West Nile Virus

Latest News

Updated August 2015

Recent results of mosquito trapping in Larimer County indicate that the numbers of mosquitoes carrying West Nile virus are increasing, raising the risk of human infection substantially.

West Nile virus is transmitted to humans through the bite of an infected mosquito. It can cause mild to severe illness and, in some cases, leads to chronic disability or death. At increased risk of serious illness from West Nile infection are people over 50, solid organ transplant recipients, and people with weakened immune systems. However, all persons who are not adequately protected are at risk of being bitten by an infected mosquito.

Weekly mosquito trapping in Larimer County continues to show a rise in the infection rates of Culex mosquitoes in Larimer County. It therefore is especially important to apply mosquito repellent before heading outdoors between dusk and dawn. Although the risk will decrease when the weather gets colder, infected mosquitoes may be around until the first frost.


West Nile virus (WNV) is transmitted to humans by mosquitoes. It first appeared in the United States in 1999 in New York. Since then, the virus has spread throughout the country. WNV first appeared in Colorado in 2003. West Nile virus infections are rare, but they can cause a variety of symptoms in humans. Most people infected with the virus do not get sick and do not know they were infected. Others develop West Nile fever or even more severe forms of the infection. West Nile fever is the most common form of illness. Symptoms vary but may include fever, headache, body aches, tiredness, skin rash on the trunk of the body, swollen lymph glands, nausea or vomiting. Signs of illness appear from three to 14 days after you are bitten by infected mosquitoes. Only about one in 150 infected people develop severe disease, which is usually caused by infection of the brain (encephalitis) or inflammation of the tissue surrounding the brain and spinal cord (meningitis).

Although infection is rare, it is a health concern in Larimer County and Colorado. In 2003, Colorado had the nation's largest number of reported cases of West Nile virus. Almost 3,000 people were infected; 63 Coloradans died (nine in Larimer County), and 622 people required hospitalization statewide. In 2006, there were 345 cases in Colorado, with 42 confirmed cases in Larimer County and one death. In 2009, there were 102 cases of West Nile virus reported in Colorado, including 25 confirmed cases in Larimer County. Sixteen of those 25 cases were reported after September 1. In 2014, there were 118 cases of West Nile virus in Colorado.

This topic was prepared with info from the City of Fort Collins, City of Loveland and the Larimer County Health Department. It is designed to help you understand West Nile virus and how to protect yourself. This site is not responsible for the information on other websites. The information here — and on all websites — is not intended to be a substitute for care given to you by a health professional.

Fort Collins, Loveland, County and State Information

City of Fort Collins
This link provides home and personal protection tips, information about City of Fort Collins action, information about managing mosquito breeding sites and more.

City of Fort Collins Notification of Mosquito Spraying
This link takes you to a page on the City of Fort Collins' website where you can subscribe to an email notification service allowing residents to be notified if/when and where truck-based mosquito spraying will take place in the city. Residents without email can sign up to receive phone notifications by calling Colorado Mosquito Control at (970) 962-2583.

Larimer County
Statistics on diagnosed West Nile virus cases in Larimer County, West Nile frequently asked questions, suggestions for protecting yourself from mosquito bites and more.

Larimer County Notification of Mosquito Spraying
This link takes you to a page on Larimer County's website where you can subscribe to an email notification service allowing residents to be notified when and where truck-based mosquito spraying will take place in unincorporated areas of Larimer County. County residents interested in spray locations also may call (970) 962-2583.

Pesticide Information, Colorado Mosquito Control, Inc.
Follow this link to learn about the techniques used locally to control the mosquito population.

West Nile Virus Information, Colorado Department of Public Health and Environment
Information about repellents as well as current-year summaries of both human West Nile virus cases and mosquito-borne testing results (as well as tables and maps from past years).

Protect Yourself — General

People become infected by the bite of a mosquito infected with West Nile virus. The risk of being infected increases during times of high mosquito activity, such as dusk and dawn. Individuals over the age of 50 and those with compromised immune systems have the highest risk of severe disease, but all age groups are at risk for West Nile virus.

Fight the Bite
Fight the Bite is the statewide West Nile virus prevention and education campaign from Colorado's state and local health departments. This site provides a wealth of information and resources. Fight the Bite also provides a statewide helpline: (877) 462-2911.

Ward off West Nile virus by remembering the 4 Ds:

  • Drain — Drain standing water around the house weekly since this is where mosquitoes lay eggs. (This includes tires, cans, flowerpots, clogged rain gutters, rain barrels, toys and puddles.)

  • Dress — Dress in long sleeves and pants during dawn and dusk and when you are in areas where mosquitoes are active.

  • Dusk & Dawn — Dusk and dawn are when mosquitoes are most active, so stay inside and avoid shrubs where they hide.

  • DEET — DEET is an effective ingredient to look for in insect repellents. (Always follow label instructions carefully and see the "Mosquito Repellent" section below for more information.)

Protect Yourself — Control Mosquito Breeding

How to Control Breeding Sites, City of Fort Collins
Suggestions on managing a variety of potential mosquito breeding sites, including bird baths, clogged rain gutters and even your wheelbarrow.

Report Standing Water in Fort Collins and Loveland
To report standing water in Fort Collins or Loveland, call (970) 962-2583.

Protect Yourself — Mosquito Repellent

Mosquito Repellent for Adults and Protection Times, Larimer County Department of Health and Environment
This link to Larimer County's West Nile virus page includes information about how long specific repellent products offer protection for adults.

Mosquito Repellent for Children and Protection Times, Larimer County Department of Health and Environment
This link to Larimer County's West Nile virus page includes information about how long specific repellent products offer protection for children.

Information About Insect Repellents, CDC
This link provides details on repellents, including information on plant-based (non-DEET) repellent products endorsed by the Centers for Disease Control.

West Nile Virus Signs and Symptoms

Disease Symptoms, Fight the Bite
This link explains the how West Nile virus is spread and discusses common symptoms of the disease.

West Nile Virus — Birds, Pets and Other Animals

Some pets and wild animals are also susceptible to West Nile virus.

Birds, Fight the Bite
West Nile virus has been found in dead birds of nearly 140 species. Although birds, particularly crows and jays, infected with West Nile virus can die or become ill, most infected birds do survive.

Horses, Fight the Bite
The Colorado Department of Agriculture has information for horse owners about vaccination for their horses.

Dogs and Cats, Fight the Bite
According to the Centers for Disease Control and Prevention, West Nile virus does not appear to cause extensive illness in dogs or cats.

Other Resources

WNV Resource List, Colorado State University Extension
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