Colorado’s wet start to 2023 brings a surge in mosquitoes — but that doesn’t guarantee a spike in West Nile virus

The warm and wet start to the year has created the perfect breeding ground for mosquitoes in Colorado, but experts say that won’t necessarily translate into a corresponding explosion in cases of the insect-borne West Nile virus in 2023.

So far this year, Denver has seen 13.67 inches of precipitation from rain and snow — more than double the city’s total this time in 2022, according to meteorologist Ayesha Wilkinson at the National Weather Service in Boulder. In all of 2022, Denver received just under 12 inches of precipitation.

Rebekah Kading, a medical entomologist at Colorado State University and an expert on mosquito-transmitted diseases, said increased rain levels can raise water into areas where mosquitoes already have laid eggs and create new areas for them to do so.

“If an increase in rain translates into more sitting water, then we end up seeing more mosquitoes,” Kading said.

While the rain increases the number of mosquitoes that can carry the virus, the relationship between precipitation and cases of West Nile is counterintuitive, said Sara Paull, a disease ecologist at the National Ecological Observatory Network in Boulder who studied connections between weather, climate and West Nile at the University of California-Santa Cruz.

“What we found is actually extremely paradoxical,” Paull said. “In Colorado, and actually across the U.S., the higher the drought level, the higher the number of human West Nile cases that we saw in the data.”

Paull said it has to do with birds — mosquitoes pick up the virus by biting infected birds before spreading it to humans.

When the water supply is limited, birds congregate around water sources, making them easier targets for mosquitoes. It can also increase birds’ stress hormones, which makes it more likely for them to contract West Nile and spread it to the mosquito population, Paull said.

While the more common route of mosquitoes becoming infected is to bite a bird with the virus, a process known as horizontal transmission because the virus is moving between different species, it is possible for female mosquitoes to pass on the virus to their offspring, Kading said.

“Vertical transmission, or the passage of virus from female mosquitoes to the eggs she lays, has been documented to happen in the field as well,” she said. “Vertical transmission is a relatively inefficient process by comparison, but is possible.”

Rising number of “danger days”

Of the 3,000 types of mosquitoes worldwide, only 20 live in Colorado, according to a news release from Greeley’s mosquito management program. And among the 20 types of mosquitoes in this state, only two are known for spreading West Nile: Culex pipiens and Culex tarsalis, Paull said.

Mosquito season in Colorado spreads across the warm months of the year, but spikes in the spring and late summer.

According to Climate Central, mosquito seasons have lengthened in many cities across the U.S. over the past several decades. From 1979 to 2022, more than 70% of the surveyed cities saw annual mosquito days increase, by 16 days on average.

“As spring and fall temperatures warm, mosquitoes can emerge earlier and survive later into the year,” Climate Central’s report stated.

The U.S. has not only seen a rise in mosquito-hospitable days, but mosquito “disease danger days.” The rate of disease transmission from mosquitoes peaks when the outside temperatures are between 79 and 84 degrees, according to Climate Central. Of 244 cities analyzed by Climate Central, 94% had a rising number of days with temperatures in that risk zone.

Most cases of West Nile are documented from late summer to early fall, when average temperatures fall into the danger zone, Kading said.

Excess rain can create breeding grounds and increase the abundance of mosquitoes able to carry the disease, but it doesn’t directly increase the percent infected with West Nile, Paull said.

“Last winter’s heavy snowpack and the wet spring we’ve had in Colorado are already causing an increase in mosquitoes in some areas,” said Natalie Marzec, a program manager with the Colorado Department of Public Health and Environment. “The biggest increase so far has been in the Aedes vexans mosquitoes, which do not transmit human diseases but are named for being aggressive biters.”

As summer gets underway, Marzec said Colorado is just starting to see a transition from these mosquitoes to the Culex mosquitoes that can carry and transmit the West Nile virus.

On Thursday, officials in Boulder and Weld counties announced they had discovered mosquitoes infected with West Nile virus.

“A state with a lot of West Nile”

Even though the rain may not increase the danger of contracting West Nile, experts say the risk is not zero and people should still take safety precautions.

“Colorado is a state with a lot of West Nile and the virus is notoriously hard to predict,” Paull said. “This is based on past data, but it is always prudent to take precautions with mosquitoes.”

According to the Centers for Disease Control and Prevention, Colorado had the most West Nile cases in the country last year with 204 cases and 18 deaths from the virus. In 2021, Colorado had the second most cases of any state.

Only 20% of people bitten by mosquitoes carrying the virus will develop symptoms — including fever, headaches, body aches, skin rashes and swollen lymph glands, according to the Center for Disease Control.

About one in 150 people who are infected develop a severe illness or life-threatening symptoms, including high fevers, tremors and convulsions, muscle weakness, paralysis and inflammation of the brain and spinal cord.

Less than 1% of people with West Nile cases die, according to the CDC.

“Severe illness can occur in people of any age; however, people over 60 years of age are at greater risk for severe illness if they are infected,” a 2022 report from the CDC stated. “People with certain medical conditions, such as cancer, diabetes, hypertension, kidney disease, and people who have received organ transplants, are also at greater risk.”

How to protect yourself

To protect yourself from West Nile, the state health department recommends using insect repellents containing DEET, picaridin, IR3535, oil of lemon eucalyptus or para-menthane-diol products.