Pesticides and the West Nile Virus: An Examination of Environmentalist Claims

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Ever since the mosquito-transmitted West Nile Virus appeared in New York City during 1999, environmental activists have been fighting efforts to keep the disease under control.  Not only have they battled against any spraying for adult mosquitoes, they have also fought methods to manage mosquito larvae.  These groups maintain that the control methods are more dangerous than the diseases they seek to control.  In the case of spraying, activists say that the chemicals imperil public health and can kill or harm wildlife.  These groups claim further that other methods, such as the use of biological agents to kill mosquito larvae, disrupt the balance of nature and thereby threaten non-target species.  Finally, while maintaining that spraying can devastate non-target organisms such as butterflies and aquatic life, many groups claim that mosquito control efforts have little or no impact on mosquito populations.

Of course, mosquito control officials should, and most arguably do, make careful consideration of the public health implications of their actions as well as any environmental tradeoffs.  After all, their programs are designed to protect the public from serious health risks; officials certainly have no interest in creating new, greater risks.

The good news is that historically, pesticides and mosquito-control efforts have provided important public health benefits.  “Contrary to the environmentalist view, public health campaigns that use insecticides against diseases have a remarkable record of public safety and a remarkable record of protecting humans from insect-borne diseases,” says Dr. Donald Roberts, professor of tropical public health at the Uniformed Services University of the Health Sciences.1  Other factors certainly also have played significant roles in reducing disease incidence in developed nations.  In particular, improved living conditions associated with growing income levels—which produced amenities such as screens and air conditioning—greatly limit human exposure to insect-borne disease.  It is difficult to quantify total benefits of vector-control programs, but there is evidence of important benefits. 

The Institute for Medicine, an affiliate of the National Academy of Sciences, documented some of these successes in a 1992 report.  For example, it notes that vector-control efforts helped mitigate an outbreak of St. Louis Encephalitis in 1966 during which a mosquito-transmitted form of viral encephalitis broke out in Dallas, Texas.  Within a few weeks, there were 145 confirmed cases and 545 suspected cases.  Dallas vector-control officials responded by spraying with the pesticide malathion over thousands of acres of land.  The Institute reports that measurements of mosquito densities before and after spraying showed that spraying had significantly reduced the mosquito populations.  The Institute concluded that: “The economic and public health consequences would certainly have been greater had pesticides not been available.”2

Similarly, U.S. vector-control efforts made possible the construction of the Panama Canal, which was impeded by yellow fever and malaria outbreaks.  In particular, U.S. engineers controlled malaria and yellow fever through a combination of drainage and application of larvicides, which reduced mosquito breeding.3 Similarly, governments have used the pesticide DDT to basically eradicate malaria-carrying vectors throughout much of the western world.4

Like anything, vector control hasn’t proven problem free.  Challenges associated with insect resistance, limitations of control methods, and impacts on wildlife continually emerge and must be managed.  Environmental activists point to such issues — often exaggerating the extent of problems — to suggest that vector control, and pesticides in particular, should be eliminated.  But just as we don’t eliminate medical antibiotics because resistance presents challenges, we should not eliminate vector control.  We should instead look for ways to manage and address such challenges.

Indeed, when governments followed anti-pesticide activists’ advice in the past, it produced deadly results.  The most egregious case involves the pesticide DDT.  Some evidence indicates adverse impacts on wildlife from widespread use of DDT for agricultural purposes, harming the bald eagle and other birds of prey in particular.  Others have suggested that DDT poses seriously adverse public health implications as well, but such claims are questionable.5 Environmental groups played an important role in raising concerns about DDT’s impact on wildlife, but rather than providing constructive advice to manage risks, they decided that DDT should never be used.  They have advanced bans around the world despite a resulting escalation of malaria deaths in developing nations.  Use of limited quantities of DDT in and around residences can keep mosquitoes outside homes at night when the malaria-carrying insects feed.  Such uses have negligible environmental impact, but can save millions of lives.

The adverse public health impacts of such extreme positions have been felt around the world.  For example, South Africa nearly eradicated malaria-carrying mosquitoes when it used DDT, but cases soared again after the nation caved to environmental activists who pressed the country to switch to other pesticides.  Cases rose from about 4,000 in 1995 to more than 27,000 by 1999 (or possibly as many as 120,000 if one considers pharmacy records).6 In response to this crisis, South Africa decided to resume DDT use.  Similarly, tropical medicine specialist Dr. Donald Roberts and his colleagues explain in a research paper that “separate analyses of data from 1993 to 1995 showed that countries that have recently discontinued their spray programs are reporting large increases in malaria incidence.  Ecuador, which has increased use of DDT since 1993, is the only country reporting a large reduction (61%) in malaria rates since 1993.”7  According to the World Health Organization, malaria alone infects 300 to 400 million people a year and kills 1 to 2 million.8  Most of its victims are children.

Because of the devastating death toll, public health officials including those at the World Health Organization took a stand against an unreserved international ban on DDT.9 These officials provided the support necessary to allow DDT use for malaria control as part of the Convention on Persistent Organic Pollutants.  Nonetheless, environmental activists continue their crusade against DDT.

Environmental groups in the United States are posing similar public health challenges—although on a less devastating scale — by fighting any reasonable use of chemicals or other socalled “unnatural” means for vector control.  Rather than offer sound advice on management of public exposure, insect resistance, or efficacy of the products and methods — all which present challenges for vector control—these groups downplay the public health risks of insect-borne disease and attempt to raise alarm about substances used for vector control.  Anti-pesticide campaigns and subsequent government regulations have impacted the ability to control emerging diseases.  “The practices of environmental advocacy groups are seriously degrading public health capabilities in the United States.  Our public health threats are real, and growing,” says Dr. Roberts.10

Activists’ extreme views are coupled with alarmist rhetoric that is designed to scare public officials and others into opposing any chemical use.  The following catalogs some of their claims and offers analysis as to where they contradict the facts.


1 Personal communication on January 28, 2002.

2 Joshua Lederberg, Robert E. Shope, and Stanley C. Oaks, Jr. eds., Emerging Infections: Microbial Threats to

Health in the United States (Washington, D.C.: National Academy Press, 1992), 166.

3 Ibid., p. 20.

4 For a discussion of the history of DDT use for malaria control see Richard Tren and Roger Bate, When Politics

Kills: Malaria and the DDT Story (Washington, D.C.: Competitive Enterprise Institute, 2000),

5 See: A. G. Smith, “How Toxic is DDT?” Lancet 2000 356 (July 22, 2000): 267-268.

6 Amir Attaran and Rajendra Maharaj, “Doctoring Malaria, Badly: The Global Campaign to Ban DDT,” British Medical Journal, no. 321 (December 2, 2000): 1403-1405,

7 Donald R. Roberts et al., “DDT, Global Strategies, and a Malaria Control Crisis in South America,” Emerging Infectious Diseases 13, no. 3 (July-September 1997), See also Amir Attaran. et al., “Balancing Risks on the Backs of the Poor,” Nature Medicine 6 (2000): 729-731.

8 World Health Organization, “Malaria: Fact Sheet,” 1998,