Colorado’s first case of West Nile virus from 2024 confirmed

Arapahoe County on Wednesday reported Colorado’s first known human case of West Nile virus for 2024, marking an early start to the mosquito-borne disease’s season.

Most of the time, Colorado doesn’t see it first cases of West Nile until July, and they leave in August. Mosquito activity increases when the days are warm and the nights remain relatively warm.

“While we know that West Nile Virus is endemic in Colorado — meaning we expect a few cases each year — we never know exactly how many cases to expect or how bad a season will be,” said Melissa Adair, communicable disease epidemiology manager at Arapahoe. County Public Health, said in a news release. “What this first case confirms for us, however, is that West Nile Virus is present in our mosquito population, and as mosquito numbers increase, so will the risk of West Nile Virus.”

Last year in Colorado, there were 634 known cases in more than half of the state’s counties, the highest number since 2003, when the virus arrived in the state. Fifty-one of those people died and 386 spent time in the hospital.

To prevent infection with West Nile virus:

• Wear insect repellents with DEET or another ingredient approved by the Environmental Protection Agency during outdoor activities
• Avoid spending time outdoors around dawn and dusk, when mosquitoes are most active
• Consider using mosquito nets when camping
• Avoid standing water sources, such as ponds
• Wear long-sleeved shirts and long pants when outdoor activities
• Drain flower pots, tires and other objects that collect water and that mosquitoes use to breed
• Keep window screens in good condition
• Maintain your septic tank, if you have one, so that mosquitoes cannot breed in it

All Colorado counties have reported at least one case of West Nile virus in humans since 2003. No one knows how many people have had the virus because most don’t get sick.