HomeMy WebLinkAboutMemo - Mail Packet - 08/05/2025 - Memorandum from Ryan Vincent re West Nile Virus – UpdateNatural Areas Department
1745 Hoffman Mill Road
PO Box 580, Fort Collins, CO 80522
970-416-2493
rvincent@fcgov.com
CC: Mike Calhoon, Parks Director; Katie Donahue, Natural Areas Director; Matt Parker, Natural Areas Ecological
Stewardship Manager 1
MEMORANDUM
Date: July 29, 2025
To: Mayor and City Councilmembers
Through: Kelly DiMartino, City Manager
Tyler Marr, Deputy City Manager
Dean Klingner, Community Services Director
From: Ryan Vincent, Natural Areas Sr. Supervisor
Subject: West Nile Virus Update
BOTTOM LINE
West Nile Virus (WNV) activity reduced significantly across the city this week.
The City will not spray for adult mosquitoes this week.
The vector index (VI) dropped to a near historical average for this time of year and did
not exceed the 0.75 VI threshold for a 5-trap floating zone.
The infection rate also dropped from 11x higher than average to 3x higher.
Cooler overnight temperatures and recent spray treatments over the past week may
have contributed to mosquitoes being less active.
WNV ACTIVITY ACROSS THE CITY
Week 30 of 37
Quadrant Previous Week VI Current VI Previous Week
Positive Traps
Current Week
Positive Traps
NW 0.47 0.11 5 1
NE 0.71 0.23 5 2
SE 1.0 0.16 11 3
SW 0.35 0.09 2 1
Citywide 0.69 0.16 23 7
Floating Zone NA 0.20*NA 2
*VI Threshold for spraying in a floating zone is 0.75.
2
WNV ACTIVITY COMPARED TO HISTORICAL AVERAGES
Because both the number of Culex mosquitoes and their WNV infection rate declined this week,
the overall vector index has returned to levels typically seen at this time of year.
HUMAN WEST NILE CASES
As of July 25, Colorado has reported 11 cases of WNV. Five of those cases involved neurologic
symptoms, three people were hospitalized, and one person died in Lafayette. Four of the cases
were reported in Larimer County, including one in Fort Collins. It is important to note that there
is a delay often several weeks between when a person is infected and when symptoms
appear or treatment begins, so the full impact of the recent spike in the Vector Index may not
yet be apparent.
NEXT STEPS
City and County staff will continue to monitor WNV using trapping and lab data and
make spray recommendations when necessary.
improving, WNV is still present. Residents should stay vigilant and follow the 4 Ds: Drain
standing water to eliminate mosquito breeding sites, Dress in long sleeves and long
pants when outdoors, Defend yourself with insect repellent, and avoid being outdoors
from Dusk to Dawn, when mosquitoes are most active.