The research group completed the S. neurona life cycle earlier this year for the first time in the laboratory with the domestic cat, and had been working on the current study since June. Two other intermediate hosts have been pinpointed in the spread of S. neurona--the nine-banded armadillo and the striped skunk--but the only other proven natural intermediate host is the armadillo, which lives in the southern region of the United States. The wide geographic range of the raccoon might explain why we see EPM in so many parts of the United States.
According to William Saville, DVM, PhD, Dipl. ACVIM, a principal investigator in the study, the researchers chose the raccoon as a potential intermediate host because it is a scavenger and because its potential role in EPM had not been examined closely before.
Saville and his colleagues took muscle from raccoons that were picked up through a pest control company and fed it to naive opossums, which in turn shed S. neurona sporocysts their feces. The sporocysts were fed to "knockout mice" (genetically altered, immunocompromised mice used for studying EPM), which then developed neurologic disease.
"We then took the sporocysts, gave them to horses which subsequently developed neurologic signs and seroconverted in serum and CSF (cerebrospinal fluid)," he said. "This accomplished in greater than ten horses, so we know it's real."The researchers also fed S. neurona sporocysts from opossum feces to raccoons in the laboratory, and for the first time were able to induce neurologic disease in this naturally infected host. This is significant because this intermediate host could be used as a model to study the disease.
He and other researchers also considered the raccoon as a potential definitive host in this study, but when fed sarcocysts from the muscle of laboratory-infected cats, the raccoon didn't shed sporocysts as the opossum readily does in its role as a definitive host. (A definitive host eats S. neurona-infected carrion and sheds the sporocysts in its feces.)Just as with other natural intermediate hosts, horse owners should not try to eliminate the live raccoon population in their area. Saville made the point, "The live ones are fine, it's the dead ones that are the problem--of which there can be many in places because of road kill." It is not yet clear to researchers whether any one intermediate host is more likely to be a factor in the spread of EPM than others.
The S. neurona exposure study, completed by the USDA and the Virginia-Maryland Regional College of Veterinary Medicine, demonstrated that 58% of 99 raccoons collected from four Eastern states (Florida, New Jersey, Pennsylvania, and Massachusetts) had antibodies to S. neurona. Antibodies were detected using the S. neurona agglutination test (or the SAT).The results from the raccoons were compared to estimated exposure levels of horses in those areas detected with the Western blot test (33-60% of horses).
"The seroprevalence of S. neurona in raccoons appears to accurately reflect the seroprevalence in horses, therefore raccoons may be good indicators of environmental contamination with S. neurona sporocysts," stated researchers in the current study.What's next for the EPM life cycle research?
"We're looking for more intermediate hosts and working on the equine challenge model," said Saville. "I think we're moving along quite well, but I still don't think we're where we want to be. We know already that the life cycle can be completed with the cat, armadillo, skunk, and raccoon, but how many other species are out there? Nobody knows."