The Westward March of West Nile
In its fourth year in the United States, the West Nile Virus (WNF) continues spreading westward through North America. The virus, which is transmitted by mosquitoes to birds, horses and humans, is now found in 36 states and Washington DC in the U.S., and Manitoba, Ontario, and Quebec in Canada. Cornell University reports the virus has now passed the 103rd meridian and has been detected as far west as Butte, County South Dakota on the Wyoming/ Montana Border.
West Nile Virus has been common in Africa and Asia for decades but was not found in the Americas until 1999, when it first appeared in New York. The New York Times reports on August 16, 2002, that health officials have identified 169 people infected this summer by the virus within the U.S., and 9 fatalities. Most people who contract West Nile virus suffer headaches and flu-like symptoms, which in elderly, chronically ill and other people with weak immune systems can develop into potentially fatal encephalitis, meningitis or other inflammations of the brain and spinal cord. According to CDC, this occurs in about one in 200 cases. The 2002 outbreak appears centered around Louisiana, with 85 cases reported. Texas and Mississippi have also reported a number of cases.
The virus appears to have appeared earlier in the season this year. In Massachusetts, WNV-positive birds were found 7 weeks earlier than last year; in Canada the virus was first found in May, vs. August last year. Seasonal conditions and weather are significant factors in the progression of WNV, as are the life cycles and biology of the mosquitoes that carry it.
In 1999 New York and other municipalities first relied heavily on pyrethroid insecticides such as Scourge (active ingredient resmethrin) and Anvil (active ingredient, phenothrin ) to kill adult mosquitoes. In a 2000 article on WNV, Rachel’s Environmental & Health Bi-Weekly reported that “The pyrethroids act as nerve toxins, and may have other long-term health effects. A 1999 study on how pyrethroids affected breast cancer cells in a laboratory setting led researchers to suggest that the pyrethroids as a group should be considered hormone disrupters. EPA is scheduled to re-evaluate the health effects of the pyrethroids in 2002.”
“The chemicals have not been adequately tested for their human health effects,” cautioned Dr. Sheldon Krimsky, a pesticide-risk specialist at Tufts University. “There is a lot of circumstantial evidence that they cause cancer in animal studies, that they are hormone disruptors. Remember, these are neurotoxins,” Krimsky said, adding that most studies done on the effects of spraying focused on agricultural spraying — not spraying in populated areas. “We simply don’t know what effects the indiscriminate spraying on human populations is going to have,” he said.
After several years experience, officials in New York are now relying less on “adulticides,” (i.e. spraying pesticides that kill adult mosquitoes) – and instead are targeting control efforts at other phases of the mosquito lifecycle, while emphasizing monitoring and surveillance. In Alameda County, California, mosquito control officials have concluded that control of mosquito larvae is key. Alameda County relies upon bacterial controls such as Bacillus thuringiensis which affect the larval digestive system, or methoprene, a juvenile growth regulation hormone that affects development of mosquito larvae. In contrast to spraying to kill adult mosquitoes, these larval compounds target mosquitoes more effectively, are less damaging to mosquito predators, and present relatively less risk to human health.
Other parts of the country are starting to adopt similar approaches. In early August, County Commissioners in Hays County, Texas canceled the spray program they had approved just two weeks before. Instead, they will begin an intensified mosquito surveillance, monitoring, education, and prevention program. In Texas, only the Southern House (Culex) mosquito has been found to carry West Nile and other diseases. This mosquito breeds in containers and polluted water, especially water polluted with sewage, making elimination of standing water a crucial control measure.
For the long term, Audrey Thier of Environmental Advocates in Albany, NY, suggests that agencies develop a decision matrix for understanding when various factors that lead an enzootic (animal epidemic causing) virus that has become established in local bird or animal populations to pose a threat of breaking out to cause a human epidemic (rather than sporadic cases). This approach is confirmed in a 2001 article in Environmental Health Perspectives by Dr. Duane Gubler, who raises the specter of new global patterns of disease born infections: “Climate change is likely to affect the transmission patterns of vector-borne pathogens, but more research is needed to clarify the interactions of weather variables and the diseases they affect.” He warns that insect and environmental monitoring programs to detect the presence of virus such as West Nile in host birds, horses, and mosquitos are not routine across the country. “Some states, such as California, Florida, New Jersey maintain programs, others do not. Support for [such] surveillance programs is frequently a very low priority in state health departments, and interest and capability regarding [such] viruses has lessened over the past 20-30 years.”
The swift advance of the West Nile Virus suggests that it is here to stay. Growing numbers of public health experts, local government officials and community groups recognize that massive spray programs based on hazardous broad spectrum pesticides in response to every detection is neither effective in the short term nor sustainable in the long. Such groups are finding common ground in advocating the development of long-term, least-toxic management strategies involving larval control, monitoring and surveillance, and increased understanding and action to address of all factors that contribute to epidemic outbreaks of viral activity.
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