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West Nile virus spreads in Europe and North America

Updated 22 times since CLSTR started tracking revisions of this situation.

What changed

2026-09-23 16:41 UTC → 2026-09-28 18:15 UTC · added ~~removed~~

In the United States, West Nile virus (WNV) continues to impact equine populations and is increasingly detected in mosquito populations. In Florida, an under-vaccinated 16-year-old Quarter Horse in Indian River County was euthanized after testing positive for both WNV and Eastern equine encephalitis (EEE), exhibiting neurological symptoms such as ataxia and extreme somnolence. In Colorado, a vaccinated yearling Paint filly in Weld County was also euthanized following neurological signs. Recent reports from the Colorado State Veterinarian’s Office confirm two additional cases: a 2-year-old Quarter Horse mare in Pueblo County exhibiting anorexia, abnormal mentation, and ataxia, and an unvaccinated 4-year-old Quarter Horse gelding in Montrose County showing muscle twitching, teeth grinding, weakness, and ataxia. Both horses remain alive. ~~Additional WNV cases were confirmed in Utah, with one horse testing positive in Weber County and two in Duchesne County.~~ New equine cases have emerged in ~~Washington~~ Washington, Kentucky, and ~~Kentucky.~~ Wyoming. In Franklin County, Washington, a 2-year-old filly ~~tested positive and~~ is recovering under veterinary care. In Washington County, Kentucky, an unvaccinated 3-year-old Tennessee Walking Horse mare was ~~euthanized after exhibiting dehydration, muscle fasciculations, and lower lip paralysis; this represents~~ euthanized, marking the first confirmed equine WNV case in Kentucky for 2026. ~~New~~ In Fremont County, Wyoming, the State Animal Health Official confirmed a WNV case in a horse. Additional mosquito detections have emerged in Illinois and Idaho. In Perry County, Illinois, the first positive mosquito sample of the season was confirmed near the Tamaroa Sewage Treatment Plant. In Bannock County, Idaho, a mosquito pool in the Swan Lake area tested positive; local officials have increased larvicide and adulticide efforts, ~~including drone and backpack spraying,~~ noting that dry, hot weather has created stagnant water conditions. In Europe, France has recorded a record number of autochthonous West Nile virus ~~cases, with 117 infections identified in mainland France between~~ cases. Between early May and September 21, ~~2026.~~ 2026, 117 infections were identified in mainland France.

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