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Causes
By Mayo Clinic staff
Infection transmitted by mosquitoes
In the United States, wild birds, especially crows and jays, are the main reservoir of West Nile virus, but the virus is actually spread by certain species of mosquitoes.
Here's how the transmission cycle works:
When a mosquito bites a bird infected with the West Nile virus, the virus enters the mosquito's bloodstream and circulates for a few days before settling in the salivary glands. When the infected mosquito bites an animal or a human, the virus then enters the host's bloodstream, where it may cause serious illness.
Although the exact mechanism of illness is unknown, West Nile virus probably enters the host's bloodstream, multiplies and moves on to the brain, crossing the blood-brain barrier — a barrier that separates the blood from the central nervous system. Once the virus crosses that barrier and infects the brain or its linings, the brain tissue becomes inflamed and symptoms arise.
West Nile virus transmission occurs mostly during warm weather, when mosquito populations are active. The incubation period — the period between when you're bitten by an infected mosquito and the appearance of signs and symptoms of the illness — ranges from two to 14 days.
Other possible routes of transmission
In rare cases, it's possible for West Nile virus to spread through other routes, including:
- Organ transplantation and blood transfusion. Donated organs are not yet screened for West Nile virus. However, blood donors are screened for the virus, substantially reducing the risk of infection from blood transfusions.
- Mother to unborn child. Although it appears that a mother may pass the virus to her unborn baby, it's not known whether this can result in abnormalities in the child.
- Breast-feeding. In a few instances, mothers have transmitted West Nile viruses to their infants through their breast milk. But experts at the Centers for Disease Control and Prevention (CDC) say that such cases are extremely rare and should not affect a woman's decision to breast-feed her baby.
- Laboratory acquisition. Some laboratory workers involved in West Nile surveillance and research have contracted the disease from infected animals.
