Immunoglobulin M-capture enzyme-linked immunosorbent assay testing of cerebrospinal fluid and serum from horses exposed to West Nile virus by vaccination or natural infection

The West Nile (WN) virus, present in the United States since 1999, is a cause of encephalomyelitis in birds, alligators, humans, and horses. No data exist regarding detection of anti-WN virus immunoglobins in equine cerebrospinal fluid (CSF). The aims of this study were to evaluate the blood-brain barrier (BBB) in WN virus-infected (WNE) horses, to compare diagnostic testing in serum and CSF, and to describe the immunoglobulin M (IgM) response in serum and CSF of vaccinated horses. CSF was collected from the lumbosacral (LS) space (n = 13) or the allanto-occipital (AO) space (n = 14) of WNE horses. The albumin quotient (AQ) and IgG index were calculated, and the IgM-capture-enzyme-linked immunosorbent assay (MAC-ELISA) was used to detect anti-WN virus IgM in serum and CSF. CSF collected from the LS site had a higher (P < .02) IgG index compared to the AO site (0.34 ± 0.04 versus 0.22 ± 0.04 [mean ± SE], respectively). The mean AQ, irrespective of collection site, did not exceed reference values. There was 100% agreement between CSF and serum testing for IgM by MAC-ELISA testing. However, the positive to negative antigen ratios were higher (P < .001) in CSF (34.5) versus serum (8.5), indicating lower nonspecific reactivity in CSF samples. Horses vaccinated against WN virus did not develop an IgM response at 1:400 mg/dL in serum; however, a few horses developed a weak IgM response in serum but not in CSF. In conclusion, MAC-ELISA testing of serum and CSF were equivocal. Also, examination of CSF data from WNE horses suggests a normal BBB integrity and increased intrathecal production of antibodies.