West Nile Virus

West Nile virus (WNv) is a disease that is transmitted from mosquitoes to people. In Colorado, only three species of the Culex group of mosquitoes that effectively transmit WNv. Culex mosquitoes feed on birds that can act as a reservoir for the virus. Once WNv reaches a high enough level in the bird population, mosquitoes are more likely to become infected, and then the virus can be transmitted to humans when bitten by an infected mosquito.

Anyone can get WNv if bitten by an infected mosquito. Most mosquitoes are not infected, so the odds of contracting the virus are relatively low, although this risk increases as the percentage of infected mosquitoes rises.

What happens if you're bitten by an infected mosquito?

• Most people - approximately 80 percent - will have no symptoms or be aware that they have been infected by the virus. The good news is these people become immune to WNv.
• About 20 percent will have symptoms similar to a bad case of the flu.
• About 1 in 150 people will develop a serious form of the disease that can result in life changing complications that can lead to permanent disability or even death.
  
  People who are over the age of 50 or those with an underlying condition are more likely to experience serious complications. Although the odds are low of contracting the severe form of WNV, the consequences are serious enough to warrant community action to reduce the risk.

West Nile virus is endemic in Colorado, meaning that it's a permanent part of local ecosystems. Although the risk for transmission varies, whenever mosquitoes are active, you should assume that West Nile virus is present and take precautions to protect yourself and your family.

When West Nile virus (WNv) first arrived in the Front Range in 2002, the following year, human cases reached epidemic levels in Colorado. Cases declined sharply in 2004, and remained relatively low compared to previous years. However, this pattern seems to be changing over the the past last few years, particularly the 2023 season. The city is exploring the reasons why this is happening and how we can continue improving the program to decrease this risk.

What is the vector index and what does it mean? The vector index estimates the risk to people for contracting WNv. It calculates the abundance of Culex mosquitoes - the type of mosquito that can become infected with WNv - with the infection rate of WNv in mosquitoes. When the vector index reaches a value of 0.75, it predicts that WNv is circulating at high enough levels that transmission to people is likely to occur. The city carefully monitors risk, and when there are indications that the risk might be increasing, the field team searches for any missed Culex breeding sites and will try to locate and drain any standing water. The city also works with other agencies to coordinate information and communication.

Residents are always advised to take protective measures to avoid mosquito bites and reduce Culex breeding on private property during every mosquito season, regardless of Vector Index results. As WNv risk increases, the city will increase communication and outreach to the community to provide information to help people stay safe.

The past three to four years, the Vector Index has been rising along the Front Range, and in 2023, it was at the the highest levels ever recorded. During this time, the city's Vector Index never exceeded the 0.75 threshold. The city's program is designed to reduce West Nile virus risk by protecting wetland health and biodiversity. The city is currently reviewing its management practices and biodiversity data to better understand why WNv levels have been so much lower in city traps than the surrounding areas.

Culex mosquitoes can breed readily in the environments found in urban properties. Both community members and the city can take actions to signficntly reduce the risk of WNv in for all of us.

The most effective way to decrease the Culex mosquito population is to reduce the breeding habitat or areas with standing water where mosquitoes lay eggs. The city maps all potential mosquito breeding sites on public lands. During mosquito season from May through September, these sites are monitored weekly and treated with the larvicide Bacillus thuringiensis israelensis or Bti. This soil bacterium targets mosquitoes and other aquatic flies to effectively kill the larvae, while they are concentrated in specific locations and before they emerge as biting adults.

The city has a well-established and effective mosquito management program. The mosquito program webpage provides the most current mosquito trap data during the summer season. This story map provides more detailed Information about mosquito ecology and how the city designs and conducts its program. The city is making improvements to the irrigation systems of agricultural systems to enable water to drain more quickly and is restoring native vegetation in wetland areas, enhancing biodiversity and mosquito predators. Restoration to healthier and more diverse wetlands reduces the number of mosquitoes and and the risk of mosquito-borne disease.

A grid of adult mosquito traps is maintained throughout the city to asses the number of mosquitoes and mosquito samples are tested for WNv to estimate the risk to people. City staff from multiple departments, experts from the city’s mosquito contractor, and Boulder County Public Heath officials work together to provide the best program possible to prevent WNv and are prepared to take quick action if risk becomes elevated.

Take the actions below to keep our community as safe as possible.