The armadillo is an intermediate host for Sarcocystis neurona, the parasite that causes EPM in horses. The first evidence that the nine-banded armadillo (Dasypus novemcinctus) is a host was reported by a team of University of Florida and Washington State University researchers.

“We chose the armadillo to study because they are an available food source for opossums,” said Andy Cheadle, MS, one of the lead researchers and a PhD candidate at the University of Florida. Opossums are a definitive host for S. neurona and shed sporocysts, the life stage of S. neurona that infects horses.

Cheadle describes a definitive host as one in which a parasite undergoes a sexual reproduction; in an intermediate host, the parasite undergoes only asexual reproduction.
“Identifying the intermediate host or hosts may provide veterinarians and horse owners with a method of controlling the intermediate host and thus the S. neurona-infected opossums on horse farms,” said Cheadle.

To determine whether the armadillo is an intermediate host, the researchers gathered road-killed armadillos—a ready source of food for the scavenger opossum--—and captured live armadillos. DNA tests of sarcocysts from armadillo muscle showed that the animals were infected with S. neurona. Sarcocysts are cysts full of bradyzoites, one of the life stages of the parasite S. neurona that become embedded in the muscle of a host, in this case, the armadillo. In this study, researchers observed sarcocysts in 30 of 48 armadillos.
In addition, Western blot tests (see IES, Vol. 1, No. 3; Vol. 2, No. 2 & 4) of armadillo plasma revealed antibodies to S. neurona.

“The fact that all wild-caught armadillos were positive for antibodies to S. neurona on Western blot test suggests that they are a significant intermediate host for S. neurona in nature,” the researchers said. These armadillos were from Texas, Louisiana, and Arkansas.

Cheadle also used an electron microscope, which can magnify 100 times that of a light microscope, to examine the sarcocysts from the armadillos. He observed a structure similar to that of S. dasypi, a known parasite of the armadillo.

“We think S. dasypi is the same as S. neurona, but we don’t have the evidence in print,” said Cheadle. “We may end up renaming one of these parasites.”

The researchers also fed sarcocyst-infected armadillo muscle to non-infected opossums. Both wild-caught and laboratory-raised opossums were used.

After eating the armadillo muscle, the opossums became infected, as indicated by sporocysts observed in their intestine and feces.

“We’re really putting our money on the results of the lab-raised opossums,” said Cheadle. “Because the wild ones did have a chance to eat armadillo.” The researchers noted that opossums may have sporocysts in their intestine and shed a barely detectable number of sporocysts in their feces.

The researchers then inoculated a two-month old colt with sporocysts taken from an experimentally infected opossum. The colt subsequently showed antibodies to S. neurona in his serum and cerebrospinal fluid, and developed slight neurological signs.

“The colt’s clinical signs resolved and it was released from the study,” said Cheadle. “But all I wanted to see were the antibodies in the CSF. The clinical signs provided more conclusive evidence that the armadillo is an intermediate host of S. neurona.”

“The armadillo’s life style is a good avenue for getting infected with S. neurona sporocysts shed by the opossum,” he said. According to Cheadle, armadillos easily can ingest sporocysts as they root in the soil for insects and grubs.

And while knowing the armadillo is an intermediate host is important, definitive hosts, such as the opossum, should remain the primary concern for prevention of EPM, according to Cheadle.

“It is more important to control the definitive host on a horse farm,” he said. “You can have armadillos, but if there are no opossums it won’t matter.” That is, unless other definitive hosts yet to be identified are present.

As the armadillo only lives in the southern US and Central and South America, its identification as a natural intermediate host for S. neurona cannot explain the high sero-prevalence of S. neurona in horses in more northern parts of the US.

“My feeling is that there are other intermediate hosts,” said Cheadle. “They are probably a wide variety of closely related animals.” Cheadle would not elaborate, saying that other research identifying those hosts would be published soon.

Cheadle and his colleagues also suggest that the use of commercially available foodstuffs that have not been heat treated — and thus possibly contaminated with sporocysts — could account for the widespread S. neurona antibody prevalence in horses.

“There is no proof to back this up,” said Cheadle. “But the number of horses that are seropositive tends to indicate something widespread, something beyond an opossum defecating in a pasture.”

This article originally appeared in the June 2001 issue of International Equine Science. IES, an eight-page newsletter, provides the latest scientific information on the athletic horse – an advertising-free and quick-to-read monthly update on equine research. Call 802-888-6189 for subscription informaiton.