Inconclusive study on DDT has potentially mortal consequences

Jasson Urbach | 15 May 2023
Health Policy Unit

Inconclusive study on DDT has potentially mortal consequences


For six decades the pesticide dichloro-diphenyl-trichlroethane commonly referred to as DDT has been used successfully for indoor residual spraying (IRS) purposes to control malaria. During the many decades that DDT has been used thousands of tonnes have been produced and distributed throughout the world and millions of people have come into direct contact with it in one way or another. Over the years a number of studies have investigated the potential adverse effects between DDT and human health. Yet despite the numerous studies the scientific world has failed to produce any substantial evidence to back claims that link DDT to health ailments in humans. 


Recently, a paper produced by the University of Pretoria examined the relationship between non-occupational exposure to DDT and sperm quality. At the outset it should be made clear that the paper finds inconclusive evidence regarding the reproductive ability of individuals in the sample. Rather, the study notes, "It is impossible to know what the effects will be on the fertility potential of this population in five to ten years with the continued use of DDT for malaria vector control." 


However, what we do know is that wherever DDT has been used in public health, disease and deaths decreased dramatically and human populations began to rise; something one wouldn't expect if DDT was as dangerous as some people make it out to be. Indeed, the recent study conducted by the University of Pretoria failed to note that the numbers of malaria cases and deaths have been decreasing since DDT was reintroduced to control malaria in the year 2000, from 9,487 cases and 68 deaths to 3,458 cases and 31 deaths in 2005.


In malaria control, sprayers apply small amounts of DDT, usually 2g of active ingredient per square metre, on the inside walls of houses and under the eaves outside. Because of its long lasting action - up to 1 year - DDT vastly improved malaria control, as previously-used shorter-acting insecticides had to be applied to dwellings every 1 to 2 weeks. DDT works in three ways; it is a spatial repellent which keeps mosquitoes from even entering sprayed houses; it is a contact irritant, so that even if a mosquito enters a house, it will often exit before feeding; lastly it is a toxic agent and will kill those mosquitoes that are neither repelled nor irritated.


South Africa maintained its IRS programme using DDT from 1946 to 1996. In 1996 the Department of Health replaced DDT with synthetic pyrethroid insecticides. A highly efficient malaria vector, Anopheles funestus, believed to have been eradicated in the 1970s, soon reappeared in South Africa. What followed was one of the worst malaria epidemics in the country's history.  Malaria cases rose from around 6,000 in 1995 to over 60,000 in 2000, with the number of deaths rising at a similar pace.  


In early 2000, South Africa reintroduced DDT to control malaria and in 2001, introduced new artemesinin-based combination therapies (ACT's) to treat malaria patients. The combination of effective insecticides and drugs ensured that malaria cases fell by almost 80% by the end of 2001. The continued reliance on the combination of IRS using DDT and ACT's reduced the number of malaria cases from 64,868 in 2000 to 7,754 in 2005, which equates to a reduced incidence rate of 15 per 10,000 to 2 per 10,000 population.


The successful malaria control programme in South Africa has shown that IRS using DDT is an essential element of the programme. Indeed, it is the success and leadership of the SA government in the fight against malaria that prompted other African countries to either re-introduce DDT to malaria control or to seriously consider the move. Given that DDT, when used properly as part of a well-managed IRS programme, is highly effective at controlling malaria and is safe for humans, this programme change should be welcomed.


The WHO has explicitly allowed for the use of DDT in malaria control precisely because it saves lives. In order to give its approval for countries to use DDT, the WHO regularly assesses scientific literature on DDT. It assesses research papers with a view to evaluating whether there is a link between DDT and harm to human health. In addition, the WHO must balance any potential risk to human health with the benefits of using DDT in malaria control. When one considers the millions of lives at stake one has to compare the real risks that people face from malaria with the often uncertain and hypothetical risks they may face from using DDT. There is clear evidence that there is a close correlation between the use of DDT and reduced mortality and morbidity and no credible evidence that DDT results in harm to human health.


Africa Fighting Malaria welcomes research into the effects of DDT on humans. We encourage malarial countries to develop the most appropriate and effective measures to control malaria given their unique circumstances. However, until such time as a viable alternative is discovered, DDT remains the most effective and least costly insecticide available and we should continue to allow countries to have a comprehensive arsenal at their disposal in the fight against malaria.


If males act on the inconclusive findings of the University of Pretoria research paper and decide to unnecessarily place their families at risk by prohibiting the minimal use of DDT in their homes, malaria mortality rates could once again rise steeply, as they did in the past when the use of DDT was discontinued with catastrophic consequences.