West Nile Virus Home > West Nile Virus Transmission

Life Cycle of the West Nile Virus

Mosquitoes become infected when they feed on infected birds. After it has been transmitted, the virus may circulate in the blood for a few days.
An infected mosquito can then transmit West Nile virus to humans and animals while biting to take blood. The virus is located in the mosquito's salivary glands. During blood feeding, the virus can be injected into an animal or a human, where it may multiply and possibly cause illness.
Following transmission by an infected mosquito, West Nile virus multiplies in the person's blood system and crosses the blood-brain barrier to reach the brain. The virus interferes with normal central nervous system functioning and causes inflammation of brain tissue.

West Nile Virus Transmission During Pregnancy

There is one documented case of transplacental (mother-to-fetus) West Nile virus transmission in a human. Although the newborn in this case was infected with West Nile virus at birth and had severe medical problems, it is unknown whether the West Nile virus infection itself caused these problems or whether they were coincidental.
More research will be needed to improve our understanding of the relationship -- if any -- between West Nile virus infection and adverse birth outcomes.
Nevertheless, pregnant women should take precautions to reduce their risk for West Nile virus and other arboviral (spread by insects and spiders) infections by avoiding mosquitoes, wearing protective clothing, and using repellents containing DEET. When West Nile virus transmission is occurring in an area, pregnant women who become ill should see their healthcare provider.

Transmission Through Other Insects and Animals

Infected mosquitoes are the primary source for transmitting West Nile virus. Although ticks infected with virus have been found in Asia and Africa, their role in the transmission and maintenance of the virus is uncertain. However, there is no information to suggest that ticks played any role in the cases identified in the United States.
Although the vast majority of infections have been identified in birds, West Nile virus has been shown to infect:
  • Horses
  • Cats
  • Bats
  • Chipmunks
  • Skunks
  • Squirrels
  • Domestic rabbits.
There is no evidence that a person can get the virus from handling live or dead infected birds. However, people should avoid bare-handed contact when handling any dead animals and use gloves or double plastic bags to place a carcass in a garbage can.
There is no evidence that West Nile virus can be transmitted to humans through consuming infected birds or animals. In keeping with overall public health practice, and due to the risk of known foodborne pathogens, people should always follow procedures for fully cooking meat from either birds or mammals.
Written by/reviewed by:
Last reviewed by: Arthur Schoenstadt, MD
Last updated/reviewed:
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