Larimer's skyrocketing West Nile tally an unsolved mystery
Larimer County has the fourth-highest number of West Nile virus cases in the country, but the high count is a mystery to experts.
The number — 28 human cases this year as of Tuesday — is the most in the state and third-most Larimer County has seen in a decade. The number is especially striking when you consider the extremely large populations of the nation’s other West Nile hubs, including Los Angeles, Dallas and Phoenix.
The strangest part of all?
Larimer County has more infected mosquitoes than usual this year, but the total number of mosquitoes is below average. Usually, those two factors go hand-in-hand.
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“I don’t know what to make of it,” said Dr. Chet Moore, faculty affiliate of the Arthropod-Borne and Infectious Disease Laboratory at Colorado State University's College of Veterinary Medicine and Biomedical Sciences. “There’s a lot of virus activity relative to the mosquito numbers. This is an odd virus, and unusual things seem to happen with it every year.”
“Unusual” has certainly been a recurring theme this season for West Nile virus. The county has spent $40,000 spraying mosquitoes in five areas of Fort Collins, twice stepping in to spray high-risk areas of the city because the two entities have vastly different ideas of when to apply pesticide. In an average year, the county doesn't spend more than $10,000 spraying, health department spokeswoman Katie O’Donnell said.
Fort Collins, which has seen 14 cases as of Sept. 1, has spent an additional $9,000 on spraying.
Local experts are perplexed at the root cause for the high infection rate.
Weather, number of infected birds, number of infected mosquitoes and human behavior are common factors for where outbreaks occur, according to the Centers for Disease Control.
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The human behavior factor makes some sense — Larimer County residents love to spend time outside in the summer. But that doesn’t explain why other Colorado counties of comparable size have fewer infected residents.
As for weather, studies show more mosquitoes become infected with West Nile during warmer periods, and Larimer County has seen a hotter-than-usual summer. Impacts of precipitation on West Nile infection remain uncertain.
West Nile virus is maintained in a bird-mosquito cycle: Infected mosquitoes transmit West Nile to birds, which in turn transmit West Nile to uninfected mosquitoes that bite them. The CDC didn’t immediately respond to questions concerning how many local birds might be infected with West Nile virus this year.
Moore offered up one theory for why Larimer County has had so many cases: Larimer County and nearby Weld and Boulder counties, which generally have high case tallies, all have lots of irrigated agricultural land.
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Irrigated agricultural land means standing water, which is offers breeding grounds for the blood-sucking pests, Moore said. The same breezes that carry the lovely smell of livestock waste from east to west in the evenings might also carry infected mosquitoes into Larimer County, said Moore, who’s been part of studies that suggest but don’t confirm the relationship.
Weld County has 12 confirmed cases as of Sept. 2.
Local officials remind residents that summer is waning but West Nile season isn’t over. September is often the peak time for infections, which usually take about three weeks to confirm.
“It’s important that people remain aware,” said Matt Parker, resource management crew chief with Fort Collins Natural Areas. “Even though the number of mosquitoes is decreasing” as temperatures cool down, “with our higher infection rate, we’re still going to see human cases.”
At noon Thursday, Banner Health is holding a 30-minute Facebook Live discussion on the signs, possible risks and prevention of West Nile virus. The discussion will feature Ken Summers, a Fort Collins resident and former state lawmaker who nearly died from the virus a few years ago.
Take part in the discussion on Banner Health's Facebook page.
Top U.S. counties for West Nile virus infection
- Los Angeles County, California (population: 10 million, largest in U.S.): 41 confirmed human cases as of Sept. 2
- Dallas County, Texas (population: 2.5 million, ninth-largest in U.S.): 35 cases as of Sept 6
- Maricopa County, Arizona (population: 4 million, fourth-largest in U.S.): 32 cases as of Sept. 1
- Larimer County, Colorado (population: about 316,000, 207th-largest in U.S.): 28 cases as of Sept. 6