Monitoring West Nile Virus
The Larimer County Department of Health and Environment works with local mosquito control teams, cities, and Colorado State University to monitor mosquito trap data and assess the risk of West Nile virus (WNV) each week during the WNV season. This information is shared weekly on our website and helps guide decisions about mosquito spraying.
What is the Vector Index?
The Vector Index (VI) measures how many mosquitoes in an area are infected with West Nile virus. This number helps predict the risk to humans. The VI is based on the number of Culex mosquitoes (the type that carries WNV) and how many of them are infected. Studies have shown that when the VI is above 0.5, the risk of human infection increases.
Spraying for West Nile Virus Control
The decision to spray for mosquitoes to protect public health is based on WNV monitoring in mosquitoes. Public health officials recommend spraying when the risk index (VI) exceeds 0.5, as this is when human cases are more likely.
In Fort Collins, spraying is considered only when the risk index reaches 0.75 or higher, based on city policy. Since it can take up to 4 weeks to confirm human cases after infection, spraying is based on high numbers of infected mosquitoes, not human cases.
If spraying occurs, it will target areas with the highest risk of West Nile infection.
Nuisance Mosquitoes
Many mosquitoes are considered "nuisance mosquitoes," meaning they are a bother but do not carry the virus. The number of infected Culex mosquitoes varies each week in different areas. We track this using weekly trap data. Sometimes, an increase in nuisance mosquitoes doesn’t mean there is a risk of West Nile virus. We monitor mosquito populations with our control contractors every week.
Private Spraying
Some cities, towns, homeowner associations (HOAs), and businesses may choose to spray for mosquitoes, both for nuisance control and to reduce the risk of West Nile virus. These efforts are separate from the county’s activities.
Spraying by Homeowner Associations and the County
HOAs may decide to spray in their neighborhoods, independent of city or county actions. If an HOA requests spraying, they cover the cost, and we coordinate with them to prevent double spraying in the same area. Colorado Mosquito Control works with HOAs to ensure the spraying schedule is well-managed.
Mosquito Control on County Land
Currently, there is no funding for broader mosquito control efforts in Larimer County. If voters decide to fund a mosquito control district, that could change. For now, mosquito control efforts focus on population centers, and municipalities like Windsor, Timnath, Loveland, and Fort Collins cover the cost of spraying.
Notifications About Spraying
Fort Collins offers a text alert service to notify residents about spraying. To sign up, simply text "FCWNV" to 888-777 to receive updates by text.
Link to Surveillance Data Map Local Spray Schedule
Larimer County works with the local cities and towns to monitor West Nile virus.
Trap data helps the Larimer County Department of Health and Environment monitor the culex mosquito count in the area. Culex mosquitoes can carry West Nile virus, so watching these numbers help recognize risk in the area.
- Ensure greenbelts and all common areas have good drainage and tall grasses are cut
- Repair broken sprinkler heads
- Report standing water in nearby irrigation canals after the water has been turned off
- Add larva-killing BTI* granules or “dunks” to standing water in places not treated by the city’s contractor
- Clean gutters to allow proper drainage
- Clear storm drains to prevent standing water
- Stock ponds and basins with fathead minnows that eat mosquito larvae
- Change water in birdbaths at least weekly (twice a week in hot weather)
- Clean and maintain swimming pools
- Prevent boats from holding rain water. Store upside down, or sprinkle BTI* granules where water collects
- Ensure outdoor toys, playground equipment, and household items don’t collect water
*BTI contains spores that produce toxins that target mosquito larvae.
