There have been 347 cases (344 confirmed, three probable) of clinical West Nile virus (WNV) infection in horses from 18 States (see Summary by state and county, below). Of 262 horses for which an outcome has been reported to date, at least 59 (22.5 percent) died or were euthanized. The numbers of equine deaths associated with WNV infection by state are: Connecticut, 3; Delaware, 1; Florida, 31; Georgia, 2; Illinois, 1; Louisiana, 3; Massachusetts, 1; Mississippi, 3; New Jersey, 5; New York, 2; North Carolina, 1; Pennsylvania, 4; Tennessee, 1; and Virginia, 1.

Of the 344 confirmed cases, 342 were positive on a WNV IgM-capture enzyme-linked immunosorbent assay (MAC-ELISA) and a WNV plaque reduction neutralization test (PRNT) performed on serum at the National Veterinary Services Laboratories (NVSL) in Ames, Iowa. One of the other two case horses had brain tissue that tested positive at the NVSL on WNV reverse transcriptase-nested polymerase chain reaction (RT-nPCR), had serum that was positive on WNV PRNT, and had WNV isolated from brain tissue. An additional horse was positive on WNV RT-nPCR on a virus isolate submitted to the NVSL for confirmatory testing.

Of the 342 serologically positive confirmed cases, brain tissues were submitted from 11 horses and tested positive on WNV RT-nPCR performed on those tissues. WNV was isolated at the NVSL from 2 of those 11 brains to date. All three probable case horses were positive on WNV RT-nPCR performed on brain tissue.

Note: Several States are currently reporting cases of clinical WNV infection in horses that have not yet been included here. Efforts are underway to gather additional information on such cases in order to be able to include as many as possible in future summary total reports