West Nile virus found in Larimer County mosquitoes

Samantha Ye

Mosquitoes in southeast Fort Collins and Loveland have tested positive for West Nile virus for the first time this year, according to the Larimer County Department of Health and Environment.

No human cases of WNV have been reported in Colorado so far this year. 


“Now that West Nile virus has been found in Larimer County, it is time to be proactive,” said Shaun May, Larimer County environmental health director, in a press release. “Make sure you are wearing insect repellent when mosquitoes are biting. Take the time to check your property, and make sure you don’t have standing water that might allow mosquitoes to breed and contribute to the spread of West Nile virus.” 

West Nile virus can be spread to humans through infected Culex mosquito bites. About 75% of people infected will show no symptoms, but the other 25% will develop West Nile fever, according to the press release. Less than 1% will develop the severe and potentially fatal neuroinvasive form, West Nile encephalitis.

Symptoms of mild infection include fever, headache, body aches and potentially skin rashes and swollen lymph nodes. 

The City recommends individuals protect themselves against the virus using the four D’s.

  • Defend against bites by using an effective insect repellent like DEET, picaridin, oil of lemon eucalyptus or IR3535.
  • Avoid being outdoors between dusk and dawn, which is the Culex mosquitoes’ peak feeding hours.
  • Dress in long sleeves and pants to minimize bitable skin surface.
  • Drain standing water in yards and gardens which serve as breeding areas for mosquitoes.

Larimer County began trapping mosquitoes to monitor for WNV risk in June. The Larimer County Department of Health and Environment, Colorado State University and mosquito abatement company Vector Disease Control International work together to monitor and assess the risk of the virus. 

Fort Collins monitors the density of Culex mosquitoes in a public map. They also publish reports on the vector index, an approximate proportion of mosquitoes trapped per night that are infected with WNV.

The most recent report shows the highest vector index is 0.10 in northeast Fort Collins. It has to reach 0.75 to trigger a City mosquito spraying in the area.

Samantha Ye can be reached at news@collegian.com or on Twitter @samxye4.