It seems that some people demand that a 'crisis' be present at all times. If one doesn't exist, it is created. Prior to the H1N1 media-fuelled panic, prior even to the bird flu panic spearheaded by WHO, there was West Nile Virus.
It was first recognized in Uganda in the late 1930's. Conspiracy-oriented belief is that it was introduced to North America by a germ warfare laboratory near New York City, and released when the lab was destroyed by a hurricane. Anyway, it's here now, to stay.
West Nile is a virus of birds. As the early epidemiological patterns showed, it's spread widely by waterfowl, ducks and geese, but is lethal to corvids - crows and jays. It's spread between birds by a few species of mosquitos that live off birds, almost totally Culex. On occasion, a human is infected by one of those mosquitos. Nearly always, human infection is minor, even totally unnoticed by those infected. And, unlike many viral diseases, influenza in particular, there is no evidence that one person with West Nile can transmit it to another.
However, in one person out of ten thousand or so, West Nile gets into their brain resulting in encephalitis. In about 10% of these cases, the person dies. The death rate is trivial compared to many other common human diseases, influenza in particular.
The reaction to West Nile, particularly that of the press, became a triumph of panic over both science and common sense. (As usual, of course, those who panic are always the first to accuse those who disagree with them as being unscientific and irrational.) Many jurisdictions, Manitoba for one, dispensed pesticides all over the landscape on the scale that Rachel Carson despaired of half a century ago. The Province of Quebec passed emergency legislation to override local health officials and bylaws, such as those in Chelsea, and spray Malathion if West Nile virus appeared in an area.
This response is grossly anti-science, and anti-health. Spraying of neurotoxins such as Malathion makes the danger to us of West Nile, and of many other viruses, worse, not less, in many ways:
First, predators are more active than their prey, or they couldn't catch them. Neurotoxins such as Malathion target the source of animal activity - the nervous system. So, spraying will kill mosquito predators faster than it will kill the mosquitos themselves. Amongst the data confirming this is some from a wetland in the State of New York: populations of mosquitos increased 15-fold over 11 years of insecticide spraying. Not only that, but the percentage of species that bite humans increased their percentage over the period. (J.Amer.Mosquito Control 13(4):315-325 1997)
It has also been shown that Malathion absorbed at only 1 ppm of body weight by frogs depresses their immune systems to the point that they are almost defenseless against diseases in their environment. (Env.Toxicol.Chem. 22(1):101-110 2003) Frogs are a significant predator of mosquitos, both larva and adult.
Second, mosquitos which transmit West Nile are suspected of having damaged stomach linings, which allow the virus to move past the lining and into the mosquito's salivary glands. Many scientists believe that mosquitos primarily develop such damage from sub-lethal doses of insecticides. That would increase the likelihood that they would transmit viruses from birds to mammals. Those 15x mosquitos are probably transmitting more than 15x the number of viruses to humans.
Third, West Nile virus only seriously affects people with impaired immune system or damaged blood-brain barrier. Even Health Canada admitted that "most people infected with West Nile virus experience no symptoms". It is only the elderly or those who are already ill who may (and even then rarely) develop serious complications. Human exposure to neurotoxins is known to impair our immune system, especially when the exposure exceeds our body's ability to detoxify them. There is also documented evidence (Ann N Y Acad Sci 917:944-50 2000) that exposure of healthy mammals to solvents such as those associated with most pesticides damages the blood-brain barrier, thus greatly increasing the risk of encephalitis by West Nile. So, exposure of people to Malathion and its solvents will increase their chances of having severe health effects if bitten by a mosquito, compared to people who have had no exposure to pesticides.
Fourth, widespread use of toxins to kill insects breeds genetic resistance. This is well documented in malarial regions of the world. (cf. Insect Mol.Biol. 11,409 2002) We should not use any toxin in a widespread manner against insects unless the health benefits are sufficient to outweigh the need to continually adapt those toxins to the resistant insects that result. In the case of West Nile Virus, no such benefit exists.
Spraying to kill mosquitos will increase the threat of West Nile to us, not decrease it. The appearance of West Nile in an area should be the signal to cease all spraying of toxins, to redouble our efforts to promote human health, rather than the reverse.
Some further reading and references:
Maine Environmental Policy Institute
Rachel's Environment & Health News
other notes on pesticides