Toxoplasma gondii seroprevalence and association with equine protozoal myeloencephalitis: A case-control study of Californian horses.

While toxoplasmosis is not commonly considered a clinical disease of equines, previous seroprevalence studies have reported differing background rates of Toxoplasma gondii infection in horses globally. The objective of this study was to evaluate possible associations between T. gondii seroprevalence and clinical signs of equine protozoal myeloencephalitis (EPM) in horses. Using a case-control study design, 720 Californian horses with neurologic signs compatible with EPM were compared to healthy, non-neurologic horses for the presence of T. gondii antibodies (using indirect fluorescent antibody tests [IFAT]). Toxoplasma gondii seroprevalence among cases and controls was determined at standard serum cut-offs: 40, 80, 160, 320, and 640. At a T. gondii titre cut-off of 320, horses with clinical signs compatible with EPM had 3.55 times the odds of a seropositive test compared to those without clinical signs (P<0.01) when adjusted for covariates. When restricted to the autumn season and at the same titre cut-off, an EPM suspect horse had 6.4 times the odds of testing seropositive to T. gondii, compared to non-neurologic horses. The association between high T. gondii titres and clinical signs compatible with EPM is potentially reflective of toxoplasmosis in equines. Serologic testing of cerebrospinal fluid and isolation of T. gondii in EPM suspect cases should be considered. Future studies investigating the relationship between T. gondii and EPM are warranted.