HomeMy WebLinkAboutMemo - Mail Packet - 09/09/2025 - Memorandum from Ryan Vincent re West Nile Virus (WNV) - UpdateNatural Areas Department
1745 Hoffman Mill Road
PO Box 580, Fort Collins, CO 80522
970-416-2493
rvincent@fcgov.com
CC: Dean Klingner, Community Services Director; Katie Donahue, Natural Areas Director; Matt Parker, Natural Areas
Ecological Stewardship Manager 1 of 2
MEMORANDUM
Date: September 4, 2025
To: Mayor and City Councilmembers
Through: Kelly DiMartino, City Manager
Tyler Marr, Deputy City Manager
From: Ryan Vincent, Natural Areas Sr. Supervisor
Subject: West Nile Virus (WNV) - Update
The Larimer County Department of Health and Environment (LCDHE) does not
recommend spraying for adult mosquitoes at this time. With only two positive traps this
week, down from 18 last week, the citywide vector index dropped sharply from 0.42 to 0.04,
signaling a lower risk of WNV for the community.
Previous spray applications and/or cooler nightly temperatures may explain the sudden drop in
WNV activity. Mosquito infection rates and vector indexes usually decline around week 35,
which is the current testing period.
As of Aug. 27, the Colorado Department of Public Health and Environment reported six deaths
and 95 cases of WNV statewide, 20 of those are from Larimer County.
WNV ACTIVITY ACROSS THE CITY
Week 35 of 37
Quadrant Previous Week VI Current VI Previous Week
Positive Traps
Current Week
Positive Traps
NW 0.47 0.00 5 0
NE 0.57 0.08 5 1
SE 0.26 0.00 4 0
SW 0.46 0.09 4 1
Citywide 0.42 0.04 18 2
Floating Zone with
highest VI
NA* 0.25** NA* 1*
*Floating zones vary in size and location and cannot be compared from week to week.**VI Threshold for spraying in a floating zone is 0.75.
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WNV ACTIVITY COMPARED TO HISTORICAL AVERAGES
The abundance of Culex mosquitoes (the carriers of WNV) remains below average, and the
citywide infection rate and vector index have dropped sharply to below-average levels.
COMMUNITY COMMUNICATIONS
The community can access the latest data anytime at fcgov.com/westnile.
NEXT STEPS
• Mosquito trapping and testing for WNV will continue for two more weeks.
• In the absence of new data, the City will not spray after September 22.
• If the LCDHE recommends another spray in the next two weeks, staff will provide
Council a memo.
Docusign Envelope ID: 70D19787-DE94-445C-9FB9-8DABE7AC1CC5