A Literature Review of the Safety and Efficacy of DDT

A Literature Review of the Safety and Efficacy of DDT

Karen Harper

National University

December 20, 2015

Abstract

Malaria infects approximately 350 million people per year and kills one million people per year, mainly in tropical areas. Sub-Saharan Africa is an area of particular concern, with 86 percent of deaths occurring there. Pregnant women, children, and the poor are disproportionately affected.

DDT has long been shown to be effective against mosquitoes carrying malaria, but due to health concerns raised by Rachel Carson in her book Silent Spring, DDT was banned in many countries. DDT has been shown to linger in the environment and can bioaccumulate in fish. It also has the potential to impact fetal development and can be passed to infants via breast milk. Prior to the publication of Carson’s book in 1962, DDT was sprayed outdoors, resulting in the deaths of fish, birds, and other wildlife.

After DDT was banned, malaria rates rose exponentially. In 2006 the World Health Organization recommended that DDT be sprayed indoors via a method known as indoor residual spraying (IRS). While IRS has largely been shown to be cost-effective and its implementation has dramatically reduced the number of new malaria cases, its long-term health effects are still unclear and many people remain skeptical about its use.

Much research needs to be done on the effects of DDT and the environment, as well as its long-term effects on human health. Public health officials in tropical areas affected by malaria also need to be aware of their population’s wariness about IRS and find ways to present accurate information to the public so that more people will adhere to treatment recommendations.

A Literature Review of the Safety and Efficacy of DDT

In 1962, Rachel Carson’s classic environmental treatise Silent Spring was published. This book was groundbreaking in its criticism of the pesticide dichlorodiphenyltrichloroethane (DDT). Up until that time, DDT was seen as a panacea for insect-borne diseases, particularly malaria. Carson’s book offered up the dark side of widespread DDT usage: dead birds, dead fish, and death and damage of other sensitive marine life (Berry-Caban, 2011). The public outcry against DDT was so strong after the publication of Silent Spring that pesticide companies including Monsanto and American Cynamid launched counterattacks against Carson. Since then, DDT has often been viewed as dangerous by the public. However, much new research has been done about the health and environmental effects of DDT since 1962.

After the publication of Carson’s book, many countries banned DDT use altogether. Some of these countries were in Africa, where the anopheles mosquito is prevalent and carries malaria. In South Africa, the number of malaria cases had leveled off by 1996 when DDT was banned; from that point on, the number of new cases skyrocketed. This was in contrast to nearby Swaziland, where DDT use was never interrupted, and malaria remained a minor problem (Bocking, 2004). Now, the conundrum was clear: widespread spraying of DDT caused harm to the environment, but also seemed to be vital in controlling the spread of malaria.

Studies about DDT’s toxicity to humans and wildlife continue to be done with varying results. A 2012 study in Bangladesh, where DDT is officially banned but is nevertheless still being used in agriculture, showed that DDT bioaccumulates in fish as a result of runoff from farming. The DDT levels in “dry fish” popular as a food staple in Bangladesh were considered to be toxic to humans (Siddique & Aktar, 2012). However, in considering the validity of this study it is also important to note that some commercial fisheries in Bangladesh use DDT as a preservative, which is not standard practice in other countries. This may have increased DDT concentration beyond levels which would have been found due to agricultural runoff alone. Thus, it important to keep in mind that different levels of toxicity may have been found if different preservatives had been used.

A long-term study of pregnant women and their daughters in Oakland, CA showed a positive correlation between prenatal DDT exposure and risk of developing hypertension later in life (Merrill, Cirillo, Piera, Terry, Krigbaum, Nickilou, Flom, & Cohn, 2013). A limitation of this study is that participants were asked to self-report their other medical conditions such as diabetes, and thus there is no way to know whether some coexisting conditions could have been incorrectly or under- reported and potentially raised the likelihood of participants having hypertension independently of DDT exposure.

Indoor residual spraying (IRS) of DDT has been suggested as a safer alternative to widespread, outdoor spraying. IRS has been used mainly in tropical countries where exposure to disease vectors is high. One study compared DDT concentrations among human populations in the Arctic versus tropical populations. In the Arctic marine mammals are a common food source and may contain residual DDT, whereas in tropical areas homes are frequently sprayed with DDT to reduce mosquito and other vector exposure. The study found that DDT concentrations were higher in people living in homes treated via IRS (Ritter, Scheringer, MacLeod, & Hungerbuhler, 2011). However, it is unclear from this study what health effects, if any, DDT exposure resulted in.

In 2011, the World Health Organization stated that more research needed to be done on the effects of IRS in women of childbearing age and their fetuses and infants. Several studies have already indicated that DDT is present in breast milk and can then be passed to nursing infants. Firstborn infants are at risk for ingesting higher amounts of DDT than their siblings. In studies of IRS-treated villages, pregnant women and nursing mothers had toxic levels of DDT in their blood, which is a concern not only for the mothers but also their developing fetuses and infants. DDT has been shown to be an endocrine disruptor and can also affect urogenital development in fetuses. The study did not find any correlation between DDT exposure and length of lactation (Bouwman, Kylin, Sereda, & Bornman, 2012). Although the study did not come up with specific health risks for infants exposed to DDT via breast milk, it is worth bearing in mind that exposure to any chemicals during a child’s early development may be risky. At the same time, it is important to note that malaria disproportionately affects pregnant women and children, and thus the benefits of IRS may outweigh potential risks (Danley, 2002).

