Immunoconversion against Sarcocystis neurona in normal and dexamethasone-treated horses challenged with S-neurona sporocysts

Equine protozoal myeloencephalitis is a common neurologic disease of horses in the Americas usually caused by Sarcocystis neurona. To date, the diseas e has not been induced in horses using characterized sporocysts from Didelp his virginiana, the definitive host. S. neurona sporocysts from IS naturall y infected opossums were fed to horses seronegative for antibodies against S. neurona. Eight horses were given 5 x 10(5) sporocysts daily fur 7 days. Horses were examined for abnormal clinical signs, and blood and cerebrospin al fluid were harvested at intervals for 90 days after the first day of cha llenge and analyzed both qualitatively (western blot) and quantitatively (a nti-17 kDa) for anti-S. neurona IgG, Four of the challenged horses were giv en dexamethasone (0.1 mg/kg orally once daily) for the duration of the expe riment. All challenged horses immunoconverted against S, neurona in blood w ithin 32 days of challenge and in CSF within 61 days. There was a trend (P = 0.057) for horses given dexamethasone to immunoconvert earlier than horse s that were not immunosuppressed. Anti-17 kDa was detected in the CSF of al l challenged horses by day 61. This response was statistically greater at d ay 32 in horses given dexamethasone. Control horses remained seronegative t hroughout the period in which all challenged horses converted. One control horse immunoconverted in blood at day 75 and in CSF at day 89. Signs of neu rologic disease were mild to equivocal in challenged horses. Horses given d examethasone had more severe signs of limb weakness than did horses not giv en dexamethasone; however, we could not determine whether these signs were due to spinal cord disease or to effects of systemic illness. At necropsy, mild-moderate multifocal gliosis and neurophagia were found histologically in the spinal cords of 7/8 challenged horses. No organisms were seen either in routinely processed sections or by immunohistochemistry. Although neuro logic disease comparable to naturally occurring equine protozoal myeloencep halitis (EPM) was not produced, we had clear evidence of an immune response to challenge both systemically and in the CNS. Broad immunosuppression wit h dexamethasone did not increase the severity of histologic changes in the CNS of challenged horses. Future work must focus on defining the factors th at govern progression of inapparent S. neurona infection to EPM. (C) 2001 E lsevier Science B.V. All rights reserved.