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Eight Reasons Why Spraying Pesticides
is Not the Solution to
West Nile Virus
by Rebecca Watson
UPdate Spring 2003
How much of a risk is West Nile Virus
(WNV)? To some extent, only time will tell. But public health experts stress
that there is no cause for fear and panic, or for panic driven
"solutions." In fact, the so-called "solution" of
spraying pesticides to kill mosquitoes will actually lead to bigger problems.
Here are eight compelling reasons why spraying pesticides is not the answer
to WNV.
1. Least Effective
Measure
The US Center for Disease Control and
other experts say that spraying or fogging is the least effective means for
slowing the spread of WNV carrying mosquitoes. For fogging to have
maximum effect, a mosquito has to be flying. Estimates are that fogging kills
only about 10% of adult mosquitoes. The federal-provincial
task force on WNV admits there is little evidence for the efficacy of insecticide
spraying. Adult mosquitoes live only about two weeks, with new larvae
hatching constantly. This means that spraying cannot be a one shot operation,
but needs to be repeated
frequently if chosen as a means of control .
2. Predators
Harmed, Mosquitoes Thrive
Aerial spraying or fogging is more harmful
to mosquito predators than to mosquitoes. Since predators are farther up the
food chain, they will take in higher amounts of pesticide. By decreasing
mosquito predator populations, aerial spraying actually leads to increases in
mosquito populations. Data from a study in New York State published in the Journal for Mosquito Control found that after 11 years of insecticide spraying,
the mosquito population had increased 15 times. Pesticide exposure also
results in immune suppression in birds, which serve as the hosts for
WNV. Birds exposed to organophosphate pesticides tend to suffer
immune suppression (as do mammals, amphibians and other animals.) This makes
them less able to fight off viral and bacterial infections, the very opposite
of what is needed. Once infected with WNV, birds are more likely to develop
symptoms and to remain ill longer than if they had not been exposed. Thus,
pesticide spraying leads to more frequent and longer infections and higher
viral loads in birds, making it more likely they will spread the disease to mosquitoes. This increases the possibility of
mosquitoes transmitting the virus to
humans and other mammals.
3. Super Mosquitoes,
Sicker Mosquitoes
For some reason, as yet unknown,
mosquitoes exposed to pesticides are more
likely to have WNV in their salivary
glands and develop a damaged gut lining which becomes more porous, allowing
WNV to pass through. Over a decade of insecticide spraying to control
encephalitis in Florida has not been effective, and mosquitoes are now 15 times more likely
to pass on the disease. Mosquitoes, which have short life spans, go through
many generations in a single year. The mosquitoes which
are exposed to pesticides and survive are more likely to develop
resistance to them. So
aerial spraying contributes to the
development of "super mosquitoes" which can only be killed by using
higher amounts or different types of pesticides.
4. Immediate Human
Health Effects
Immediate health effects on humans from
exposure to sprayed pesticides are
considerable. A letter from 26 prominent
physicians and scientists in Quebec released last summer states,
"Indiscriminate spraying of pesticides, especially in heavily populated
urban areas, is far more dangerous to human health and the natural
environment than a relatively small risk of West Nile Virus.... Ironically,
such spraying is especially dangerous to those with impaired immunity for
whose 'protection' such spraying is mainly being done. ..Those individuals
who are most vulnerable in this chemical action against mosquitoes include
children, pregnant women, the elderly, chemically sensitive and immuno-suppressed individuals, such as patients with AIDS
and cancer, and people suffering with asthma and other
allergies."
Organophosphates are the most common class of pesticides used in mosquito
control sprays. According to the United States Environmental Protection
Agency (EPA), they are "efficiently absorbed by inhalation, ingestion
and skin penetration" and were "the class of pesticides most often
implicated in symptomatic illnesses among people in 1996."
5. Long Term Health
Effects
Pesticides used in mosquito control can
contribute to immune suppression in humans. A report from the World Resources
Institute notes, "Impairment of the immune system by chemical pesticides
can lead to allergies, auto immune disorders such as lupus, and cancer. It
may also lead to infections to which one may be normally
resistant." People with weakened immune systems are the most
vulnerable to WNV. Thus, in the long term, aerial spraying may actually
increase the number of people who become
seriously ill from WNV. And immune system suppression has serious
implications for other diseases as well, including SARS.
