Sarcocystis neurona is the most common cause of equine protozoal myeloencephalitis (EPM) in horses in America. It is a single celled parasite belonging to the group called coccidia (Apicomplexa: Sarcocystidae) with opossums as the definitive hosts and a variety of mammals as aberrant or natural intermediate hosts . Only asexual stages have been identified in the aberrant intermediate hosts, and they are confined to the brain and spinal cord, and any part of the central nervous system (CNS) may be affected. In histologic sections of CNS, individual merozoites are about 3-5 um long and contain a single, centrally located vesicular nucleus. The sarcocysts are microscopic (~700 um long) with a 1-3 um thick sarcocyst wall. The bradyzoites are slender and tiny (~ 5 um long). Sarcocystis neurona sporocysts from opossum faeces are ~ 10 x 8 um in size (USDA Agricultural Research Service: Sarcocystis neurona).
4. Microbial Pathogenesis
The pathogenesis of EPM is not clear because the complete life cycle is unknown. Sarcocystis neurona can parasitize all regions of the CNS, from the anterior cerebrum to the end of the spinal cord. Sarcocystis neurona schizonts and merozoites are found in neurons, mononuclear cells, glial cells, and perhaps other neural cells (USDA Agricultural Research Service: Sarcocystis neurona).
5. Host Ranges and Animal Models
Opossums (Didelphis virginiana, D. albiventris) are its definitive (reservoir) hosts and excrete oocysts and sporocysts (environmentally resistant stage)in their feces. Raccoons, armadillos, sea otters, skunks, cats and possibly other mammals are intermediate hosts (USDA Agricultural Research Service: Sarcocystis neurona).
6. Host Protective Immunity
Cell mediated immunity is an important component of controlling this intracellular parasite (Marsh et al., 2004).
Molecule Role Annotation :
Horses were vaccinated with adjuvanted recombinant SnSAG1 (rSnSAG1)and 5 control (sham vaccinated) horses were vaccinated with adjuvant only. The effect of vaccination with rSnSAG1 on in vivo infection by S. neurona was evaluated by challenging all the horses with S. neurona merozoites. The study showed that vaccination with rSnSAG-1 produced antibodies in horses that neutralized merozoites when tested by in vitro culture and significantly reduced clinical signs demonstrated by in vivo challenge (Ellison and Witonsky, 2009).
Vaccination Protocol:
The horses were vaccinated on days 0 and 21 with 1 mL adjuvanted (Polygen; MVP Laboratories, Omaha, Nebraska, USA) rSnSAG1 (50 μg) or 1 mL adjuvant alone by IM injection in the left side of the neck (Ellison and Witonsky, 2009).
Challenge Protocol:
Horses in all groups were challenged on study day 36 with S. neurona merozoites (Ellison and Witonsky, 2009).
Efficacy:
Vaccination with rSnSAG-1 produced antibodies in horses that neutralized merozoites when tested by in vitro culture and significantly reduced clinical signs demonstrated by in vivo challenge (Ellison and Witonsky, 2009).
IV. References
1. Crowdus et al., 2008: Crowdus CA, Marsh AE, Saville WJ, Lindsay DS, Dubey JP, Granstrom DE, Howe DK. SnSAG5 is an alternative surface antigen of Sarcocystis neurona strains that is mutually exclusive to SnSAG1. Veterinary parasitology. 2008; 158(1-2); 36-43. [PubMed: 18829171].
2. Ellison and Witonsky, 2009: Ellison S, Witonsky S. Evidence that antibodies against recombinant SnSAG1 of Sarcocystis neurona merozoites are involved in infection and immunity in equine protozoal myeloencephalitis. Canadian journal of veterinary research = Revue canadienne de recherche veterinaire. 2009; 73(3); 176-183. [PubMed: 19794889].
3. Elsheikha and Mansfield, 2004: Elsheikha HM, Mansfield LS. Sarcocystis neurona major surface antigen gene 1 (SAG1) shows evidence of having evolved under positive selection pressure. Parasitology research. 2004; 94(6); 452-459. [PubMed: 15517384].
4. Marsh et al., 2004: Marsh AE, Lakritz J, Johnson PJ, Miller MA, Chiang YW, Chu HJ. Evaluation of immune responses in horses immunized using a killed Sarcocystis neurona vaccine. Veterinary therapeutics : research in applied veterinary medicine. 2004; 5(1); 34-42. [PubMed: 15150728].
