Understanding The West Nile Virus Cycle

City of Eastvale


What is the West Nile Virus?

West Nile (WN) virus is a mosquito-borne virus that has been found in parts of Asia, Eastern Europe, Africa, and the Middle East. The virus was first detected in the United States in 1999 (in New York City). The majority of people and animals that are infected with the virus have a mild illness or no symptoms. In rare cases, the virus can cause a more serious inflammation of the brain. The elderly are at a higher risk for disease caused by West Nile virus.
How Do People And Animals Get West Nile Virus?
WN virus is transmitted to people and animals by infected mosquitoes. Only certain species of mosquitoes carry the virus and very few mosquitoes are actually infected. A mosquito first acquires the infection by feeding on a bird with the virus in its blood. The virus lives in the mosquito and is transmitted to a new host in the mosquito’s saliva when the insect bites a person or animal. Humans and horses are incidental hosts for the West Nile virus. The virus is most prevalent from May to October when mosquitoes are most abundant.

What Are The Symptoms Of West Nile Virus In People?

Most people who are infected with WNV have no symptoms whatsoever. However, of those who become ill, symptoms can include fever, headache, nausea, body aches, mild skin rash, or swollen lymph nodes. In a few cases, the disease will progress to encephalitis (inflammation of the brain). The time between the mosquito bite and the onset of the illness, known as the incubation period, ranges from 5-15 days in humans. It is estimated that 1 in 150 people who are infected with WNV will require hospitalization. The elderly are particularly susceptible to clinical illness caused by WNV. There is no specific treatment for infection with WNV, although supportive care is important.

Which Animals Get West Nile Virus?
An infected mosquito can bite any animal, but not all animals will become infected. The disease most often affects birds, but occasionally causes disease in other animals as well.

  • Birds: Wild birds are the animals from which the mosquito vector primarily acquires the virus. Infection has been reported in more than 138 bird species. Although many birds that are infected with WNV will not appear ill, WNV infection can cause serious illness and death in some birds. The most severe illnesses are seen among the corvid birds, which include crows, jays, ravens, and magpies. American crows constitute the majority of the birds reported dead due to WNV.
  • Horses: Horses are also susceptible to WNV. The disease does not seem to be specific to a particular breed or age of horse. Clinical signs of disease consist of central nervous system abnormalities similar to those caused by infection with eastern equine encephalitis (EEE) and western equine encephalitis (WEE). EEE and WEE vaccines are available for horses and are recommended for use in the spring. An equine WNV vaccine is now also available.

West Nile Virus Prevention And Control
To decrease exposure to mosquitoes and the infections they may carry:

  • Avoid outside activity at dawn and dusk during the mosquito season (May to October). This is particularly important for elderly and small children.
  • Wear protective clothing (long pants and long sleeves) and apply insect repellant when outside.
  • Make sure that doors and windows have tight fitting screens. Repair or replace screens that have tears or holes in them.
  • Drain all standing water on private property and stock permanent ponds with fish that eat mosquito larvae.
  • Make sure roof gutters drain properly. Clean clogged gutters in the spring and fall.

What Do I Do If I See A Dead Bird?

If you find a dead bird, particularly a dead crow or other corvid (e.g., jay, magpie, raven), please call the number below promptly. Do not touch the bird.

Dead Bird Surveillance and Contact Information:  West Nile Virus Dead Bird Surveillance Program
Toll Free Telephone: 877-WNV-BIRD (877-968-2473)
Website: http://westnile.ca.gov