The epidemiology of equine protozoal myeloencephalitis (EPM)

Equine Protozoal Myeloencephalitis (EPM) is the most common neurologic disease of horses presented to the OSU Veterinary Hospital. The purpose of these studies was to investigate the epidemiology of EPM. We examined the prevalence of serum antibody to Sarcocystis neurona using a random sample of horse serum. The rate of exposure increased with age. Breed and gender were not associated with seroprevalence, but there were geographic differences. We followed horses (n = 6) that were neurologically normal and negative for serum antibody to Sarcocystis neurona for approximately one year to investigate natural exposure to this parasite. Horses naturally exposed to S. neurona may not develop clinical signs of EPM for long periods and some may never develop clinical signs. A retrospective case-control study was performed to examine risk factors for EPM and factors associated with improvement and survival in EPM cases. Risk factors that appear to increase the risk of EPM include age, season, horses located on premises which had previously diagnosed cases of EPM, premises where opossums are commonly seen, the occurrence of stressful events prior to admission, and race or show horses. Factors that reduced the risk of developing clinical signs of EPM included protection of feed from wildlife access and presence of a creek or river on the premises. The likelihood of improvement in clinical signs was lower in breeding and pleasure horses, but higher in horses treated with antimicrobials. Two factors were associated with the likelihood of survival in EPM cases, severity of clinical signs at presentation and improvement in clinical signs. Neurologic and non-neurologic horses were examined prospectively to compare agreement between board certified internists. There was good overall agreement when examining for presence or absence of neurologic signs, but poor agreement in diagnosing specific clinical signs. Prospective examination of ante-mortem tests used for diagnosis of EPM found that Western blot appears to have reasonable sensitivity but poor specificity in CSF. PCR testing appears to be insensitive. CSF indices also may not be helpful in diagnosing neurologic disease. There were no differences between clinically normal and horses with neurologic disease in parameters evaluated during routine CSF analysis.