by Rachel Massey*
As we saw two weeks ago (REHW #709), West Nile virus (WNV)
appeared in the U.S. for the first time in 1999.** WNV was
previously unknown in the Western Hemisphere, but it has now
spread to seven states, most recently North Carolina. Carried
by mosquitoes that can infect humans, this virus often produces
no symptoms at all but can sometimes lead to serious illness. In
some cities, public health authorities have responded by spraying
entire neighborhoods with pesticides intended to kill mosquitoes.
These mass pesticide sprayings pose threats to human health and
do not necessarily reduce populations of disease-bearing
Experiences with another mosquito-borne virus, eastern equine
encephalitis (EEE), indicate that pesticide sprays do not
necessarily achieve the desired effect. For example, a 1997 study
looked at trends in populations of CULISETA MELANURA, the
mosquito primarily responsible for transmitting EEE among birds.
Over a period of eleven years, Cicero Swamp in central New York
state was sprayed fifteen times with one insecticide and once
with another. Instead of declining, the population of CS.
MELANURA grew fifteen-fold during this period. The study suggests
that the pesticides may have altered the ecological balance of
the swamp, killing organisms whose presence would ordinarily help
limit the CS. MELANURA population. In general, spraying can
kill fish and other natural mosquito predators, and repeated
spraying can produce pesticide-resistant mosquitoes.
Pesticides meant to kill flying insects are often applied as an
ultra-low volume (ULV) spray. ULV spray equipment creates tiny
pesticide droplets that can remain aloft for long periods and,
due to their light weight, readily drift away from the target
area. Scientists have estimated that less than 0.0001% of ULV
pesticide sprays actually reach the target insects.[4,pgs.18,22]
So for every droplet that reaches a mosquito, hundreds of
thousands more droplets circulate pointlessly in the environment.
Effective mosquito control uses knowledge of mosquito ecology to
minimize opportunities for human exposure. One important tactic
is reducing mosquito breeding habitats. CULEX PIPIENS, also known
as the northern house mosquito, has been the principal though not
the only mosquito species transmitting WNV in the U.S. in 1999
and 2000. CX. PIPIENS breeds readily in standing water found in
places like wading pools, birdbaths, puddles, ditches, and
standing surface water from septic systems. I ts typical
flight range is a quarter to a half mile. This means that a
mosquito that bites you on your front porch may well have hatched
in your back yard -- and that you and your neighbors can have a
direct effect on local mosquito populations.
On a community level, guidelines for effective mosquito control
** Do not use "adulticides," or pesticides meant to kill adult
** Focus on controlling mosquitoes in their immature forms: eggs,
larvae, or pupae. Stock ponds and other bodies of water with
mosquito-eating fish, and keep waterways clean so that fish and
other mosquito predators can survive. In some cases, it is
appropriate to use bacterial larvicides or mechanical controls
such as vegetable-based oils that smother mosquito eggs floating
on the surface of the water. Mechanical control of adults may
be an option as well. Traps exist that may attract and kill
mosquitoes over areas of up to an acre. (See, for example,
** On a municipal or county level, set up a system for citizens
to report standing water near their homes.
** Establish monitoring programs to pinpoint where mosquito-borne
diseases are occurring. Monitoring can rely both on trapping
mosquitoes and on "sentinel birds," such as chickens, tested
regularly for signs of infection.
** Continuously evaluate the effectiveness of all mosquito
** Make sure the public knows what people can do at home to
minimize mosquito exposure and eliminate breeding sites. Public
health education is a good investment of resources and will pay
off better than quick-fix expenditures on chemical sprays. ^Here
are some steps individuals can take around their homes:
** Get rid of any unnecessary items on your property that can
hold stagnant water, such as old tires.
** Empty water from buckets, toys, and containers, and store them
in places where they will not collect rain.
** Drill holes in the bottoms of recycling bins and any other
containers that must be kept outdoors.
** Drain the water from bird baths, fountains, wading pools,
plant pots and drip trays twice a week.
** Check for other ways water may be collecting around your
house, such as puddles beneath air conditioners.
** Clean out your gutters and fix gutters that sag or do not
drain completely. Check for areas of standing water on flat
** If you have a swimming pool, outdoor sauna, or hot tub, make
sure rainwater does not collect on the cover.
** Clear vegetation and trash from any drains, culverts, ponds or
streams on your property so that water drains properly.
** Keep grass cut short and trim shrubs to minimize hiding places
for adult mosquitoes.
** Eliminate standing water in your basement.
** To minimize the likelihood of being bitten inside your house,
make sure window and door screens fit properly and replace
outdoor lights with yellow "bug lights."
** To avoid being bitten outdoors, wear hats, long sleeves and
long pants in the evenings, when mosquitoes are most active.
Insect repellents can help, but some of them are dangerous.
Products containing the pesticide DEET should be avoided. The
U.S. Environmental Protection Agency (EPA) acknowledges fourteen
cases in which individuals reported seizures associated with
exposure to DEET. Twelve were children, three of whom died.[9,
pgs. 22-23] DEET can also interact with other chemicals to
produce severe toxic effects on the nervous system, and may have
played a role in Gulf War Syndrome (see REHW #498 ). Based on
existing information about DEET's health effects, EPA determined
in September 1998 that the labels on some DEET-containing
products were misleading. Under EPA's new requirements, it is
illegal to label DEET-containing products as designed for
children or "safe for kids." However, EPA chose to allow a grace
period of more than four years during which products with old
labels can be sold [9, pg. 41], so stores can still sell products
with misleadin g safety claims.
