Fort Collins mosquitos test positive for West Nile virus

Ravyn Cullor

Mosquitos caught in Fort Collins, Weld County and Berthoud have tested positive for the West Nile virus (WNV), according to a news release from the City of Fort Collins.

According to the release, a variety of mosquito called Culex mosquitos tested positive with a vector index of 0.004. The vector index is the approximate proportion of mosquitoes trapped per night which are infected with WNV, according to the Center for Disease Control.


The vector index has to be 0.75 in all four trapping zones for the City to deploy adulticide, a pesticide which specifically targets the adult of the species, according to the City’s release. However, Larimer County has a lower threshold for adulticide of 0.5 and the Director of the Health and Environment Department may recommend earlier action.

According to Larimer County, investigations into West Nile outbreaks have shown that once a vector index is over .5, the risk of humans getting infected with West Nile virus increases. If Larimer County decides to spray adulticide, it does so to target areas at the highest risk of West Nile infection.

While the risk of getting WNV is still low, the City recommends draining standing water near homes, wearing long-sleeved shirts and full-length pants outside, using insect repellent and limiting time outdoors between dusk and dawn to avoid contact with infected mosquitos.

The Four D’s of West Nile virus prevention, according to the City:

  • Drain: Since mosquitoes breed in water, you should drain any standing water in your yard each week. Bird baths, clogged gutters and kiddie pools are common breeding sites.
  • Dress: Wear lightweight, long-sleeved shirts and long pants while outdoors. Spray clothing with insect repellent since mosquitoes may bite through clothing. 
  • Defend: Apply insect repellent sparingly to exposed skin. Use an approved repellent according to its label.  You can visit the Environmental Protection Agency’s website to find the repellent that fits your protection needs. 
  • Dawn/Dusk: Limit time spent outdoors at dawn through dusk, when mosquitoes are most active and feeding.

WNV can cause neurological disease and death in people, according to the World Health Organization

While 80 percent of people do not develop any symptoms after being infected with WNV, some develop fever, headache, body aches, joint pains, vomiting, diarrhea or rash, according to the CDC. Most people who present with symptoms recover fully, but one in 150 people may develop illness which affects the central nervous system.

Those over the age of 60, those who have certain medical conditions, such as cancer, diabetes, hypertension or kidney disease and those who have had organ transplants are at a higher risk of developing serious symptoms, according to the CDC.

For more information about the concentration of Culex mosquitos in Fort Collins, check out the City’s density map, which shows the number of Culex females in each trap location during West Nile season

To learn more about WNV and how to protect your family, visit

Collegian reporter Ravyn Cullor can be reached at or on Twitter @RCullor99.