Residual spraying                         Photo courtesy

Pretoria University’s Department of Urology recently released a study headed by Dr. Riana Bornman purporting that there is a link between an insecticide used for malaria control, commonly known as DDT, and urogenital birth defects (UGBDs) in boys born in the Limpopo region of South Africa. After reading the study, we find there are a number of shortcomings and question the validity of the methodology adopted.

 

Media coverage of this study has been extremely biased, with not a single voice heard from a programme manager or any other scientist involved with malaria control. Studies that provide the public health community with an improved understanding of public health interventions should be welcomed by all, however, this study is neither informative nor instructive and could severely undermine malaria control not only in South Africa but in the entire Southern African region.

 

DDT was introduced in the 1940’s and has been used successfully in malaria control programmes for six decades. Since its introduction, thousands of tonnes of the chemical have been produced and used throughout the world with millions of people coming into direct contact with it in one way or another. Heavy use of DDT in agriculture and for control of household pests has occurred in many developed countries with advanced health systems. We are unaware of any replicated scientific studies of those advanced health systems having picked up that UGBDs occur in boys born in areas with high levels of exposure.

 

Dr Paul Müller, whose vital discovery of DDT’s insecticidal properties, was awarded a Nobel Prize for his work. One of the main attributes of DDT is its long residual action. This attribute vastly improved malaria control because prior to this, insecticides such as natural pyrethrum had to be sprayed approximately every two weeks compared to DDT which needs to be sprayed only once a year. DDT’s persistence made malaria control more effective and cost-effective and has protected millions of people from this preventable disease. But DDT’s persistence has also given rise to the suspicion that it is harmful to humans.

 

Over the years, numerous studies have investigated the potential adverse effects of DDT on human health. Yet despite the voluminous research, no scientific study has been able to prove that DDT is harmful to people. Most studies find no evidence of harm or find only weak and un-replicated associations between DDT and possible human health harm. Unfortunately, those weak and un-replicated studies, all too often, are used by anti-insecticide activists to lobby for restrictions on public health insecticides.

 

In Africa, Malaria kills a child every thirty seconds. Approximately 85 per cent of deaths are children under the age of five years. Effective malaria control saves lives, prevents the trauma of unnecessary deaths in families, and improves local economies. Worldwide there are at least 247 million cases of malaria annually (212 million in Africa alone). Malaria sufferers have great difficulty in carrying out sustained work, which intensifies human misery and poverty in areas where the disease is prevalent. Malaria is thus not only a human tragedy; it is an economic one as well. In malarial countries it is estimated that the disease reduces per capita economic growth by 1.3 percent per year. This equates to approximately $12 billion in forgone income.

 

In South Africa, DDT was introduced in 1946, and, where it was applied, annual cases of malaria fell from 1,177 (1945-46) to just 61 by 1951. In 1996, South Africa stopped using DDT due to international pressure and alternative control mechanisms were tried. Soon, a massive malaria outbreak occurred and, from 1996 to 2000, malaria deaths increased eight-fold. The number of malaria cases increased at a similar rate from 5,000 per annum to more than 60,000. In 2000, South Africa reintroduced indoor residual spraying with DDT, despite international pressures to ban the insecticide, and the number of malaria cases and deaths dropped by a remarkable 80 per cent.

 

There can be no disputing the extraordinary impact that DDT has made, and continues to make, in disease control programmes around the world. The remarkable degree to which DDT saves lives has allowed communities to grow and prosper – contrary to the expectations of critics who argue that DDT is harmful to humans. Broadly, the concerns that DDT may be harmful to human health are not supported by any meaningful data on deaths or diseases as a consequence of coming into contact with DDT, and are further contradicted by evidence over many decades of reduced disease and rising populations wherever the chemical has been used.

 

In their study, Bornman and her colleagues claim that 10.8 per cent of the sample (357 out of 3,310) had UGBDs. The birth defects occurred in 11.0per cent (264 out of 2,396) of the sample that had been exposed compared to 10.2per cent (93 out of 914) of those that had never been exposed – hardly a statistically significant difference. In fact, hypospadias was the numerically dominant birth defect (representing 47.9 per cent of all birth defects) but the rate of hypospadias was actually higher in the unexposed population than in the exposed population.

 

The authors did not control for history of malaria infection and a host of other variables. Malaria is mostly a problem of poor rural populations. Poverty and poor nutrition also have severe and adverse affects on a whole spectrum of biological parameters. The study compares populations exposed to DDT with populations not exposed to DDT. Presumably, the unexposed populations do not suffer from malaria; whereas exposed populations have problems with malaria.

 

Malaria causes severe stress on pregnant women and malaria infections are a confounding variable for the developing foetus. Indeed, the authors state, “it is possible that some other unknown factor that differed between people in the sprayed and unsprayed villages could account for some of the associations seen.” Indeed, the differences in counts of birth defects were so small between, supposedly, exposed and unexposed populations that almost any one of a number of uncontrolled variables could account for those differences. Correlation does not automatically translate into causation. 

 

Robust, evidence-based discussion and debate over the role of DDT and other man-made chemicals in malaria control is needed. Further research into DDT would help focus attention on the long-term lack of investment in the search for legitimate replacement chemicals. Until viable replacements are found, support for the ongoing use of DDT, based on the scientific evidence of its effectiveness, must continue.

 

Given how the data are manipulated in order to get the reported results, and the authors' failure to adequately consider all potential confounders, this study should be classified as highly biased and the conclusion that DDT is a cause of UGBDs as invalid until such time as an independent peer-reviewed study can be conducted to support the findings of this paper. Considering the immediate and real risks posed by malarial mosquitoes, particularly to young children, this study should not be used to argue against the use of DDT in malaria control in South Africa or elsewhere.

 

By Jasson Urbach,

Director of the Health Policy Unit and Africa Fighting Malaria.