Abstract
Immunoglobulin M (IgM) and G (IgG) capture enzyme-linked immunosorbent assays (ELISAs) were used as possible adjuncts to hemagglutination inhibition (HI) and virus neutralization (VN) tests to differentiate between reaction to recent exposure to eastern equine encephalomyelitis (EEE) virus and those due to prior vaccination. Serum samples were evaluated by the IgM-capture ELISA, and the results were compared with those of HI and VN tests. Of 381 serum samples, 51% (195 samples) were positive by HI test (≥1:40) and 54% (205 samples) were positive by VN test (≥ 1:10), but only 35% (132 samples) were positive by IgM-capture ELISA (≥ 1:100). With only a few exceptions, the sera with IgG ELISA titers had a VN titer of ≥1: 100. When EEE virus isolation and serology were compared, the EEE cases were divided into three categories: 1) peracute cases- the serum was negative for EEE IgM and IgG by the ELISA, negative for VN antibody, but HI antibody positive; 2) acute cases-IgM and HI antibody positive but negative for IgG and VN antibody; and 3) transitional cases–positive for IgM and IgG antibodies, HI titers of 1:40–1:160, and VN titers of ≥ 1: 100. IgM antibodies of EEE virus were monospecific and did not cross-react with western or Venezuelan equine encephalomyelitis viral antigens by the ELISA. Isolation of EEE virus from tissue or detection of IgM antibodies to EEE virus in a single serum sample is evidence of recent EEE infection and differentiated serum from previously vaccinated horses. However, the presence of IgM in the absence of virus isolation could also be due to a recent viral vaccination.