Many essential oils derived from plants have insect repellent
properties, and some plant-based formulations provide protection
from bites. CONSUMER REPORTS magazine says a product called
Bite Blocker is effective for 1 to 4 hours. With all insect
repellents, it is worth minimizing your exposure. Treat clothing,
rather than skin, whenever possible, and wash off repellents with
soap and water after returning indoors. ^If WNV has not yet
appeared where you live, you may want to get ready in case it
appears next year. You can start now to educate your town, city,
or county officials about pesticide hazards and encourage them to
develop a comprehensive non-chemical mosquito control program. It
makes sense to contact these officials during the winter, when
they are not under pressure to act quickly.
^Find out whether your city or town already has a mosquito
control program, and try to identify an individual in your public
health department who is responsible for mosquito-borne diseases.
Ask whether the department has a written plan for responding if
infected birds or mosquitoes are found in your area. ^This could
also be a good opportunity to find out about and work to
eliminate "nuisance" mosquito control programs, in which routine
spraying is carried out with no public health rationale.
^Finally, you may wish to raise awareness about the links between
global warming and emerging infectious diseases. Because global
warming creates opportunities for tropical diseases to spread or
move northward, the appearance of WNV in the U.S. may be a
harbinger of things to come. If your municipality has a program
to reduce emissions of "greenhouse gases" such as carbon dioxide,
talking about links to WNV could help jumpstart the program or
give it new publicity.
^Paul Epstein of Harvard Medical School's Center for Health and
the Global Environment argues that the spread of mosquito-borne
diseases like WNV is aided by several phenomena associated with
global warming, including mild winters, hot summers, and drought.
The "globalized" economy and increasing international travel also
create new opportunities for exchange of diseases across regions.
According to Epstein, back-to-back weather extremes in 1998 and
1999 probably encouraged the proliferation of WNV and the
mosquitoes that carry it. In a recent article in SCIENTIFIC
AMERICAN he writes, "The mild winter of 1998-99 enabled many of
the mosquitoes to survive into the spring, which arrived early.
Drought in spring and summer concentrated nourishing organic
matter in their breeding areas and simultaneously killed off
mosquito predators, such as lacewings and ladybugs, that would
otherwise have helped limit mosquito populations. Drought would
also have led birds to congregate more, as they sha red fewer and
smaller watering holes, many of which were frequented, naturally,
by mosquitoes."[12, pg. 54] Later in the summer, heavy rain
created new mosquito breeding opportunities. Higher temperatures
also tend to increase mosquito activity and accelerate the
reproduction and maturation of viruses within their bodies.[12,
^As communities make decisions about WNV, public officials must
be prepared to talk frankly about the uncertainties they face.
For example, it is not acceptable to provide ample information on
risks associated with WNV while witholding parallel information
on the toxicity of pesticides used to combat mosquitoes. As
always, our ability to make good decisions depends on honesty
about scientific uncertainties combined with open public
discussion of the full range of alternatives available to us.
* Rachel Massey is a consultant to Environmental Research
** Thanks to Audrey Thier of Environmental Advocates
(www.envadvocates.org) for helping clarify several points in this
 "West Nile Virus Found in North Carolina," REUTERS, October
 John J. Howard and Joanne Oliver, "Impact of Naled (Dibrom
14) on the Mosquito Vectors of Eastern Equine Encephalitis
Virus," JOURNAL OF THE AMERICAN MOSQUITO CONTROL ASSOCIATION Vol.
13, No. 4 (December 1997), pgs. 315-325
 For information on resistance and alternative mosquito
control measures, see Environmental Advocates and others, "Toward
Safer Mosquito Control in New York State," January 2000,
available at http://www.envadvocates.org/public_html/temp/
 David Pimentel, "Amounts of Pesticides Reaching Target Pests:
Environmental Impacts and Ethics." JOURNAL OF AGRICULTURAL AND
ENVIRONMENTAL ETHICS Vol. 8, No. 1 (1995), pgs. 17-29.
 "Mosquito Control In and Around the House," fact sheet
available at http://www.rci.rutgers.edu/~insects/sompam.htm;
"Mosquito Control in Maryland," fact sheet available at
 "Biological Data on 25 Common Species of Mosquito Found in
Coastal North Carolina," North Carolina Public Health information
 For one example, see http://www.erie.gov/
 See U.S. Environmental Protection Agency, "Mosquitoes: How to
Control Them," available at http://www.epa.gov/pesticides/
citizens/mosquito.htm and "West Nile Virus: The Facts,"
available at http://www.erie.gov/west_nile_virus.phtml
 U.S. Environmental Protection Agency Office of Prevention,
Pesticides, and Toxic Substances. "Reregistration Eligibility
Decision (RED): DEET," Washington, D.C.: US EPA, September 1998,
EPA publication 738-R-98-010. Available at http://www.epa.gov/
 Mark S Fradin, "Mosquitoes and Mosquito Repellents: A
Clinician's Guide," ANNALS OF INTERNAL MEDICINE Vol. 128 (June,
1998), pgs. 931-940. See www.acponline.org/journals/annals/
 "Buzz Off," CONSUMER REPORTS Vol. 65, No. 6 (June 2000),
 Paul R. Epstein, "Is Global Warming Harmful to Health?"
SCIENTIFIC AMERICAN Vol. 283, No. 2 (August 2000), pp. 50-57.