DENVER (CN) — West Nile Virus is the leading cause of mosquito-borne disease in the continental United States, and this year is a record-setting one.
Water, heat and blood are all that Culex mosquitoes need to reproduce, and an exceptionally wet spring and early summer this year, with extreme heat, have produced a record number of infections in Colorado.
“So far this season, state health officials have confirmed 11 human deaths from West Nile virus,” said AnneMarie Harper, communications director for Colorado’s Division of Disease Control and Public Health Response. “CDPHE has also confirmed 122 human cases of the virus, 73 of which resulted in hospitalization.”
A Denver physician this week cited a recent spate of the disease in counties north of Denver. Eleven of Colorado’s 64 counties have recorded West Nile infections in humans this summer.
Larimer County, centered on Fort Collins, has seen “an unprecedented risk of West Nile Virus” since late July, with two deaths so far, according to Larimer County Public Health spokeswoman Kori Wilford.
“We’ve had 34 reported cases, 11 hospitalizations and 9 neuro-invasive cases," Wilford said. "Those are the ones that make you really sick.”
In neuroinvasive cases, the virus enters the brain casing, the spine or the brain itself.
"There’s no treatment,” Wilford said, and “unfortunately there’s no vaccine. They just treat the symptoms.”
Rachel Herlihy, Colorado’s state epidemiologist with the CDPHE, said, “The trends we are seeing in our West Nile virus tracking data are unprecedented.
“The number of West Nile virus-infected mosquitoes we've detected this season is the highest we've seen in years. This is especially concerning now that August is here and September is just around the corner, as this is usually when human cases peak in Colorado.”
In Larimer County, Wilford said, “We have a lot of agricultural land, and that’s where they like to breed.”
What is West Nile Virus?
West Nile Virus is transmitted by Culex mosquitoes. The Culex mosquito may acquire the disease by biting an infected bird, usually a crow or a jay, or by biting an infected person, then transmit it by biting another one.
A female mosquito may bite five or six times a day, and will stop only when she is full. Males mosquitoes do not bite; they live on nectar and live for only about six weeks. Females may live as long as six months and even hibernate over the winter.
For reasons unknown, only about 20% of people infected with West Nile show symptoms, according to Johns Hopkins Medical School. Symptoms of a “mild” case may include fever, headache, body aches, skin rash and swollen lymph glands.
The severe form usually affects older adults, when the virus crosses the blood-brain barrier. Symptoms may include headache, high fever, neck stiffness, muscle weakness, stupor, disorientation, tremors, convulsions, vision loss, coma and paralysis.
“About 1 out of 150 infected people develop a serious, sometimes fatal illness,” according to Johns Hopkins.
A severe, or neuro-invasive, case may involve encephalitis (inflammation of the brain), meningitis (inflammation of the brain lining and spinal cord) and meningoencephalitis (inflammation of the brain and its surrounding membrane).
Mosquitoes: an ancient scourge
Mosquitoes are believed to have killed more humans throughout history than any other vector. Malaria is the leading mosquito-borne killer; it is spread by Anopheles mosquitoes, not Culex.
“Malaria is a human tragedy: by one estimate it may have killed half of all the people who ever lived,” according to a June 2015 article in Nature, a leading scientific journal.
Many demographers consider that claim a wild exaggeration. But if even 5% of the estimated 110 billion people who have ever lived on Earth died of malaria. That’s 5.5 billion deaths, far more than the number who have been killed by war. Malaria continues to cause as many as 1% of worldwide deaths each year, according to the Centers for Disease Control and Prevention.
The first case of the virus was reported in the West Nile region of Uganda in 1937, according to the National Library of Medicine. In the United States, it was first reported in New York City in 1999. Since then it has been reported in all 50 states and the District of Columbia.
It is now a nationally reported disease, meaning states and counties are supposed to report cases to the Centers for Disease Control. However, due to reporting lags, state and county health departments generally have more accurately updated numbers than the CDC.
“We get numbers of infected mosquitoes each week,” said Kristeen Bevel, health specialist in charge of mosquito studies for Weld County, Colorado, whose seat is Greeley, which has recorded one death and 13 human cases this year.
“The residents who tested positive are not just from one area, rather they represent a wide geographical area,” Bevel said.
Weld County tests 15 mosquito traps a week, five from each of its three zones. From these zones, they establish three separate “pools,” which are vials of mosquitoes from each of the three zones, which are not intermixed. The pools are tested to see if West Nile Virus occurs in them.
Similar studies were done in the early 1980s, when AIDS became an emerging disease. Researchers allowed mosquitoes to feed on baboons that had been infected with the AIDS virus, then waited a few days and liquified the mosquitoes, to see if the AIDS virus could live in the mosquitoes’ guts: a possibility that could have turned a nightmare into a catastrophe. Fortunately, it could not.
All the specialists consulted for this article attributed Colorado’s outbreaks to this year’s wet spring and early summer and the intensive heat. A similar outbreak of West Nile Virus occurred in Arizona in 2021, when Maricopa County, centered on Phoenix, received three times the amount of rain generally recorded from June to September (6.6 inches compared to 2.2 inches) and Maricopa County alone recorded 1,463 human cases of West Nile Virus, according to Climate.gov.
Nearly all sources consulted for this article consider global warming, combined with this year’s Pacific El Niño wet weather patterns, as prime suspects for this year’s rise in West Nile disease.
“People are doing research on linking (various) diseases to weather conditions," said Bevel, of Weld County.
For prevention, all sources cited the “three Ds:”
Drain standing water around your house, where mosquitoes breed, at least once a week, including clogged gutters, bird baths, spare tires and any other containers.
Mosquitoes are especially active at dawn and dusk, so you might limit your outdoor activities then, or if you go outside, observe the other precaution.
Defend: DEET is an effective insect repellent, as is Picaradin, which some specialists say may be safer than DEET. For those who prefer natural repellents, oil of lemon eucalyptus is recommended.
Dress in long-sleeved shirts and pants, especially at dawn and dusk. This may be difficult in a record-breaking hot summer, but clothes may be sprayed with insect repellent. Follow directions on the product labels.
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