Etiology/Life Cycle

Sarcocystis neurona has been cultured from CNS lesions of eight horses from several different locations: New York, California (3), Panama, and Kentucky (3). Preliminary morphologic, immunologic, and DNA comparisons have detected only minor differences among isolates. This stage of Sarcocystis spp. is not known to be transmissible to other animals. Transplacental infection has not been reported but cannot be ruled out. Many normal foals have been produced by EPM suspect mares. Recently one of the mares was euthanized and a diagnosis of EPM was histologically confirmed. Her foal, taken at euthanasia, died of pneumonia at eight weeks of age without neurologic signs. No lesions were present in the CNS. Although the mare was immunoblot positive in serum and CSF, the foal was immunoblot negative. The earliest EPM case reported occurred in a two month old foal. If transplacental transmission does not occur, the minimum incubation period may be eight weeks. However, a recent case suggests the incubation period may be much shorter. Serum and CSF collected 4 days after onset of clinical signs were both negative for antibodies to S. neurona. Serum and CSF collected 31/2 weeks later were both positive. This indicates that the parasite was ingested and caused clinical signs in the 10-12 days required to produce a detectable antibody response.

Sarcocystis spp. belong to the phylum apicomplexa which includes several genera of coccidia that utilize an obligatory, predator-prey life cycle. The host range for an individual species of Sarcocystis is usually narrow. Sarcocystis spp. produce sporulated oocysts in the gut wall of the appropriate predator or definitive host (opossum). However, the oocyst wall is very fragile and usually ruptures before being passed in the feces. Infective sporocysts are introduced into the food and water supply of the prey animal or intermediate host by fecal contamination from the predator. Birds and insects may serve as transport hosts to further disseminate sporocysts. Original work performed both in the laboratory and out using Sarcocystis gigantea sporocysts suggested that there were environmental factors that may affect the life of the sporocyst. It appears that the organism does not survive well in extremes of hot or cold, freezing and thawing, nor extremes of ultraviolet radiation. Nevertheless, there was speculation that the organism could survive and remain viable up to one year. Recent studies using Sarcocystis cruzi sporocysts, a different species, corroborates what was found in the earlier work, however, there are indications that the sporocysts may remain viable for shorter periods of time. Although these are not S. neurona sporocysts, the effects of climate may also apply, and therefore may affect transmission of the disease to your horse.

Once ingested by the intermediate host (birds), sporocysts release sporozoites which penetrate the gut and enter arterial endothelial cells in various organs. Meronts develop rapidly and eventually rupture the host cell releasing merozoites into the bloodstream. This is usually followed by a second round of merogony in capillary endothelial cells throughout the body. Second generation merozoites are released into the blood stream and usually enter skeletal muscle cells where they develop into specialized meronts known as sarcocysts. Mature sarcocysts contain bradyzoites which are only able to complete the life cycle when ingested by the appropriate predator or scavenger.

Sarcocysts of S. neurona have not been found in affected horses, precluding transmission of the parasite to the definitive host. The horse is an aberrant, dead-end host. Sarcocystis neurona probably cycles normally between 2 or more wildlife species. The list of animals that may serve as the true intermediate host(s) is extremely long. Although many species have been suggested as the true definitive host, skunks, raccoons, or opossums were the most probable. Like EPM, these species are unique to the Western hemisphere. Immunoblot testing of serum from several skunks demonstrated the presence of antibodies to S. neurona-specific proteins. Raccoons and opossums tested negative. Skunks may be inadvertently exposed, similarly to horses, but the absence of antibodies in other wildlife species suggests that parasites are penetrating the gut and stimulating an immune response in the skunk only.

In 1995, results of PCR analysis provided strong evidence that the opossum is the definitive host of Sarcocystis neurona. This study revealed a 99.67% homology with the opossum sporocyst, Sarcocystis falcatula. These results have been corroborated by others. Experimental induction of EPM has just been completed by investigators at the University of Kentucky, which further corroborates that the opossum is the definitive host for this organism. The opossum is indigenous to North, Central and South America which coincides with the fact that cases of EPM have only been reported in horses that have lived in areas that the opossum inhabits.

