Analysis of IgG responses to Sarcocystis neurona in horses with equine protozoal myeloencephalitis (EPM) suggests a Th1-biased immune response

Equine protozoal myeloencephalitis (EPM) caused by Sarcocystis neurona is one of the most important neurological diseases of horses in the Americas. While seroprevalence of S. neurona in horses is high, clinical manifestation of EPM occurs in less than 1 % of infected horses. Antemortem diagnosis has proven challenging as serum antibodies against S. neurona are an indicator of infection but not necessarily disease. Factors governing the occurrence of EPM are largely unknown, although horse immunity might contribute to EPM pathogenesis. Immunoglobulin G is the predominant antibody class in equine serum and consists of four subisotypes; IgG1/2 (IgGa) and IgG4/7 (IgGb) are thought to be indicative of a Th1, cell-mediated immune response, and isotypes IgG3/5 (IgG(T)) and IgG6 (IgGc) are thought to be indicative of a Th2, humoral response. Herein, we examined the hypothesis that EPM occurs due to an aberrant immune response, which will be discernable by IgG subisotypes. A modified ELISA was used to quantify S. neurona antigen-specific IgG sub-isotypes 1/2, 3/5, and 4/7. Based on documented serum concentrations of IgG subisotypes, standard curves were generated using sera from 21 healthy horses and S. neurona-specific IgG subisotype levels were determined in serum and cerebrospinal spinal fluid from infected diseased (n = 93) and infected normal (n = 116) horses. The mean IgG3/5 serum concentration and IgG1/2:IgG3/5 ratio against S. neurona were found to be significantly different between diseased and normal horses, suggesting that the immune response to S. neurona in EPM horses is skewed towards a Th1, cell-mediated response. Unfortunately, these differences were not sufficient for developing a serum-based immunoassay for EPM diagnosis.