A FEMALE mosquito sucks her dinner out of a malaria patient recuperating in
bed. Engorged with blood and weary after her meal, she flies over to rest on the
wall before taking off in search of her next victim. Little does she realise
that the wall is coated with a thin layer of the insecticide DDT that will leave
her disoriented and then kill her within minutes.
This scene is played out daily in Belize, Brazil, Ecuador, Ethiopia, India,
Kenya, Thailand and elsewhere. In the West, DDT has been demonised by
governments since 1962, when Rachel Carson鈥檚 book Silent Spring linked
its use as an agricultural pesticide to massive ecological disruption. But in
many tropical countries, where mosquito-borne diseases such as malaria and
yellow fever still infect around 270 million people each year, DDT remains a
crucial public health tool. 鈥淚 hate to be defending a chemical that causes such
terrible environmental problems,鈥� says Renato Gusm茫o of the Pan American
Health Organization in Washington DC, part of the WHO. 鈥淏ut it鈥檚 useful, safe
[to humans] and cheap.鈥�
Not everyone would agree. DDT鈥檚 detractors point out that while it may keep
malaria at bay, it is one of many artificial chemicals believed to interfere
with people鈥檚 endocrine system, which secretes hormones into the blood. Such
disruption could cause cancer, although the issue remains controversial. Two
recent studies in The New England Journal of Medicine (vol
337, p 1253) and the British Medical Journal (vol 315, p 81) failed to
find an association between DDT and cancer.
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In the 1950s, DDT was one of the chemicals that revolutionised agriculture.
But the discovery that it builds up in the environment, is highly toxic to
wildlife鈥攅specially invertebrates鈥攁nd passes up the food chain
resulted in many countries prohibiting its use. In June, 110 countries will
begin negotiations for a worldwide ban on DDT and other persistent organic
pollutants such as PCBs and dioxins, under the auspices of the UN Environment
Programme (UNEP).
Public health officials from the tropics will do everything they can to avert
a ban. They will argue that they use only small amounts of DDT in confined
areas, doing little environmental damage while saving lives. Donald Roberts of
the Uniformed Services University of the Health Sciences in Bethesda, Maryland,
who has done extensive fieldwork in Belize, says that abandoning DDT can result
in large increases in malaria cases.
Belize鈥檚 malaria problem spiralled out of control after it stopped using DDT
six years ago, having virtually eliminated the disease. The country has a
population of around 200 000, and in 1994 it reported over 10 000 malaria cases,
more than at any other time in the country鈥檚 history. In 1996, the country
started spraying again and malaria rates dropped. Roberts concedes that other
factors such as human migrations due to political unrest might have contributed
to the epidemic, but he has analysed data from Brazil, Ecuador and elsewhere and
found a similar pattern.
Roberts and others stress that using DDT for mosquito control is very
different from using it for agriculture. Treating homes across the whole of
Guyana, for example, which covers 215 000 square kilometres, uses no more DDT
than would be used to spray 4 square kilometres of cotton during a single
growing season. The pesticide is also confined indoors. 鈥淚f it is used as
recommended, we expect the environmental contamination to be minimal,鈥� says
Pushpa Herath of the WHO鈥檚 malaria control programme in Geneva.
But environmentalists and northern governments say that a complete ban on DDT
would force health workers to explore other, less risky ways to control
mosquito-borne diseases. 鈥淥ne of the issues that will be very hot [at the
negotiations in June] is whether we have safer alternatives to the chemicals for
public health,鈥� says Tryggvi Felixson of the Iceland Ministry for the
Environment. The Nordic governments are pushing more strongly than any for a ban
because many persistent chemicals that evaporate in the south are carried north
and condense in the cold Arctic air.
Some countries have already tried replacing DDT with organophosphates and
pyrethroids. Although these substances break down more quickly and do not linger
in the environment, they are more acutely toxic and can injure health workers,
who rarely wear full-body protection in the hot, humid tropics. And since they
must be frequently reapplied, these insecticides are more expensive. They may
also be less effective than DDT. Experience in several countries suggests that
mosquitoes can rapidly develop multiple resistance to the seven or eight
available alternative pesticides, says Herath. 鈥淯ntil we have enough
alternatives, DDT should be one of the options.鈥�
Environmentalists would like health officials to use more non-chemical
methods to prevent the spread of malaria, such as bed nets, managing waterways
to eliminate pools of still water where mosquitoes breed, and using predatory
fish, bacteria and other biological controls to kill larvae. Health workers
claim that many of these methods, while laudable, are simply not practical in
the field. But proponents of the ban believe that new opportunities would arise
as DDT was phased out. 鈥淓xperience has shown that when there is pressure to make
changes, it yields positive results,鈥� says Felixson. 鈥淲e鈥檝e seen that with the
ozone-depleting chemicals. We must not have an agreement with a lot of
loopholes. There would be no incentive to find alternatives.鈥�
Some countries have already been forced to discontinue or minimise their use
of DDT because of pressure from foreign aid organisations. Others, such as
Indonesia and Bangladesh, have banned the chemical because traces have appeared
in agricultural exports, endangering their international trade links.
Black-market trade in pesticides often results in DDT that has been earmarked
for public health purposes ending up on farmers鈥� fields. Julie Langer of the
World Wildlife Federation Canada in Toronto, which is developing ways to control
malaria without resorting to DDT, says that only a total ban will prevent this
illegal trade.
A ban would come too late for communities in the Arctic, whose women already
have concentrations of DDT and other persistent pollutants in their breast milk
and umbilical cord blood far in excess of recommended safe levels. The chemicals
condense in the Arctic air, fall as rain and move up the long marine food chain
(鈥淣orthern exposure鈥�, 快猫短视频, 31 May 1997, p 24).
By the time they reach seals鈥攚hich are eaten by Artic people鈥攖hey
are often concentrated a millionfold.
Some public health officials already see the writing on the wall. 鈥淭here is
no need for forcefully reducing DDT because it is happening automatically,鈥� says
Herath. Fewer and fewer companies are manufacturing the pesticide, he says, and
small countries are reluctant to use anything that might endanger their foreign
aid.
Herath hopes that a UNEP package to ban DDT will include releasing
much-needed funds for poor countries to study alternatives to DDT. Langer and
other supporters of a ban claim that assistance for poor countries would
automatically follow. 鈥淓veryone is aware of the fact that in dealing with
persistent organic pollutants there may be a need for financial means to ease
the transition,鈥� says Felixson.
Yet those who have used DDT as a public health weapon still shudder at the
thought of giving up something that has worked for four decades. 鈥淲e still have
enormous confidence in DDT,鈥� says Gusm茫o. He acknowledges that the
eventual abandonment of DDT would be no bad thing, but stresses: 鈥淟et鈥檚 finish
malaria first.鈥�
