Experimental induction of equine protozoal myeloencephalitis in horses using Sarcocystis sp. sporocysts from the opossum (Didelphis virginiana).
Sarcocystis sp. sporocysts isolated from 8 feral opossums (Didelphis virginiana) caught in Kentucky, USA, were pooled and fed to 18 commercially reared budgerigars (Melopsittacus undulatus), 14 wild-caught sparrows (Passer domesticus), one wild-caught slate-colored Junco (Junco hyemalis) and 5 weanling horses (Equus caballus). All budgerigars died within 5 weeks pi. Histological examination revealed meronts within the pulmonary epithelia and typical S. falcatula sarcocysts developing in the leg muscles. Sparrows were euthanized 13 and 17 weeks pi and their carcasses were fed to 4 laboratory-raised opossums. Sporocysts were detected in the faeces of 2 opossums on 15 days pi and in a 3rd opossum on 40 days pi. Faecal samples from the 4th opossum remained negative; however, sporocysts were found in intestinal digests from all 4 opossums. Sporocysts were not found in the faeces or intestinal digest of an additional opossum that was fed 3 uninoculated sparrows. Five foals were fed sporocysts (Foals 2, 3, 4, 5 and 7) and 2 foals were maintained as uninoculated controls (Foals 1 and 6). Sporocysts from 2 additional feral opossums also were fed to foals. Foal 5 was given 0.05 mg/kg dexamethasone sodium phosphate daily beginning 2 days before inoculation for a total of 2 weeks. Horse sera were tested 3 times/week, and cerebrospinal fluid (CSF) samples were tested biweekly for anti-Sarcocystis neurona antibodies by Western blot analysis. No foals had any S. neurona-specific antibodies by Western blot analysis prior to sporocyst ingestion. Seroconversion occurred in Foals 3, 5 and 7 by 24 days pi, followed by positive CSF tests on 28 days pi. Foals 2 and 4 seroconverted by 40 days pi. Cerebrospinal fluid from Foal 2 tested positive by 42 days pi, but Foal 4 remained seronegative throughout the study. Sera and CSF from control Foals 1 and 6 remained seronegative. All foals with positive CSF developed neurologic clinical signs. Neurological disease was evident in Foals 2 and 3 by 42 days pi and in Foal 7 by 28 days pi. The severity of clinical signs progressed to marked spasticity, hypermetria and ataxia in Foal 7 by the end of the trial. Necropsy examination of inoculated foals did not reveal gross lesions; however, microscopic lesions consistent with equine protozoal myeloencephalitis (EPM) were found in Foals 2, 3 and 7. Protozoa were not observed in the tissue sections. Microscopic lesions consistent with EPM were not found in Foals 4 and 5 nor in uninoculated control Foals 1 and 6. Foal 5 had unilateral non-inflammatory lesions in the cervical and thoracic spinal cord consistent with cord compression. It is concluded that the opossum is a definitive host of S. neurona.