West Nile Virus (WNV) is a mosquito-borne disease named for the district in Uganda where it was first identified in 1937. Its appearance in New York in 1999 was the first record of this virus in the Western Hemisphere. Since that time, the virus has spread rapidly through the eastern United States: north into Connecticut in 1999, into Massachusetts and New Hampshire in mid-summer of this year, and, most recently, south through New Jersey, Pennsylvania, and into the Maryland-D.C. area. Probably, the spread of the disease is mediated by the movement of infected birds. Birds are the primary reservoir of WNV, although the disease has also been found in mammals, including horses, several species of rodents, and humans.

WNV is transmitted by mosquitoes. A mosquito acquires the disease by biting an infected organism. The mosquito then passes the disease to the next organism that it bites. The primary mosquito carrier in the northeastern US is Culex pipiens, a species complex common to urban areas. WNV also has been found in seven other species of mosquitoes in the US, and at least one of these species is a competent transmitter of the disease. Recently, USGS scientists reported that the disease could be spread among birds without a mosquito intermediary, although the mechanism of transmission remains a mystery.

WNV poses a direct threat to bird populations in the Western Hemisphere. To date, the virus has been found in 59 species of birds in the eastern U.S. Among the thousands of dead birds that have tested positive for the virus, crows are most numerous (620 American Crows). Laboratory tests have demonstrated that in American Crows, the virus is 100% lethal, but mortality rates are likely to vary across species. Unfortunately, introduced viruses typically cause high mortality. Migrating birds infected with West Nile Virus could carry the disease to their wintering grounds, possibly leading to the infection of resident birds in Central and South America, as well as other wintering birds from the central and western U.S. and Canada. The ultimate effect on bird populations in the Western Hemisphere is unknown but potentially devastating.

In humans, WNV is usually benign. Most infections occur without symptoms. However, in immunocompromised individuals, typically the elderly, WNV can cause encephalitis or meningitis. In 1999, the virus caused serious illness in 62 people, and seven people died. This year, the number of serious illnesses is 17, with one fatality.

Along the east coast of the U.S., the spread of WNV has been rapid. Many cities and counties in states where the virus has been detected have implemented mosquito control measures. These measures include spraying of pesticides for control of adult mosquitoes and application of larvacides to kill mosquitoes in their aquatic, larval stage. The National Audubon Society has urged that spraying of pesticides be restricted to areas where WNV has been detected. The American Bird Conservancy, in a recently released statement, has opposed the use of pesticides in response to WNV. Instead, the ABC calls for the controlled use of larvacides and increased efforts to eliminate potential mosquito breeding sites (i.e., pools of stagnant water) near urban and suburban areas. The ABC argues that the spraying of pesticides is ineffective in controlling the spread of WNV, harmful to birds and other wildlife, and a health risk to humans.

The most commonly used pesticides in mosquito control are pyrethrin and pyrethroids. These compounds work by interfering with sodium channel function in insects and are effective at killing adult mosquitoes, but also kill most other insects. Insects provide food for many vertebrates, including frogs, bats, and birds, so pesticide use triggers a cascade of detrimental side effects in the local ecosystem. Also, these pesticides are directly toxic to fish and aquatic invertebrates and are suspected of causing endocrine disorders in vertebrates, including humans. In addition, widespread spraying could result in the evolution of pesticide resistance in the mosquitoes.

The ABC points out that the use of pesticides has a negative impact on birds directly, through toxic effects, and indirectly, by reducing the birds' food supply. Pesticide use will increase the stress on birds in migration, potentially increasing rather than decreasing WNV infection rates by making individual birds more vulnerable to infection. In this way, pesticide use can enhance the spread of WNV, argues the ABC.

» printer friendly version