(North America) Equine Arbovirus season is upon us and 23 cases of West Nile Virus (WNV) have been diagnosed in the past month at the AHDC. Two cases have been reported in New York. The first was in Steuben County in an unvaccinated 15-year-old gelding who was hospitalized for ataxia and generalized muscle fasciculations. The horse recovered. The second was in Suffolk County, in a 5-year-old mare who presented with a fever, dysphagia and recumbency, and died within 24 hours. The mare was also unvaccinated.

Other cases have been diagnosed in PA, CT, DE, MD, OH, WI, GA, MT, TX and Alberta, Canada. Most horses have no recent West Nile vaccine history. Clinical signs have been varied and include the following: fever, ranging from low grade to 106 F, moderate to severe ataxia, muscle fasciculations, occasional body tremor, urine dribbling, absent tail tone and anal tone, recumbency, lip droop and eyelid droop, inability to swallow an esophageal tube, and head shyness. Horses have ranged in age from 2 years to 27 years old and include a variety of breeds.

The WNV IgG/IgM capture ELISA, run on serum, is the test of choice. A positive IgM antibody value is diagnostic for recent infection with WNV. Each report contains the following:

''West Nile Virus IgG and IgM Capture ELISA interpretation statement:
<2.5 = Negative
2.5-3.3 = Inconclusive
>3.3 = Positive

WNV IgG antibodies in horse serum can originate from infection with WNV or from vaccination. Most infected horses with WN IgM positive results also have WNV IgG antibodies in their serum.''

IgG antibodies can be used to measure vaccine response; a value of >20 on the WNV IgG ELISA is considered a reasonable response to vaccination.

Distribution of WNV IgG/IgM capture ELISA results over the past six years: