West Nile Virus cases climb in Colorado; most cases in

West Nile Virus infections and deaths are both up from last year, and there have been more cases so far than at any point since 2016, according to data from the Colorado Department of Public Health and Environment. 

As of this week, 139 Coloradans have contracted the mosquito-borne illness, which has resulted in six deaths. Those figures are up 297% and 500%, respectively, when compared to 2020. In fact, this year’s reported cases is the third highest since 2010, according to the state health department.

While COVID-19 has claimed 2,574 lives in Colorado as of Oct. 7 since the beginning of the year, according to CDPHE data, and a relative mild 2020-21 influenza season took another six lives, the West Nile numbers show that there is another potentially deadly virus out there and experts say that precautions need to be taken. 

But despite the spike in West Nile cases and deaths, many health officials said it is not abnormal for the numbers to vary from year to year and emphasized preventive measures can reduce the likelihood of contraction.

“There is fluctuation from year to year, and geographically, there’s fluctuation,” said Daniel Pastula, associate professor of neurology, medicine and epidemiology at the University of Colorado School of Medicine and Colorado School of Public Health. “Some years we have no cases, and some years we have a whole lot of cases.”

The virus was first discovered in a woman in the West Nile district of Uganda in 1937 and later identified in birds in the Nile delta region in 1953, according to the World Health Organization. 

The mosquito-borne illness is known to cause fever and headaches, and in severe cases, can cause neuroinvasive disease or death. Yet, 80% of those who contract the virus don’t show symptoms, said Pastula.

About one out of every 150 people bitten by an infected mosquito develops neuroinvasive diseases such as encephalitis, a brain infection, or meningitis. About 10 percent of severely infected people die from the illness, and survivors can have long-term nervous system problems, according to the U.S. Centers for Disease Control and Prevention. 

However, this year, 79 of the cases in Colorado — 57 percent — are categorized as neuroinvasive cases, which is the most since 2013. Those greatest risk for severe cases are people over 60 years old or those with immunosuppressive diseases, according to CDPHE. 

For decades, the virus lingered in the Eastern Hemisphere, but in 1999, it began making its way across the Atlantic Ocean and into the Americas. 

Colorado’s first reported case was confirmed in 2002, and by the following year, the number of cases reached “epidemic levels” with 2,948 confirmed cases, said State Public Health Veterinarian Jennifer House.

“We had what we call an epidemic year in 2003, and that is when pretty much everything in Colorado was naive to the virus, so there was no immunity in our birds, horses or people at all,” House said. “It’s been present in the state ever since then, and we have continued to see cases in people every year since our outbreak in 2003.” 

Mosquitos contract the potentially deadly virus through birds and spread the disease to people, horses and, in rare instances, pets. Although health professionals know how the insects contract and spread the disease, House said it’s impossible to eradicate.

“We can’t eliminate it because it is transmitted by birds and birds migrate, so there’s just no way of eliminating the virus,” House said. 

Even though the virus cannot be eradicated, health officials have devised several ways to lower the chance of infection, such as through mosquito trapping, testing and outreach education, officials said. 

For example, Larimer County, which reported 17% of the total reported cases in Colorado between 2003 and 2020, has assembled over 100 mosquito traps across the county.

Larimer is one of five “hot spot” counties where infection is highest at-risk. Boulder, Delta, Mesa and Weld counties are the others, and the group has collectively accounted for 53% of all cases in the state since 2003, according to state data.

Each June, Larimer County officials conducts surveillance measures, monitors mosquito traps and analyzes captured insects, said Kori Wilford, a spokeswoman for the Larimer County Health Department.

“They’ll look at the mosquitos and look at the Culex mosquitoes, which are the ones that transmit West Nile Virus, and then start testing them and looking for infection,” Wilford said. 

Other methods used in Larimer County vary by municipalities. Loveland regularly performs aerial spraying, while Fort Collins does not spray until the number of mosquitos reaches a certain threshold.

The aerial spraying, a popular mosquito mitigation method, “minimizes mosquito annoyance problems and helps stop the spread of mosquito-borne diseases such as West Nile, Wilford said.

Other communities such as Broomfield County will contract with mosquito control agencies to perform mitigation such as aerial spraying.

Although spraying, testing and analyzing  reduce the number of infected mosquitos, they do not 100% eradicate them. Health officials also issue public service announcements about the virus and have established the “Four D’s”: 

  • Drain: Mosquitoes breed in water. Drain any standing water in your yard each week. Bird baths, clogged gutters and kiddies pools are common breeding sites.
  • Dress: Wear lightweight, long-sleeved shirts and long pants while outdoors. Spray clothing with insect repellent since mosquitos bite through clothing.
  • Defends: Apply insect repellent sparingly to exposed skin. Use an approved repellent according to its label. During the peak season infected mosquitoes can be found all along the Front Range so use repellent where you live, work and recreate.
  • Dawn/Dusk: The best way to avoid West Nile virus is to prevent mosquito bites. Stay indoors, if possible during peak mosquito hours — sunset to 1.5 hours after sunset appear to be the most active “feeding time” for the species that carry West Nile virus.

These rules should be followed beginning in June and through the first frost of the year, House said.

House said she expected the number of cases to continue to rise until the first frost. 

And while officials know how the virus spreads and some factors that contribute to a “bad year,” House said it’s nearly impossible to predict how each season will happen.

“There’s a lot of factors that go into it and includes climate, early spring rain fall, stagnant water and then followed by hot, dry summers. So it’s actually really complicated,” House said.

Even though 80% of those infected with the virus do not show symptoms, Pastula said people should still take the potentially deadly virus seriously.

“You can’t get West Nile if you don’t get bit by a mosquito,” Pastula said. “Wear your insect repellent, avoid activities around mosquito prone areas around dusk and dawn, consider wearing longer pants or sleeves, consider permethrin on your clothes and stay indoors when mosquitoes are active.”


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