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West Nile Virus Crops Up Again in the U.S.
July 20, 2001

For the third year in a row, the West Nile Virus (WNV) has been detected in birds and mosquito pools throughout the Northeast and Mid-Atlantic regions of the United States. WNV is a mosquito-borne disease that generally causes mild flu-like symptoms but can cause more serious and sometimes fatal infections of the brain.

WNV was first detected in North America in August, 1999 in New York City. By the year’s end, the virus had been detected in people (62 acute cases, including 7 deaths), mosquitoes, birds, and other animals across the region. That year, government agencies responded to the virus with the aerial and ground spraying of three different insecticides: the organophosphate malathion and the pyrethroid insecticides resmethrin and sumithrin.

Responding to the public’s criticism of the government’s use of pesticides as a remedy, affected municipalities in New York City and surrounding counties, and the states of Connecticut and New Jersey instituted preventative and least toxic control measures in the spring of 2000, (including source reduction, targeting larvae with biological agents, monitoring and surveillance). All state and local response plans that year described using insecticides to target adult insects as a tactic of last resort, but when the virus was detected in migratory birds in the New York City region, heavy spraying followed. Urged by the U.S. Centers for Disease Control, most (but not all) municipalities pursued an eradication-oriented strategy, spraying for adult insects up to a 2-mile radius around each infected bird and mosquito-pool detected.

Despite chemical spraying, the virus was neither contained nor eradicated, and last year it spread rapidly — detected in Washington D.C. and twelve states. Although the range expanded and the number of viral detections increased, the incidence of disease fell to 21 acute human cases across three states.

This year, WNV has been detected as far south as Florida and Georgia. In contrast to previous years, however, pesticides that target adult mosquitoes have so far only been deployed in a few neighborhoods in Ocean Township, New Jersey, where the pyrethroid insecticide resmethrin was sprayed in the second week of July. In all other areas for which information is currently available, preventative measures have been stepped-up in response to detections of the virus, but no pesticide spraying for adult mosquito control has yet occurred. The situation, however, changes daily.

A recent review of WNV in Europe cited in the Morbidity and Mortality Weekly Report confirmed that a high level of viral activity — such as in infected migratory birds or mosquito pools — can occur without a corresponding human outbreak and is therefore not a reliable indicator of risk to humans. The review also reports that a large human outbreak in a given region is often followed by a year with sporadic or no disease incidence, a pattern that reflects the experience to date in the United States.

The risk of a large human outbreak of WNV depends on a complex interaction of numerous factors — including weather patterns and the balance between preferred hosts and vector species — rather than the simple presence of the virus. Individual human cases do not necessarily signify a looming outbreak, but such cases can be difficult to prevent, since WNV is continually reintroduced into regions by migratory birds and could possibly infect the native bird population as well. Interestingly, in a recent press release, the U.S.-based conservation group National Audubon Society reported that a study conducted by a New York state wildlife official found that although the virus was a factor in some deaths, the leading cause of death in over 80,000 birds examined for WNV was pesticide poisoning.

Thus far, no states except for New Jersey have sprayed insecticides that target adult insects this year. In other areas, authorities have addressed early virus detections in birds and mosquito pools by increasing source reduction measures and preventative larval control measures. The current New York State WNV response plan reflects this more ecologically-based approach by stating that spraying insecticides that target adult insects should only be considered when there are numerous adult vectors and when multiple detections of viral activity suggest a high risk of human infection. The plan also says that finding a WNV positive bird or mosquito pool does not by itself constitute evidence of an imminent threat to human health and does not warrant targeting adult mosquitoes with insecticides.

WNV has established itself in the U.S. and the permanent emergency mode of chemical pesticide spraying in New York State and surrounding areas appears to have been set aside for now. Yet the possibility remains that if viral detections escalate, the current policy of caution could give way to a new round of widespread spraying. Meanwhile, many outstanding questions and concerns remain regarding pesticide efficacy, the public health impact of the pesticide spraying and a host of regulatory violations by pesticide applicator companies and manufacturers.

For more information on WNV, see Rachel’s environment & health biweekly #709 and #710 (October 12 and 26, 2000) available through Environmental Research Foundation, P.O. Box 5036, Annapolis, MD 21403; fax (410) 263-8944; email erf@rachel.org; Web site http://www.rachel.org.

Sources: Audrey Thier, Environmental Advocates; Cartter , M. et al. April 13, 2001. “Human West Nile Virus Surveillance — Connecticut, New Jersey, and New York, 2000.” Morbidity and Mortality Weekly Report. 50(14);265-8; Hubalek, Z. 2001. “European Experience with the West Nile Virus Ecology and Epidemiology: Could It Be Relevant for the New World?” Viral Immunology. 13(4):415-426; New York State Department of Health. 2001. New York State West Nile Virus Response Plan — Guidance Document. Albany, NY http://www.health.state.ny.us/nysdoh/westnile/index.htm; National Audubon Society Press Release, “Audubon Learns Pesticide is Leading Cause of Bird Deaths in New York Audubon calls on Other States to Test for Pesticides, Release Data” June 22, 2001.

Contact: Environmental Advocates, 353 Hamilton St., Albany, NY 12210; phone (518) 462-5526 ext. 236; fax (518) 427-0381; email webeditor@eany.org; Web site http://www.eany.org. National Audubon Society, National Audubon Society; 700 Broadway; New York, NY 10003; phone (212) 979 3000; fax (212) 979 3188; email audubonaction@audubon.org; Web site http://www.audubon.org.

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