Malathion, Naled and Resmethrin are pesticides commonly used in mosquito
control. Malathion, an organophosphate, is neurotoxic. It is the most common pesticide used in
aerial spraying. In studies on rats, pesticides were shown to impair
the blood-brain barrier. In humans, the more serious effects of WNV occur
when the virus crosses the blood-brain barrier. Malathion,
like all members of the organophosphate family, disrupts nervous system
function. Besides causing headaches, nausea and diarrhea, it has been
linked to gene damage causing attention deficit hyperactive disorder (ADHD).
Other heath hazards identified in laboratory studies include damaged sperm,
altered immune function, increased incidence of breast tumors, and increased
risk of non-Hodgkin's lymphoma. Naled is another
organophosphate which disrupts nervous system function, also causing
headaches, nausea and diarrhea. Naled is most toxic
when exposure occurs by inhalation. Lab tests connected exposure to Naled's breakdown product, dichlorvos,
to aggressiveness and deterioration of memory and learning. Dichlorvos is also classified as a carcinogen, and
interferes with prenatal brain development.
Resmethrin is considered by the World Health Organization as
a "neuropoison." Its effects on the human
nervous system are similar to its effects in insects. Lab studies on rats
showed that Resmethrin interfered with
reproduction, increasing numbers of stillborns even at the lowest exposure
tested.
6. Long Term
Environmental Effects
Most of the pesticides presently used for
mosquito control do not selectively target mosquitoes. Malathion,
Naled and Resmethrin kill
all insects. This includes hundreds of beneficial insect species that
pollinate crops and keep pests under control. Malathion
is known to contaminate water, and is classified as highly toxic to most
species of fish. In 1999, 90% of adult lobsters in Long Island Sound were
killed by malathion used on land. Fish kills in the
thousands have been reported following mosquito spraying. Since some species
of fish feed on mosquito larvae, this is
doubly counterproductive. Other organisms
that feed on mosquito larva are also killed. Bird populations are also
threatened. According to New York State wildlife
pathologist Ward Stone,
more of the birds sent to his unit for examination in 2000 died from
pesticides than from WNV. Among the more frequent causes of bird death were
broad band insecticides from the organophosphate category such as Dursban, diazinon and ethylparathion. Organophosphates used in mosquito control
add harmful volatile organic compounds (VOCs) to
the atmosphere, and are precursors of ozone (smog) forming chemicals. This
means they are contributors to global warming.
7. Keep Risk in
Perspective
While the image of a new killer virus from
the tropics is scary and makes for good media material, public health experts
at all levels are attempting to help people put WNV in perspective. West Nile
Virus is less dangerous than the flu. Only 1% of mosquitoes carry the WNV,
even in places where WNV has been common for years. Because of our climate,
the virus is not expected to overwinter, but would
likely be reintroduced each year through bird migration. Less than 1% of
people bitten by infected mosquitoes will have any symptoms, and most of
those will be equivalent to a one day flu or headache. Studies in New York
when WNV was most widespread found thousands of people who tested positive
for WNV but had never experienced any symptoms of illness. People bitten by
infected mosquitoes, even those who
experience no symptoms, will develop a lifetime immunity to the disease. In Africa and Europe,
the virus occurs in cycles, with typically three years of human infections in
late summer, with the majority of infections in the first year of a cycle.
Then the virus fades into the background, and may not reappear for many
years. In Africa, WNV is a childhood disease; adults have developed
immunity.
8. Taking a Long-term
Approach
WNV may be one of a number of tropical
diseases which will spread to our geo- graphic area with global warning.
Instead of panic and sensationalism, we need a rational, long term
problem-solving approach which is healthy for humans and the
environment. Reducing mosquito breeding sites (standing water), known
as source control, is the most effective mosquito control method. Since adult
mosquitoes
seldom travel more than 1 kilometer,
source control in a neighborhood can be extremely effective and quite
non-toxic. Experts stress the value of source controls such as
mechanical flushing of sewer catch basins, and introduction of dragonfly
larvae in nearby ponds and lakes. These methods have been practiced with
great success in Wells, Maine for 26 years. Maintaining healthy
mosquito predator populations is an important part of a mosquito control strategy.
Eliminating mosquito larvae, through predators and
biological means and if absolutely necessary via pesticides, is far
more effective than trying to kill adult mosquitoes. And ultiimately,
the most effective defense against WNV is a healthy ecosystem and a healthy
immune system in humans, birds and other species.
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Rebecca Watson
is a writer with a special interest in environmental health.
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