On a continent where a child dies of malaria every 30 seconds, a handful of southern African countries have managed to slash incidence and mortality rates. Compared with the 2000-01 season, there has been a 96% reduction in malaria cases in the South African province of KwaZulu Natal; a 91% fall in neighbouring Swaziland; and a decrease of 86% in southern Mozambique.

There is huge local pride at these achievements, but such feelings are tempered by frustration at the reluctance of WHO and powerful donors to acknowledge that much of the success is due to residual wall spraying with insecticides such as dichloro-diphenyl-trichloroethane, or DDT.

"We have two vector control measures: insecticide-treated nets and indoor residual spraying. Yet a large proportion of the world seems to ignore one of them", says Brian Sharp, director of the South African Medical Research Council's malaria programme. "It is ludicrous", he adds.

Residual spraying of DDT to the inside surfaces of house walls helped eliminate malaria from much of Europe and North America in the 1940s and 1950s, and slashed malaria cases in India, Sri Lanka, China, and tropical South America during the 1950s and 1960s.

In Africa, DDT helped Madagascar overcome a devastating epidemic that left up to 40 000 people dead during the 1980s. Ethiopia has relied heavily on DDT ever since a 1958 epidemic killed 150000 people.

Fears over the environmental impact of massive crop dusting with DDT led to the pesticide being outlawed in the USA in 1972. Castigated as one of the "dirty dozen" chemicals, it was banned worldwide under the Stockholm Convention on Persistent Organic Pollutants, which entered into force in May this year.

As a result of lobbying by some developing countries--backed by WHO--the UN convention provides for an exemption for continued use of DDT for the purpose of disease-vector control. Countries wishing to use DDT must make detailed reports to WHO every 3 years. The UN expects around 20 countries to apply for an exemption, although so far only one, the Marshall Islands, has submitted a request.

The recommended dose for indoor spraying is 1-2 g/m²--widely considered too low to have an environmental impact. Even so, the UN insists that exemptions from the global ban should be of limited duration.

For WHO and much of the donor community, the future of malaria vector control lies with insecticide-treated nets. "Because of much greater long-term population acceptability, [treated nets] are the recommended method in settings of intense, ongoing transmission", says Allan Schapira of WHO's Roll Back Malaria department, adding that exhaustive tests have proven their effectiveness.

Schapira maintains that residual spraying is suitable for only a small part southern Africa, where there are seasonal epidemics of malaria.

In a global drive to halve malaria by 2010, around 20 African countries have scrapped or reduced taxes on bednets to make them more affordable. The ideal would be for universal free distribution of bednets, but given the chronic lack of resources for malaria control this remains a pipe dream. Uptake has increased, but still only 2% of African children are sleeping under treated nets, according to the advocacy group Massive Effort.

Schapira and other health officials hope that grants from the Global Fund to Fight AIDS, Tuberculosis, and Malaria will lead to a dramatic increase in the numbers protected by bednets at night.

WHO often cites the example of Vietnam for successful vector control. Between 1991 and 1997-98, according to WHO, the number of people using treated bednets rose from 300000 to more than 10 million. An integrated antimalaria package tailored to local needs contributed to mortality and morbidity reductions of 97% and 60%, respectively, and local outbreaks were virtually eliminated

However, influential malaria experts in southern Africa contend that WHO, its African regional office, AFRO, and other key players like UNICEF and USAID, have ignored the value of residual wall spraying in the rush to promote treated bednets.

"It is an appalling arrogance that WHO is ignoring what is working in southern Africa", says Richard Tren, director of the Johannesburg-based pressure group Africa Fighting Malaria. "We are halfway through Roll Back Malaria and cases and deaths are going up and up. It is time for a policy rethink."

At the recent AFRO meeting in Brazzaville, South African health minister Manto Tshabalala-Msimang publicly complained about the regional office activity report's silence on indoor residual spraying. He also pointedly queried the practicality of cooking and doing other evening tasks under a bednet.

Scientists also cite problems with ensuring bednets are properly used. "You issue bednets and have no control over what happens. You give the net to pregnant women and children and then find the father ends up sleeping under it because he is the breadwinner who needs to stay healthy", comments Rajendra Maharaj, senior specialist scientist with South Africa's malaria research programme. "With the spray, you treat the house and know everyone has protection."

South Africa started using DDT for residual spraying in 1946, but stopped in 1995 because of environmental concerns and local objections to the white residue on walls. The government switched to spraying with pyrethroids, generally regarded as being more environmentally friendly, only to suffer from a subsequent invasion of pyrethroid-resistant mosquitoes. Malaria cases soared from 4117 cases in 1995 to 64 622 in 2000.

"When there was the epidemic in 2000, community leaders approached authorities and said they wanted the chemical 'that left the white powder behind on the wall'. After we introduced DDT the number of cases went down, the disease is under control and everyone is happy", Maharaj says.

Sharp believes South Africa would like to spray on a rotational basis with other pesticides to try to reduce the risk of resistance. But he notes that, at least for now, DDT rates highest in terms of longevity, impact, and price.

South Africa is lucky. Its malaria belt is small; it boasts a well-developed infrastructure; and it has the money to buy the combination therapy ACT, backed up by reasonable access to primary care.

Many African countries don't enjoy those luxuries. Malaria kills an estimated 1 million people in sub-Saharan African each year and is the leading cause of under-5 mortality, according to WHO. The disease costs Africa more than US$12 billion per year in lost economic output.

With incidence and mortality rates still unacceptably high, there is a surge of interest in DDT despite the tepid response from WHO and its partner agencies.

Swaziland has relied on DDT for house spraying for more than six decades. Simon Kunene, manager of the national malaria programme, says that the latest surveys indicated a parasite prevalence of 0·25%, with mortality at less than 10 people a year.

Zambia is extending the wall- spraying programmes, which slashed malaria cases in the Copper Belt mine clinics in 2000, to major cities like Lusaka and Livingstone, according to Tren. This is likely to be boosted by a grant of US$20·3 million dollars to "scale up equitable and sustainable interventions" in malaria prevention and control.

Tren hopes Zambia's successful management of DDT will help ease fears of Ugandan environmentalists that the pesticide will find its way to the agricultural industry and seep into lake Victoria, thereby jeopardising valuable Nile perch exports. After prolonged arguments between the ministries of environment and health, it looks like the Ugandan government will resume residual spraying with DDT--subject to securing the necessary funds.

Zimbabwe is receiving South African assistance in its spraying programmes, although these are hampered by lack of basics resources, such as fuel to reach outlying communities.

Wall-spraying proponents are encouraged by assurances that the Global Fund will consider funding projects that include use of DDT, such as the one in Zambia. And above all, that the injection of cash from the Global Fund will at last breathe new hope into vector control.

"I personally think that the Global Fund will now open up access to large scale funding as this has been one of the main problems until now", says Sharp, a South African house-spraying proponent. "It's the best break we've had."

Clare Kapp