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West Nile Virus Detected in McHenry County Mosquitoes for First Time This Season

Officials are urging residents to follow strict mosquito-control guidelines to prevent human infections during the active season.

HEMET, CA – APRIL 26: A field sample of mosquitoes that could carry West Nile Virus is seen at offices of the Riverside County Department of Environmental Health on April 26, 2007 in Hemet, California. California health officials announced this week that West Nile Virus season is starting earlier than usual because of an unusually warm March. Mosquitoes that carry the virus have begun breeding earlier than usual and the West Nile Virus has been detected in mosquito pools, birds, or horses in eight California counties. Although the virus has not been detected in humans so far this year, 24 people have died and 1,200 sickened by the virus over the past two years in California. West Nile can be transmitted from infected birds, squirrels, and other animals to humans and animals such as horses through several varieties of female mosquitoes. The disease first appeared in the United States in 1999 in New York and killed hundreds of people during its westward expansion before gaining a firm foothold in California in 2004. In reaction to the early start of West Nile Virus season, the California State Legislature this week declared April 23-29 “West Nile Virus and Mosquito and Vector Control Awareness Week”. The effected counties include Imperial, Los Angeles, Orange, Riverside, Santa Clara, San Diego, Sonoma and Stanislaus. (Photo by David McNew/Getty Images)
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Overview

  • Mosquito sample from Lake in the Hills tested positive for West Nile Virus on June 2, marking the first detection of the season in McHenry County.
  • No human cases have been reported in McHenry County or anywhere in Illinois in 2025, according to the Illinois Department of Public Health.
  • The McHenry County Department of Health will continue mosquito surveillance through October, conducting weekly tests to monitor virus activity.
  • Residents are urged to follow the '4 Ds of Defense'—drain standing water, defend with EPA-approved repellents, dress in protective clothing and limit exposure from dusk to dawn.
  • While most infected people show no symptoms or mild flu-like signs 2–14 days after a bite, Illinois recorded 69 West Nile cases and 13 deaths in 2024, with severe illness more likely in older adults or those with underlying conditions.