West Nile Virus

The West Nile virus is a member of the family Flaviviridae, a virus that has become more prominent in Europe and North America in the past decade. The virus, which is closely related to the St. Louis encephalitis virus found in the United States, causes an encephalitis (swelling of the brain) in domestic animals (such as horses, dogs, cats), wild animals, and wild birds. When transferred from an infected animal to a human, the viral infection can produce encephalitis as well as inflammation of nerve cells of the spinal cord (meningitis).

In 1937, the virus was isolated from a woman in the West Nile District of Uganda. This locale was the basis for the designation of the virus as the West Nile virus. During the 1950s, the ability of the virus to cause the serious and life-threatening human disease was recognized. In the 1960s, the virus was established as a cause of equine encephalitis.

Whether the virus has spread geographically from Uganda, or whether increased surveillance has detected the virus in hitherto unsuspected regions is not clear. However, the pattern of detection has been that of a global dissemination. Long found in humans, animals, and birds in Africa, Eastern Europe, West Asia, and the Middle East, the virus was first detected in North America in 1999.

The virus has come to prominent attention in North America following its 1999 appearance on the continent. That year, 62 cases of the disease were reported in New York City. Seven people died. The following year 21 more cases occurred, and two of the people died. In 1999 and 2000, the West Nile virus was confined to the northeastern coastal states of the United States. However, an inexorable spread to other regions of the country and the continent has begun. In the summer of 2001, dead birds that tested positive for the virus were found as far north as Toronto, Canada, as far south as the northern portion of Florida, and as far west as Milwaukee, Wisconsin. Scientists anticipate that the virus will continue to disseminate. During the summer of 2002, more than 300 cases and at least 14 deaths were reported—with a continued spread of the virus into the western United States. By August 2002, West Nile virus was reported in 41 states.

The mosquitoes are the prime vector of the West Nile virus. When mosquitoes obtain a blood meal from an infected animal or a bird, they acquire the virus. The virus resides in the salivary glands of the mosquito, to be passed on to a human when the mosquito seeks another blood meal. The cases in New York City, especially those in 2000, are thought to have been caused by the bite of virus-infected mosquitoes that survived the cold winter months. The emergence of the mosquito in the spring can facilitate the re-emergence of the virus. For example in North America, there were large die-offs of crow populations due to West Nile virus in the Spring of 2000 and then again in the Spring of 2001.

Upon entry to a host's bloodstream, multiplication of the virus in the blood occurs. Then, by a mechanism that is not yet deciphered, the virus crosses the barrier between the blood and the brain. Subsequent multiplication of the virus in brain tissue causes nervous system malfunction and inflammation of the infected brain tissue.

Although a large population of mosquitoes may be present, the chances of acquiring West Nile virus via a mosquito bite is small. Data from the examination of mosquito populations indicates that less than one percent of mosquitoes carry the virus, even in areas where the virus is known to be present.

The mosquito to human route of infection is the only route known thus far. The virus is known to infect certain species of ticks. However, as of early 2002, tick-borne out-break of the disease has not been documented in humans. Person to person contact cannot occur. Even exchange of body fluids between an infected human and an uninfected person will not transmit the virus.

Currently no human vaccine to the West Nile virus exists. Prevention of infection consists of repelling mosquitoes by conventional means, such as the use of repellent sprays or creams, protective clothing, and avoiding locations or times of the day or season when mosquitoes might typically be encountered.

See also Viruses and responses to viral infection; Zoonoses