West Nile Virus

What is West Nile virus?

West Nile virus is a viral infection primarily spread through the bite of an infected mosquito.

In the continental United States, West Nile virus is the most common mosquito-borne infection. West Nile circulates in nature between birds and mosquitoes. People become infected when a mosquito feeds on an infected bird and later bites a person.

West Nile virus can be transmitted anytime when mosquitoes are active. In Colorado, mosquitoes can be active as early as May but are typically in higher numbers during summer through fall. Mosquito season ends after the first hard freeze of the year. This can take place in October or November, but can happen as late as December.

Humans and other mammals are dead-end hosts for the virus, meaning that humans won’t develop enough virus to infect a healthy mosquito when bitten. In very rare cases, West Nile virus has been spread through laboratory exposure, blood transfusions, organ transplants, and from mother to child during pregnancy, delivery, or breastfeeding. Blood collection agencies in Colorado actively screen blood donations for West Nile virus.

West Nile virus is not spread by:

Coughing or sneezing (it is not spread from person-to-person)
Touching or handling live or dead infected animals, although it’s still best to avoid bare-hand contact when handling any dead animal
Eating infected animals

Symptoms

Thankfully, most people who are infected with West Nile virus experience no symptoms and may likely never know that they had it. Approximately one in five people who are infected will develop a fever and other symptoms that can include:

Headache
Body Aches
Joint pain
Vomiting
Diarrhea
Rash
Flu-like symptoms

Most people who develop these symptoms will recover from it completely without needing any medical treatment. These symptoms may last from a few days to a couple of weeks. Approximately 1/150 people infected with the virus may develop severe illness that affects the central nervous system such as encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord). Symptoms of neuroinvasive illness can include:

High fever
Headache
Neck stiffness
Stupor
Disorientation
Tremors
Coma

Severe illness can impact people of any age. It should be noted that people over the age of 60 are at greater risk for severe illness. Immunocompromised people with preexisting medical conditions such as cancer, diabetes, hypertension, kidney disease, and people who have received organ transplants are also at an increased risk.

If you suspect that you or a family member has West Nile virus, please contact your healthcare provider immediately.

Treatment

There are no specific cures, medications, or treatments available to treat West Nile virus. Antibiotics do not treat viral illnesses.

Plenty of rest, fluids, and over-the-counter pain medications can help relieve symptoms of WNV. People with severe WNV illness may need to be hospitalized to receive supportive healthcare.

There is currently no available vaccine for West Nile virus.

It is generally believed that most people infected with West Nile virus obtain lifelong immunity to the virus, protecting them from getting the disease again. Those with preexisting medical conditions or compromised immune systems may be able to get infected again.

Prevention

The best way to prevent West Nile virus is to avoid being bitten by mosquitoes. Water is essential in the mosquito breeding cycle and is key to eliminating areas where mosquitos are produced. One 6-inch puddle of water can produce 1,000 mosquitoes!

Mosquito-Proof Your Home

Maintain Screens: Install or repair window and door screens to keep mosquitoes out.
Check for Water Collectors: Regularly inspect and empty any object that can hold water, like flowerpots, saucers, toys, or outdoor décor.
Clear Gutters: Keep gutters clean and free-flowing to avoid water buildup.

Remember the 4Ds of mosquito protection:

Defend- Use effective insect repellants when you are outdoors. Repellants that contain DEET, picaridin, IR3535, and oil of lemon eucalyptus provide effective protection against mosquitoes. If you are also wearing sunscreen, use the sunscreen first followed by the insect repellant. Apply insect repellant to your clothes in addition to any exposed skin. Always follow the labels instructions when using insect repellants.
Dress- Wear closed-toe shoes, long sleeves, and pants to cover as much exposed skin as possible. Loose-fitting clothes are effective at preventing mosquito bites.
Drain- Drain any standing water around your property at least once a week to eliminate mosquito breeding grounds. Keep your yard clear of debris that can hold water.
Dusk & Dawn- Mosquito activity typically peaks at dusk and dawn. To reduce your exposure to mosquitoes, limit any outdoor activities at these times. During the day, mosquitoes tend to linger around cool and shaded areas.

Eliminate Mosquito Breeding Sites

Drain Standing Water: At least once a week, remove standing water from containers, tires, birdbaths, pots, saucers, gutters, buckets, and toys.
Avoid Overwatering: Don’t overwater your yard, which can create puddles in turf, storm sewers, and landscaping.
Fix Leaks: Repair leaky faucets and broken sprinkler systems that may create pooling water.
Maintain Water Features: Keep fountains and pools circulating properly, or drain and cover them when not in use. If this isn’t possible, apply larvicide to prevent mosquito development.

West Nile Virus in Colorado and Denver

West Nile virus has been a reportable disease in the state of Colorado since 2002 when the first case of the disease was identified. The virus has since been endemic to Colorado, meaning that we can expect to see cases every year. Since its introduction to the state, there have been two significant outbreaks occurring in 2003 and 2023. These outbreaks resulted in over 3500 confirmed cases with over 100 deaths. In 2023, Colorado accounted for nearly a quarter of all West Nile virus cases in the United States.

It’s impossible to predict what the year may look like for West Nile virus. Throughout the mosquito season, Denver’s Mosquito Management Program uses an approach to reduce mosquito breeding sites and inhibit larval mosquito survival to help keep mosquito populations at a safe level. However, mosquitoes can take advantage of particularly wet spring and summer seasons to increase their populations, as was evident with the 2023 mosquito season. Statewide West Nile virus data is available through the Colorado Department of Public Health and Environment.