West Nile Virus Basics

What is West Nile virus?

West Nile virus is a mosquito-borne virus naturally found in bird populations. Occasionally, the infection can be transmitted to other animals and humans through the bite of an infected mosquito.

How can I get it?

Picture of the West Nile Virus Transmission cycle West Nile virus is typically transmitted though the bite of an infected mosquito, however in rare cases transmission may occur from human to human by:

  • Transplacental transmission from mother to unborn child
  • Breastfeeding
  • Blood transfusion from an infected donor
  • Organ transplantation from an infected donor

The principal transmission cycle of West Nile virus involves several species of birds and the virus has been isolated from 43 species of mosquitoes nationwide. The virus is transmitted when a mosquito feeds on an infected bird and later feeds on another animal. Some varieties of bird may carry a high enough concentration of virus in the blood to pass on to mosquitoes which feed on their blood and are known as reservoirs. However, some animals, including humans, carry much lower levels of virus if infected. At these levels, it is thought that virus cannot in turn be passed on to biting mosquitoes. These groups are known as incidental hosts.

How dangerous is it for humans?

For 80 percent of people infected with West Nile virus, there are no symptoms at all. The majority of the other 20 percent will show symptoms associated with West Nile fever. These include flu like symptoms such as:

  • Fever
  • Fatigue
  • Headache
  • Swollen lymph glands
  • Rash on the trunk of the body

Approximately one percent of people infected with West Nile virus may go on to develop severe neuroinvasive disease such as West Nile meningitis, West Nile encephalitis or West Nile poliomyelitis. Clinical symptoms include:

  • High fever
  • Weakness
  • Stiff neck
  • Severe headache
  • Change in mental status
  • Flaccid paralysis
  • Gastrointestinal symptoms
  • Seizures

Symptoms are seen 3-14 days after infection.

How dangerous is it for domestic pets, horses and other animals?

Horses can contract West Nile virus and in 40 percent of cases, an infection may result in death. In areas where West Nile virus activity has been detected, horse owners should consult their veterinarians regarding the West Nile virus vaccine licensed for use in equines.

Cats and dogs may be infected with West Nile virus and in a small number of cases, which can result in illness. Most healthy cats and dogs will not show any symptoms and will recover naturally. There are no documented cases of West Nile virus transmission from cats or dogs to humans. Low virus levels in dog saliva suggest a low risk of virus transmission from the bite of a West Nile virus infected dog.

Should I report dead birds?

It is not necessary to report dead birds to the Health Department at this time.

West Nile virus may cause the death of several types of birds. In particular, crows, blue jays and raptors. In the past, dead birds provided an early sign of the circulation of West Nile virus within an area; however, mosquito surveillance is more reliable as the first indicator of virus circulation in Fairfax County.


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