Features

Give DDT a chance

Indian malaria experts are making a strong case for the much-maligned insecticide despite international lobbies piling criticism over its use in public health programmes.

Subhra Priyadarshini

doi:10.1038/nindia.2008.160 Published online 28 March 2008

DDT has no known mutagenic effect on humans. It has no adverse impact on neonatal performance as well.

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Rachael Carson's watershed book 'Silent Spring' unleashed the anti-DDT wave in USA in the early sixties triggering off revolutionary changes in the prevailing law. The U.S. Environment Protection Agency banned Dichloro-Diphenyl-Trichloroethane within a decade considering its adverse 'ecological impact'.

Though DDT continues to be used in public health emergencies such as malaria outbreaks (the ban exempts this), Indian scientists are arguing that the insecticide is not such a big demon that it has been made out to be.

"DDT still remains unquestionably one of the most effective ways to control the mosquitoes that transmit malaria," says A. P. Dash, director of the National Institute of Malaria Research in New Delhi. Contending that the benefits of DDT far outweigh the perceived dangers related to human health and general ecology, he says if well managed, it poses no harm whatsoever to wildlife or man.

In an editorial in the Indian Journal of Medical Research, Dash cites a number of studies undertaken over the last decade that could tilt the balance in favour of DDT. For instance, it is not known to have any mutagenic effect or impact on the unborn foetus. Similarly, another study last year had concluded that though breast feeding by mothers with high DDT burden may considerably increase DDT levels in the children, it did not have any adverse impact on neonatal performance.

Unquestionably, it's one of the most effective ways to control malaria mosquitoes

A team from Royal Society for Protection of Birds has also shown that thinning of egg shells of some birds had occurred 50 years before the use of DDT began. This study counters the strongest argument of anti-DDT environmentalists — that bird population was declining due to DDT use — which ultimately led to the ban.

DDT residues remain in top soil and rarely contaminate ground water. In a study in Delhi soils, the half-life of DDT residues was estimated to be 6-14 months in tropical and subtropical soils as against 2-15 years in temperate soils. Eminent medical epidemiologist M. K. K. Pillai, who was part of the team studying the soils, says DDT is an insecticide that hasn’t really killed anybody. "Moreover, temperate countries don’t have the diseases we do. And DDT is a very cost effective means to counter malaria and kala-azar," he told Nature India.

In all, 31 countries have opted for exemption from the total ban on DDT, including China and India. The trend reflects the faith of the usefulness of DDT in vector-borne disease control though the UN Stockholm conference in May 2004 had urged complete elimination of DDT production, Dash points out.

In India, about 10,000 metric tons of the insecticide is used against malaria vectors susceptible to DDT in the north eastern states and hilly regions every year. Anopheles culicifacies, the major vector of malaria highly resistant to DDT since 1960s, now shows increased susceptibility in Gujarat and other parts of the country where DDT was withdrawn since 1969. "This dispels the misinformation that DDT is no longer effective in entire India," Dash says.

It would be scientific to use DDT in rotation with pyrethroids

Pending national level feasibility studies for the use of DDT in other areas, Pillai advocates a rotation-type spraying programme along with synthetic pyrethroids. The WHO/UNICEF roll back malaria (RBM) initiative to reduce 50 per cent deaths by 2010 in Africa had used pyrethroids in place of DDT. A review of this programme showed increase in malaria deaths in Africa during 1999-2003. The ambitious programme proved to be 'a receipt for failure' as feared earlier by some scientists, Dash feels.

“We don’t want that kind of a situation in India. So the scientific thing, after we study its feasibility, would be to use DDT in rotation with pyrethroids,” Pillai says.

With introduction to newer areas on the anvil, DDT seems to be set for enhanced use in future vector control programmes in India.


References

  1. Dash, A. P. et al. Resurrection of DDT: A critical appraisal. Indian J. Med. Res. 126, 1-3 (2007)
  2. Samuel, T. et al. Persistence and binding of DDT and gamma-HCH in a sandy loan soil under field conditions in Delhi, India. Pestic. Sci. 22, 1-15 (2008)