West Nile virus (WNV) is carried by birds, and in the wild, has only been known to be transmitted from bird to bird by mosquitoes. Mosquitoes become infected when they bite, or take a blood meal, from a bird carrying WNV; the infected mosquito then transmits the virus to another bird through a second bite. Birds in the "enzootic cycle" are relatively resistant to disease, and serve to maintain the virus in the avian population. Susceptible avian species and mammals may be infected when bitten by a "bridge vector" mosquito (one which will feed on birds and mammals), which essentially takes the virus out of the enzootic cycle. These bridge vector species of mosquitoes that bite mammals as well as birds may transmit the virus to a horse or human, or other mammal.
Research has shown that it is possible for bird to bird transmission to occur as a result of oral and cloacal secretions contaminating water and food. There may be a direct bird to bird transmission but the cause is unknown. Research has also shown that the virus may be passed to raptors by eating infected birds. However, the most likely route of transmission is still the mosquito.
In Michigan the most common species of mosquito associated with WNV transmission is the Culex sp. These are mosquitoes that primarily bite birds, and breed in small collections of stagnant water. When levels of WNV virus circulating amongst birds is high, people and horses are susceptible to accidental infection through the bite of an infected mosquito. Horses and humans are considered dead-end hosts because they do not produce enough virus in their blood to infect biting mosquitoes. There is no evidence that infected horses, people, or other animals are able to transmit the virus to other animals, people, or mosquitoes through normal contact.
In 2002 a positive Coquillettidia perturbans pool was detected for the first time in the state. This species may be important as a potential bridge vector as it bites both birds and mammals.
Migrating birds carrying WNV in their blood have had a significant impact on the spread of the virus across the United States. Birds in the family Corvidae-crows, blue jays, and ravens-have proven to be especially susceptible to WNV infection, and have been the focus of WNV surveillance efforts across the United States and Canada. Residents in areas where WNV activity has been identified are at risk of contracting West Nile encephalitis or West Nile fever. However, very few mosquitoes actually become infected with the virus, and less than 1% of people who are bitten become infected and severely ill. In Michigan the virus was detected in 73 out of 83 in 2002 Upper and Lower Peninsula counties, making the virus endemic in the state. People over 50 years of age are found to be at the highest risk of developing severe disease. In addition, immunocompromised persons or people with underlying health conditions have an increased risk of developing more severe forms of WNV disease.
In 2002 the CDC confirmed additional routes of transmission which include:
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infection through transplanted organs
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one case of transplacental (mother-to-child) infection
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one case of transmission through breast milk
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blood transfusion-associated transmission
These routes of transmission represent only a very small proportion of WNV cases.
In attempts to reduce blood transfusion-associated transmission of WNV, blood collection agencies began experimental screening of donated units for WNV using nucleic acid amplification tests (NATs) in July 2003. Collection agencies subsequently identified 6 blood donors from Michigan who had WNV in their blood at the time of donation, 4 of which later developed West Nile illness. Currently, all blood banks are screening donor units for WNV. In addition, enhanced donor deferral questions have been implemented. Blood collection agencies specifically ask potential donors about occurrence of fever or headache the week before attempting to donate and defer those reporting such symptoms.
Please visit the CDC website links below for more information.
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