Danley (2002) also notes that alternatives to DDT are not always safer or as effective. Bed nets are frequently treated with pyrethroids, which mosquitoes have become resistant to in some areas. While mosquitoes can also become resistant to DDT, it is less common. Using biological controls such as mosquito-eating fish can also threaten biodiversity if not managed correctly. Draining marshes and wetlands can also be harmful to the environment. DDT is in many cases a simpler, cheaper, and more effective way to control mosquitoes, despite its risks to environmental and human health.

DDT is known to linger in the environment, which is one reason IRS is a concern. Another option, though not nearly as cost-effective, is treating bed nets with DDT. This would potentially reduce the chemical’s impact on the environment. However, bed nets may not prevent as many malaria infections as IRS (Pedercini, Blanco, & Kopainsky, 2011).

In 2006, the World Health Organization recommended IRS as a preventative measure in areas with high rates of malaria infection (WHO, 2006). They noted that one million people die from malaria infection every year, and 86 percent of these deaths occur in sub-Saharan Africa. In these areas, the poor are disproportionately affected. Following this recommendation, a study in Mozambique found that a household’s socioeconomic status impacted their perception and adherence to IRS recommendations. In general, households with lower socioeconomic status had less trust in the public health authority and did not believe IRS worked well against mosquitoes. People in these households favored bed nets over IRS. By contrast, households with higher socioeconomic status were more likely to view IRS favorably and were more likely to comply with treatment recommendations. People were also more likely to allow their homes to be sprayed if they personally knew the person spraying. There was also a misperception among many people that it did not make sense to spray indoors since mosquitoes live and breed outdoors (Munguambe, Pool, Montgomery, Bavo, Nhacolo, & Fiosse, 2011). This study illustrates the importance of education and developing trust among the population in order to improve outcomes.

Conclusion

More research needs to be done on the health effects of DDT, particularly in women of childbearing age and their infants. Since DDT lingers in the environment and can bioaccumulate in fish, more research needs to be done on the implications for the environment and humans. Public health campaigns in malaria-prone areas should also address IRS adherence issues such as distrust of the government and fears of the insecticide.

References

Bocking, Stephen. (2004). One cheer for DDT? Forty years after Silent Spring, resurgent malaria gives new life to an old killer.(Political Science). Alternatives Journal, 30(4), 28.

Bouwman, Hindrik, Kylin, Henrik, Sereda, Barbara, & Bornman, Riana. (2012). High levels of DDT in breast milk: Intake, risk, lactation duration, and involvement of gender. Environmental Pollution, 170, 63-70.

Cristobal S. Berry-Caban. (2011). DDT and Silent Spring: Fifty years after. Journal of Military and Veterans’ Health, 19(4), Journal of Military and Veterans’ Health, 01 October 2011, Vol.19(4).

Danley, J. (2002). Balancing Risks: Mosquitoes, Malaria, Morality, and DDT. Business and Society Review, 107(1), 145-170.
Merrill, Michele La, Cirillo, Piera M., Terry, Mary Beth, Krigbaum, Nickilou Y.,

Flom, Julie D., & Cohn, Barbara A. (2013). Prenatal exposure to the pesticide DDT and hypertension diagnosed in women before age 50: A longitudinal birth cohort study.(Research)(Clinical report). Environmental Health Perspectives, 121(5), 594.

Munguambe, K., Pool, R., Montgomery, C., Bavo, C., Nhacolo, A., Fiosse, L., . . . Alonso, P. (2011). What drives community adherence to indoor residual spraying (IRS) against malaria in Manhiça district, rural Mozambique: A qualitative study. Malaria Journal, 10, 344.

Pedercini, M., Blanco, S. M., & Kopainsky, B. (2011). Application of the malaria management model to the analysis of costs and benefits of DDT versus non-DDT malaria control. PLoS One, 6(11) doi:http://dx.doi.org/10.1371/journal.pone.0027771

Ritter, Roland, Scheringer, Martin, MacLeod, Matthew, & Hungerbuhler, Konrad. (2011). Assessment of nonoccupational exposure to DDT in the tropics and the North: Relevance of uptake via Inhalation from indoor residual spraying.(Research)(Report). Environmental Health Perspectives, 119(5), 707.

Siddique, M., & Aktar, M. (2012). Detection of health hazard insecticide dichlorodiphenyltrichloroethane (DDT) in some common marine dry fish samples from Bangladesh. Health, 4(4), 185-189.

WHO gives indoor use of DDT a clean bill of health for controlling malaria. (2006, September 15). Retrieved December 20, 2015, from http://www.who.int/mediacentre/news/releases/2006/pr50/en/