West Nile Virus (WNV) is one of a family of Flaviviruses which can cause encephalitis in humans and horses. The virus is carried in the blood of birds and is transferred to humans and horses via a bite from a mosquito which has recently bitten an infected bird.
Although the usual effects of the disease are mild, it can be fatal to humans who are very old or young, or who have compromised immune systems. Some public health officials in the Northeast have decided, often in the face of vigorous protest, to spray pesticides in areas where there are infected adult mosquitoes to reduce incidence of the disease. In 2000 some areas of Boston were sprayed and the potential exists for more Massachusetts spraying in 2001.
The Virus
The virus is so-named because it was first isolated in 1937 in the West Nile district of Uganda. An outbreak in Israel in 1957 caused cases of severe meningoencephalitis (inflammation of the spinal cord and brain) and another outbreak in New York in 1999 (the first time it was encountered in the Western Hemisphere) killed 7 elderly people. The disease is present now in parts of Africa, Europe, the Mid-East, West Central Asia, Oceania, and North America.
Many people mistakenly believe that mosquito-borne diseases are only a danger in tropical regions. But yellow fever, dengue and other such diseases can be threats in temperate regions during the mosquito season. In the Little Ice Age, when temperatures were significantly lower than now, malaria was known as far north as England (Oliver Cromwell, in fact, died of malaria.)
The symptoms of WNV are usually those of a low grade flu: headaches, fever, and muscle weakness. Many cases are so mild that the victim does not know he or she is infected. The Center for Disease Control (CDC) advises residents of WNV infected areas that most mosquito bites still do not transmit the virus, and most infected people do not become ill. By comparison, a typical severe flu season in the US (in spite of the distribution of 80 million does of flu vaccine) kills 20,000 people, with over 110,000 hospitalized.
A small proportion of the people infected with WNV, however, cannot easily fight the disease and suffer serious results: disorientation, convulsion, paralysis, coma, even death. The seven mortalities in 1999 were all individuals 68 or older. Of the other elderly who were seriously affected, many had lingering neurological damage including confusion, muscle weakness, and memory loss.
Use of Pesticides to Control West Nile Virus
In response to the 1999 outbreak in New York City, Mayor Rudolph Giuliani began a series of helicopter and truck sprayings of the pesticide Malathion. Surrounding areas such as Nassau, Westchester and Suffolk Counties, as well as Fairfield County in Connecticut, sprayed Malathion or one of the synthetic pyrethrins - Resmethrin or Sumithrin. Critics of the sprayings charge that they are an over-reaction motivated by political rather than public health concerns. They argue that the sprayings are in themselves more of a threat to public health than the virus they are designed to prevent.
Malathion is an organophosphate, the class of pesticides originally designed as nerve and immune system poisons in Germany for chemical warfare during World War II. Exposure can occur through ingestion, inhalation, and skin absorption. Malathion is a cholinesterase blocking agent, keeping this key nervous system enzyme from functioning properly. Instead, nerve electrical signals become uncontrolled, leading to muscle spasms, paralyzed breathing, cardiac irregularities, and potential death. It has also been associated with long-term reproductive and endocrine disrupting effects, and mutations in human T lymphocytes, crucial immune system components.
Synthetic pyrethroids such as Resmethrin, Permethrin, and Sumithrin act by interfering with sodium ion channels to block nerve impulses, as well as affecting both male and female reproductive systems by binding to hormone receptors. Breast cancer, lowered sperm counts, thyroid damage and childhood brain cancers are linked to exposure to pyrethroids. Respiration is the quickest way to experience ill effects. Inhaling pyrethroids can cause burning and itching, sneezing, asthmatic breathing, headache, nausea, and convulsions. Aquatic organisms and bees are particularly vulnerable to damage from pyrethroids.
Compounding the human ill-effects of the active ingredients in these sprays is the presence of piperonyl butoxide as a synergist. Piperonyl butoxide is to be reassessed by the EPA in 2002, but has been linked to endocrine disruption and cancers. The combination of pesticide and synergist chemical makes the mix significantly more toxic than either alone.
Prevention
Many health professionals feel that prevention of exposure to WNV is a more reasonable goal than eradication. Prevention involves behavioral measures such as staying inside or wearing long-sleeved and -legged clothes when walking outdoors during mosquito season, wearing repellants, keeping window and door screens repaired, and eliminating sources of stagnant water (to breed, mosquitoes require water which has been stagnant for at least 4 days.)
Critics of spraying charge that pesticides are ineffective against WNV-carrying adult mosquitoes because as little as 1% of the active ingredient ever reaches the insects, which then quickly develop resistance. These compounds are also broad spectrum killers which destroy many of the natural predators of mosquitos as well as the pests themselves. More effective in mosquito control, many believe, are the biological larvacides such as Bacillus thuringiensis israelensis (Bti) and Bacillus sphaericus (Bs). These are naturally occurring bacteria which, if eaten by mosquito larvae, disrupt the gut of the mosquito by binding to specific insect receptor cells. These biological products are of no danger to mammals and have not been shown to damage non-target species, wildlife, or the environment.
Devices are also on the market which confuse or trap mosquitoes by emitting puffs of carbon dioxide (the gas which mosquitoes detect as part of their homing-in on warm-blooded prey.) While these devices are beyond the reach of most homeowners -- running up to $1000 to protect an area the size of a house lot -- municipalities may find them affordable for parks and other busy urban areas likely to harbor mosquitoes.
Preventing Spraying
Whether pesticides are sprayed in your town depends on several things. Public health officials are actively monitoring local WNV levels in mosquitoes, birds, and humans. If they detect a rise in virus levels in any of these, they may call for spraying to begin. Once human cases are encountered, particularly when a human death occurs, residents understandably become frightened. Many will call for public spraying unless they understand the risks from being exposed to these pesticides. At this stage it is very difficult for politicians to resist such calls for action. So widespread knowledge and preparation before the problem develops is the best way to prevent spraying.
In Massachusetts, 140 cities and towns are part of the 9 state Mosquito Control Districts (MCD), while 210 municipalities have not joined such districts. Officials in a MCD are trained and licensed to spray for mosquitoes. No such officials are licensed in non-MCD towns. In 2000, the state Department of Food and Agriculture (whose Pesticide Bureau regulates use of pesticides in the Commonwealth) began a program to train and license non-MCD town employees to apply larvacides in catch basins where stagnant water collects.
Determine how your town fits into the mosquito control structure. Is it a member of a MCD? Is anyone in town tracking the levels of WNV in overwintering mosquitoes? Do you have employees licensed to spray larvacides? Do you have employees licensed to spray adulticides? Who in your town will make the ultimate decision whether or not to spray?
Early efforts to reduce mosquito breeding areas and educate people about minimizing exposure to mosquitoes are important. Inclusion of opponents to spraying in discussions with town officials before problems develop will keep you in the loop where you can raise alternatives. Getting doctors, environmentalists, proponents of hunting and fishing, chemically sensitive individuals and organic advocates to speak up will let town officials know there is a constituency opposed to spraying. Letters to the editor, petitions to boards of health, and educational materials at public forums all can let others of your persuasion know that there is an alternative and where they can go to offer help.
The bad news is that while the risks of death or long-term neurological damage from West Nile Virus are not large, they do exist. So do risks associated with spraying pesticides to control mosquitoes. The good news is that alternatives of prevention and early intervention exist which can reduce the likelihood that either infection or spraying happen in your area.
For more information:
For information on groups opposed to spraying:
This page was last modified on January 14, 2008 at 12:25:30 PM.