Are Sarcocystis neurona and Sarcocystis falcatula synonymous? A horse infection challenge

Equine protozoal myeloencephalitis (EPM) is a debilitating neurologic disea se of the horse. The causative agent, Sarcocystis neurona, has been suggest ed to be synonymous with Sarcocystis falcatula, implying a role for birds a s intermediate hosts. To test this hypothesis, opossums (Didelphis virginia na) were fed muscles containing S. falcatula sarcocysts from naturally infe cted brown-headed cowbirds (Molothrus ater). Ten horses were tested extensi vely to ensure no previous exposure to S. neurona and were quarantined for 14 days, and then 5 of the horses were each administered 10(6) S. falcatula sporocysts collected from laboratory opossums. Over a 12-wk period, 4 chal lenged horses remained clinically normal and all tests for S. neurona antib ody and DNA in serum and cerebrospinal fluid were negative. Rechallenge of the 4 seronegative horses had identical results. Although 1 horse developed EPM, presence of S. neurona antibody prior to challenge strongly indicated that infection occurred before sporocyst administration. Viability of spor ocysts was confirmed by observing excystation in equine bile in vitro and b y successful infection of naive brown-headed cowbirds. These data suggest t hat S. falcatula and S. neurona are not synonymous. One defining distinctio n is the apparent inability of S. falcatula to infect horses, in contrast t o S. neurona, which was named when cultured from equine spinal cord.