite that causes equine protozoal myeloencephalitis, or EPM – may lead to improved control of a disease that, according to the most recent National Animal Health Monitoring System survey, cost the U.S. horse industry $27 million in 1998.

“Basically, horses become infected with the parasite that causes EPM by eating feed or drinking water that is contaminated by stages of the parasite released in opossum feces,” said veterinary researcher Andy Cheadle, whose findings appeared in the April and June issues of the International Journal for Parasitology. “What we’ve found is that opossums can become infected by eating skunk and armadillo muscle that contains the parasite.”

The message to horse owners: Control the opossum, skunk and armadillo population on your farm. “The single most important animal to remove would be the opossum,” said Cheadle, who recently received his doctorate from the University of Florida.

Each year, several thousand new cases of EPM are reported, with 14 out of every 10,000 horses in the United States developing the disease, according to the National Animal Health survey. The number can be as high as 60 cases per 10,000 in performance horses, said UF equine medicine Professor Rob MacKay. No reliable estimates are available for how many horses live with the condition, but studies have shown that only 40 percent of horses appear to recover completely.

“In some horses, the signs may be very subtle and difficult to distinguish from a musculoskeletal problem or an obscure lameness,” said Stephen Reed, professor and head of equine medicine and surgery at The Ohio State University College of Veterinary Medicine. “Many affected horses show muscle wasting and profound weakness.”

Treatment for the disease involves the use of various medications, as well as other therapies that may act to increase the horse’s natural defense mechanisms, Reed added.

The parasite’s life cycle has confounded scientists nationwide who have worked diligently in recent years to identify all the stages of the disease cycle and be able to reproduce the disease in horses in order to better study it.

The opossum’s role as a definitive host for the parasite, a single-celled organism called Sarcocystis neurona, has been known since a 1995 discovery by University of Kentucky researchers. The animal that the opossum was eating to become infected remained unknown.

UF scientists have maintained an extensive EPM research program since the mid-1990s seeking to identify the intermediate hosts of the EPM parasite. Molecular tools developed by UF researchers Susan Tanhauser and John Dame helped lay the groundwork for further studies conducted by the EPM group.

While working in the laboratory of UF researcher Ellis Greiner, graduate students developed the idea of establishing an unusual “road-kill hot line” to investigate the program. Radio ads encouraged residents of Central Florida to phone the hot line when they spotted dead opossums along the roadside. Student assistants were charged with the unenviable task of collecting the opossum remains for scientists to gather samples of the EPM parasite.

“We’ve always wanted to know what the opossums were eating to become infected,” Cheadle said. “The intermediate host continues the life cycle of the parasite and makes it available to the opossum, so without it, the opossum would not become infected and pass the parasite on to the horse.”

After evaluating many different kinds of animals, Cheadle found that opossums became infected with, and released, S. neurona in their feces after being fed infected armadillo muscle. Colleagues at Washington State University fed the parasites to a horse, which developed antibodies to the parasite and transient EPM.

“It was very important to show that the parasites we collected from opossums fed armadillo muscle would cause clinical signs in the horse,” Cheadle said.

Cheadle also found that the striped skunk was in intermediate host to S. neurona. Skunks, deliberately infected with the EPM parasite, developed sarcocysts of S. neurona in their muscles. Opossums fed these infected skunk tissues produced the stage of the parasite infective for horses.

Researchers suspect that other animals, including the brown-headed cowbird and the domestic cat, may also play a role in the parasite’s life cycle.

“We now have methods for producing the parasite, which paves the way for further development of animal models, as well as new molecular tests, vaccine development and treatments for the parasite,” Cheadle said.

The UF study was supported by grants from the Florida Department of Business Regulation’s Division of Pari-mutuel Wagering and the U.S. Department of Agriculture.