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Transmission of West Nile Virus FAQs

1.  What is the basic transmission cycle?

Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for a few days. Infected mosquitoes 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 may be injected into the animal or human, where it may multiply, possibly causing illness.

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2.  If I live in an area where birds or mosquitoes with West Nile Virus have been reported and a mosquito bites me, and I likely to get sick?

No. Even in areas where the virus is circulating, very few mosquitoes are infected with the virus. Even if the mosquito is infected, less than one percent of people who get bitten and become infected will get severely ill. The chances you will become severely ill from any on mosquito bite are extremely small.

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3.  Can you get West Nile Encephalitis from another person?

No. West Nile Encephalitis is NOT transmitted from person-to-person. For example, you cannot get West Nile Virus from touching or kissing a person who has the disease, or from a health care worker who has treated someone with the disease.

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4.  Is a woman's pregnancy at risk if she gets West Nile Encephalitis?

There is one documented case of trans-placental (mother-to-child) transmission of WNV in humans. Although the newborn in this case was infected with WNV at birth and had severe medical problems, it is unknown whether the WNV 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 WNV infection and adverse birth outcomes.

Nevertheless, pregnant women should take precautions to reduce their risk for WMV and other arboviral infections by avoiding mosquitoes, using protective clothing and repellents containing DEET. When WNV transmission is occurring in an area, pregnant women who become ill should see their health care provider. Those whose illness is consistent with acute WNV infection should undergo appropriate diagnostic testing. For more details regarding the case described above, please see Centers for Disease Control and Prevention (CDC) MMWR Dec. 20, 2002.

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5.  Is West Nile Virus (WNV) transmitted by blood transfusion or organ donation?

A recent investigation has identified transplanted organs as the source of WNV infection in four recipients or organs from a single donor. How the organ donor became infected is unknown. The organ donor might have become infected from a mosquito bite or possible acquired the infection through transfusion; an investigation of the numerous transfusions received by the organ donor is ongoing. Since the report of these cases, the Centers for Disease Control and Prevention (CDC) has been informed of other patients who developed WNV infection within several weeks of receiving blood products or organs. Investigations are ongoing to determine whether WNV was transmitted by transfusion or transplantation in any of these cases.

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6.  Besides mosquitoes, can you het West Nile Virus directly from other insects or ticks?

Infected mosquitoes are the primary source for West Nile Virus. Although ticks infected with West Nile Virus have been found in Asian 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.

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7.  How many types of animals have been found to be infected with West Nile Virus?

Although the vast majority of infections have been identified in birds, WNV has been shown to infect horses, cats, bats, chipmunks, skunks, squirrels, and domestic rabbits.

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8.  How does West Nile Virus actually cause severe illness and death in humans?

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.

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9.  What proportion of people with severe illness due to West Nile Virus die?

Among those with severe illness due to West Nile Virus, case-fatality rates range from three percent to 15% and are highest among the elderly. Less than one percent of persons infected with West Nile Virus will develop severe illness.

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10.  If a person contracts West Nile Virus, does that person develop a natural immunity to future infection by the virus?

It is assumed that immunity will be lifelong; however, it may wane in later years.

(Information on this page obtained from the Centers for Disease Control and Prevention)

For West Nile Virus information updates call toll free: 877-WNV-BIRD (877-968-2473)

Centers for Disease Control information on West Nile Virus

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