West Nile Virus Transmission

Q. How do people get infected with West Nile Virus (WNV)?

A. The principal route of human infection with West Nile Virus is through the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for a few days. The virus eventually finds its way into the mosquito's salivary glands. During subsequent blood meals, the virus may be injected into humans and animals, where it can multiply and possibly cause illness.

Additional routes of infection have become apparent during the 2002 West Nile epidemic. It is important to note that these other methods of transmission represent a very small proportion of cases. A recent investigation has confirmed WNV transmission through transplanted organs. Investigations of other patients who developed WNV infection within several weeks of receiving blood products or organs are ongoing to determine whether WNV was transmitted by transfusion or transplantation in any of these cases. There is one reported case of transplacental (mother-to-child) WNV transmission. This case is detailed in MMWR Dec 20, 2002. Although transmission of WNV and similar viruses to laboratory workers is not a new phenomenon, two recent cases of WNV infection of laboratory workers have been reported. These cases are detailed in MMWR Dec 20, 2002.

Q. What is the basic transmission cycle of West Nile Virus?

A. 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.

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

A. 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 1% of people who get bitten and become infected will get severely ill. The chances you will become severely ill from any one mosquito bite are extremely small.

Q. Can you get West Nile encephalitis from another person?

A. 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 healthcare worker who has treated someone with the disease.

Q. Is a woman's pregnancy at risk if she gets infected with West Nile Virus?

A. There is one documented case of transplacental (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 WNV and other arboviral infections by avoiding mosquitoes, using protective clothing and repellents containing DEET (See Insect Repellent Use and Safety). When WNV transmission is occurring in an area, pregnant women who become ill should see their healthcare provider. Those whose illness is consistent with acute WNV infection (see Symptoms of West Nile Virus) should undergo appropriate diagnostic testing.

For more details regarding the case described above, please see: MMWR Dec 20, 2002.

Q. Can West Nile Virus be transmitted through blood transfusions?

A. Please refer to Blood Transfusions and Organ Donations.

Q. Are laboratory workers in contact with WNV-positive specimens at risk for WNV infection?

A. Yes, and approximately 20 laboratory-acquired WNV infections have been reported in the medical literature over many decades. In the two most recently reported cases, WNV infection of two microbiologists working with WNV-positive samples resulted from percutaneous inoculation (pierced through the skin). Both persons had mild, self-limited illnesses.

As more laboratories have recently become involved in WNV diagnostic and reference activities, the risk for laboratory-acquired WNV infections has probably increased. Laboratory workers handling materials that are potentially infected with WNV should use every precaution to minimize their risk for exposure. For more details, please see: MMWR Dec 20, 2002.

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

A. Infected mosquitoes are the primary source for West Nile Virus. Although ticks infected with West Nile 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.

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

A. 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.

Q. Can you get West Nile Virus directly from birds?

A. There is no evidence that a person can get the virus from handling live or dead infected birds. However, persons should avoid bare-handed contact when handling any dead animals and use gloves or double plastic bags to place the carcass in a garbage can.

Q. Can you get infected with West Nile Virus by caring for an infected horse?

A. West Nile Virus is transmitted by infectious mosquitoes. There is no documented evidence of person-to-person or animal-to-person transmission of West Nile Virus. Normal veterinary infection-control precautions should be followed when caring for a horse suspected to have this or any viral infection.

Q. Can you get WNV from eating game birds or animals that have been infected?

A. There is no evidence that WNV 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 food-borne pathogens, people should always follow procedures for fully cooking meat from either birds or mammals.

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

A. 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.

Q. How long does the West Nile Virus remain in a person's body after they are infected?

A. There is no scientific evidence indicating that people can be chronically infected with West Nile Virus. What remain in a person's body for long periods of time are antibodies and "memory" white blood cells (T-lymphocytes) that the body produces to the virus. These antibodies and T-lymphocytes last for years, and may last for the rest of a person's life. Antibodies are what many diagnostic tests look for when clinical laboratory testing is performed. Both antibodies and "memory" T-lymphocytes provide future protection from the virus.

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

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

See also:

West Nile Virus and Horses

West Nile Virus and Birds

West Nile Virus and Wild Game Hunters