Colorado’s West Nile virus season is starting a bit early, but experts aren’t sure whether a wet spring will translate into a tough year for the sometimes-deadly mosquito-borne illness.
Denver reported its first mosquito that tested positive for West Nile on Thursday, becoming the second Front Range county to confirm the presence of the virus this summer.
Not all counties set out mosquito traps, so Colorado residents can’t assume their local bugs don’t carry the virus if they haven’t heard about reported cases.
Mosquitoes need standing water to breed, so storms that leave puddles could benefit them. But the amount of rain is only one factor in how many people will become severely ill with West Nile, said Dr. Daniel Pastula, chief of neuroinfectious diseases and global neurology at University of Colorado School of Medicine.
Summer temperatures influence how active mosquitoes are, and how many pick up the West Nile virus depends on whether birds passing through Colorado have active infections, he said.
“We haven’t been able to predict ahead of time when there’s going to be a bad West Nile season,” he said.
Residents along the Front Range may get a bit of warning from counties’ mosquito traps, though. When bugs test positive for West Nile, that signals the risk is up, and people need to be especially vigilant about avoiding mosquito bites, Pastula said.
Larimer County found its first infected mosquito of the year on June 23. Typically, West Nile season starts in early to mid-July.
Most West Nile cases in Colorado happen between July and September, though mosquitoes can live until the first snow or hard freeze, Pastula said.
About 80% of West Nile cases are asymptomatic or have mild symptoms that people typically don’t seek treatment for. In about one in 100 cases, though, the virus invades the brain or spinal cord, and about one in 1,000 infected people die, Pastula said. Those who survive a severe case sometimes experience paralysis or cognitive damage, he said.
Colorado recorded 76 West Nile cases and zero deaths last year, according to health department data. But the state saw 634 cases and 51 deaths from the virus in 2023 and 207 cases and 20 deaths in 2022.
People who are over 50, have compromised immune systems, or have chronic conditions such as diabetes or kidney disease are at higher risk for the virus to infect their nervous systems. But occasionally, people with no obvious risk factor become seriously ill, Pastula said.
The western Great Plains and the Front Range have higher than expected rates of severe West Nile, possibly because the mosquito species that live here are more aggressive toward people than those on the coasts, he said.
“I have seen healthy 20- and 30-year-olds with severe West Nile disease and permanent neurological damage,” he said. “It is much better to avoid getting this.”
People can protect themselves by wearing insect repellent and long pants and sleeves while outside. Limiting outdoor activity around dawn and dusk also lowers the chances of a bite.
Using air conditioning or screens over open windows will keep mosquitoes out, and dumping any standing water on your property will limit opportunities for them to breed.
Sign up for our weekly newsletter to get health news sent straight to your inbox.