Additional research by investigators from California indicates that there may be other organisms that cause EPM. This work suggested that Neospora sp. may be one of those causes. Neospora is a worldwide cause of abortion particularly in cattle. In contrast, all previous research indicates that EPM is a disease of the Americas which is related to the definitive host, the opossum, who happens to be indigenous to the Americas. Therefore, the likelihood of Neospora being a major cause of EPM is very small.

Unlike most Sarcocystis spp., S. neurona (falcatula) may aberrantly infect a large number of intermediate hosts such as it does in the horse. Reports in the literature indicate infection with this organism may be possible in dogs, sheep, cats, mink, raccoons, striped skunks, golden hawks, rhesus monkeys and chickens. The natural intermediate hosts are cowbirds and grackles, however earlier work suggests that the organism may form sarcocysts in the muscle of many species of birds from several different Orders. This wide host range is atypical for Sarcocystis spp., however, this behavior is similar to Toxoplasma gondii which may be phylogenetically similar. This would suggest the possibility of more than one species of S. falcatula or perhaps there are subspecies. Recent work generated by the University of Florida would suggest that S. neurona is distinct from S. falcatula and that the natural intermediate host for S. neurona is not the cowbird or grackle. It would appear that the opossum does harbor more than one species of Sarcocystis. More recent evidence would suggest that the Opossum harbors at least three species of Sarcocystis. The three species likely include S. neurona, S. falcatula and S. speeri.

This information was generated using nude mice and gamma-interferon knockout mice, therefore, further research is needed to determine the effect of all three species in the horse. More research is needed to determine the exact life cycle of this organism. The most recent evidence regarding the life cycle of S. neurona has taken the top spot in the news. Back in the fall of 2000, it was demonstrated that experimentally the domestic house cat can be used to complete the life cycle of this organism. In this study, the sporocysts were administered to both IFN-gamma KO mice and to ponies resulting in death in the mice and seroconversion and neurologic deficits in the ponies.

As far as the cat role in nature, that is yet to be determined. Subsequent to the cat discovery, it was demonstrated that the nine-banded armadillo is naturally infected with S. neurona. Serum samples from 19 wild-caught armadillos were all positive for anti-S. neurona antibody. Sarcocyst-infected muscles from the armadillo were fed to laboratory raised opossums resulting in shedding of sporocysts. The sporocysts were administered to a two-month old foal resulting in immunoconversion and development of neurologic deficits. The latest intermediate host determination involves the striped skunk. The naive skunks were administered S. neurona sporocysts from a naturally infected opossum and subsequently developed sarcocysts in the muscle. Sarcocyst-infected muscle was then fed to laboratory-raised opossums which shed sporocysts in their feces. The sporocysts were the administed to IFN-gamma KO mice and a two-month old foal with similar results to what has been demonstrated previously with the cat-derived sporocyst. Since early work done by Granstrom demonstrated S. neurona antibody in wild-caught skunks, likely they do play a role in nature.

In addition to the many intermediate hosts, there is speculation that other vectors may be involved. It does not seem feasible that there are enough opossums to disseminate this organism all over the US. The birds are widely distributed across the country and very well may serve as transport vectors. As well, there are other suspected transport vectors such as the cockroach and perhaps other insects.

It seems likely that sporozoites penetrate the horses' intestinal tract, enter vascular endothelial cells, and must pass through the vascular endothelium of the blood-brain barrier to reach the CNS. It is uncertain whether merozoites pass through the blood-brain barrier within leucocytes or cross directly through the cytoplasm of endothelial cells.

The clinical severity of experimental sarcocystosis, in appropriate intermediate hosts studied, is directly related to the number of sporocysts fed. It appears that the ability of any individual to resist infection is related to the size of the infective dose, immune competency, environmental stress, and the species of Sarcocystis. Some individuals may be inherently more susceptible to infection, which may have variable heritability. It seems premature to speculate about this possibility until more is learned about the pathogenesis of clinical infection.