LARIMER COUNTY, Colo. — On Thursday, the Larimer County Department of Health and Environment warned people of an "unprecedented risk" of the West Nile virus due to an unusually high number of mosquitoes that can carry the virus.
This announcement came shortly after the Denver Department of Public Health and Environment confirmed that mosquitoes collected in Denver tested positive for West Nile virus.
The Larimer County health department also confirmed the first case of West Nile virus in a Larimer County resident. They said the person lives in the southern part of Fort Collins. This year's first case was confirmed one month earlier than last year's, which is probably due to the abundance of virus-carrying mosquitoes in the area, the department said.
Weld County said they are investigating a potential case of the virus as well.
The Larimer County health department said the Vector Index (VI), which is used to measure the risk of the West Nile virus in a specific area, is high in municipalities across the county. A VI above 0.5 predicts that human cases of the virus will occur, and a VI above 1.0 is "strongly correlated with an increase in human cases." Berthoud has a VI that ranges between 1.2 and 3.029, the department said.
"We have a long way to go, we're still in July," said Tom Gonzales, public health director for Larimer County. "This could keep amplifying and we could have a really terrible August."
He added, "I'm very concerned we're going to have many human cases, so its really important for us to be aware of the risk from dusk to dawn."
Aside from Berthoud, other affected municipalities in Larimer County have started spraying to reduce the number of mosquitoes to protect the public, the department said.
"We will continue to alert the public to the concerning and continued risk of West Nile virus, especially in the Berthoud area," Gonzales said. "Residents and visitors should be taking extra precautions to prevent mosquito bites."
The Department of Health asked people in Larimer County to practice the "4 D's" to prevent West Nile virus disease:
- Defend: Use an EPA-registered mosquito repellent that has been proven to be effective against West Nile virus-carrying mosquitoes.
- Dusk to Dawn: Avoid exposure during peak Culex mosquito feeding times, which is from dusk through dawn.
- Dress: Wear long sleeves and pants to keep mosquitoes from biting.
- Drain: Remove standing water in your yard or garden to minimize mosquito breeding areas.
The West Nile virus is spread through Culex mosquito bites. Symptoms usually appear three to 14 days after an infection, the health department said. Though symptoms vary, the most common include fever, nausea and vomiting, muscle weakness and aches, rash and headaches.
"Mother Nature has provided this ideal circumstance [this year] to breed lots of mosquitoes and to especially breed lots of Culex mosquitoes," said Dr. Bob Hancock, a professor at Metropolitan State University of Denver who studies mosquito behavior.
"Because of our rainfall, and because of the restoration of our water table, you know, we've kind of worked our way out of drought, we have so much more long-standing water, that we're providing a lot more breeding space and breeding opportunity for Culex mosquitoes," he said.
Hancock compared annual data on mosquito samples and said this year is notable.
"They are just completely off the charts," he said.
"A few weeks ago, I made a comparison of one trap in Johnstown, Colorado, I looked at the 2014 trap for the first week of July, and compared it to the same trapping in 2023 for the first week of July. And the differences [were] six mosquitoes in the 2014 trap, and 600 mosquitoes in the 2023 trap. So that's massive."
In 2022, Colorado had 206 reported human cases of West Nile virus, including 20 deaths, according to the Colorado Department of Public Health and Environment. People aged 60 and older and those with some pre-existing medical conditions have a higher risk of serious illness, the Larimer County health department said.
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