Weekly Vector Index#
WNV vector index levels updated weekly, June 8 through Sept. 20.
Last updated: August 8, 2025. Next scheduled update: August 12, 2025.
Vector Index by Zone (This Week vs. Last Week):
- Northwest: 0.32 ↑ (from 0.11)
- Northeast: 0.42 ↑ (from 0.23)
- Southeast: 0.25 ↑ (from 0.16)
- Southwest: 0.1 ↑ (from 0.09)
Citywide Average: 0.28↑ (from 0.16)
5-Trap Floating Zone: 0.88
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This week’s floating zone (or the area with the highest VI value) was located between Prospect Road and Carpenter Road, and between Timberline Road and I-25.
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Trap Reports- The first step toward determining vector indexes#
These trap reports come from Vector Disease Control International, the company that helps the City manage mosquitoes, and monitor the City's 53 mosquito traps. The reports show how many of the mosquitoes caught are Culex—the type that can spread West Nile virus (WNV)—compared to other kinds. The mosquitoes are sent to Colorado State University to test how much WNV they’re carrying. When there are a lot of Culex mosquitoes and many of them have the virus, the risk to people—measured by the vector index—goes up.
Most recent three weeks' trap data:
How does the vector index determine a spray area?
The decision to spray for adult mosquitoes is made by the Larimer County Health Department. They recommend spraying when at least five mosquito traps in an area show a high risk of West Nile virus—specifically, when the vector index is 0.75 or higher.
The City of Fort Collins runs 53 mosquito traps across the city and checks them every week to see how many mosquitoes are carrying West Nile virus.
Even though we report risk levels by large areas called quadrants, mosquitoes don’t follow these boundaries. So, spraying zones are based on where the data shows the most risk—not on maps or neighborhoods. These zones may be smaller or cross over quadrant lines. We call this a, "floating zone."
Spraying might happen even if the overall risk for a big area looks low, because smaller pockets of high-risk mosquitoes can still be dangerous. We only consider spraying if there are at least five traps showing enough data to back it up.
Our goals are to:
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Use pesticides carefully and only when needed
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Spray the smallest area possible
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Act early enough to stop the virus from spreading too much
Interactive Mosquito Surveillance & Control Map#
Our mosquito control contractor, Vector Disease Control International, has interactive mosquito surveillance and control maps you can review.
Want to learn even more?#
Vector Disease Control International provides the following reports with lots of great information-
Monthly Reports: