Malaria May Be Twice As Prevalent As Previously Thought
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http://www.nature.com/news/2005/050307/full/050307-10.html
Published online: 8 March 2005;
Malaria map paints stark picture
Study suggests the disease may afflict twice as many people as
thought.
By Helen Pearson
The number of malaria cases worldwide may be close to double
that previously estimated, according to a new tally of the kil-
ler disease.
The study, which is one of the most comprehensive efforts to map
the prevalence of malaria, shows that over half a billion people
could have the disease. This elevated count could increase pres-
sure on governments to pump money into prevention efforts.
Malaria, which is caused by a mosquito-borne parasite, is one of
the world's biggest infectious killers. But it has been tough to
figure out exactly how many people succumb to the disease. Ear-
lier estimates relied to some extent on health clinics that vol-
untarily report their cases. But this can be inaccurate because
many people with malaria do not go to clinics - and many clinics
do not submit figures.
In the new study, published in Nature1, Robert Snow at the Well-
come Trust Research Laboratories in Nairobi, Kenya, and his col-
leagues created a computer model to build a detailed world map
showing how many people are likely to be experiencing malaria
symptoms.
To do this, they first divided the world into regions in which
the disease is present, based on medical advice to travellers.
They excluded areas above a certain altitude, where the parasite
is less able to survive, and very built-up areas where there is
less clean water in which mosquitoes can breed.
The team refined this map by adding estimates of the population
density in each area, the risk of picking up an infection from a
mosquito bite, and medical reports on the likelihood that an in-
fection would blossom into the full-blown fever and other symp-
toms. "Then we pressed a button and the number came out," Snow
says.
Best estimate
The researchers reckon that there were around 515 million clini-
cal cases of malaria in 2002, although the actual figure could
lie anywhere between 300 and 660 million. This is not far off
double the estimate of 273 million cases produced by the World
Health Organisation (WHO) in 1998. And in areas outside Africa,
the new figures are at least three times as high as those previ-
ously estimated by the WHO.
Snow's numbers are still only a rough approximation of malaria's
prevalence. But "it's probably the best estimate we have", says
Andrew Spielman, a specialist in tropical infectious diseases at
Harvard School of Public Health in Boston. "It's known to be a
terrible burden and now it appears it's even more terrible than
we thought."
Snow hopes that the new study will help work out the amount of
money, medicines and other resources needed to combat malaria.
Without it, for example, pharmaceutical companies struggle to
know how many drugs to make and public health agencies find it
hard to gauge how well they are curbing the disease. The map
could also help guide where in the world money and aid should be
targeted.
International momentum
The report comes at a time when international momentum to tackle
malaria is building. For example, the Global Fund to Fight AIDS,
Tuberculosis and Malaria, which coordinates government and pri-
vate funding, has channelled $3.1 billion into these diseases
since its establishment in 2002.
Even so, public health researchers and advocates say that they
need more. They point out that unlike other infectious diseases
such as HIV and tuberculosis, we already know how to defeat ma-
laria using simple measures such as insecticide-treated bed nets
and drugs. Spielman predicts that some groups will use the study
as "ammunition" to ask for more funding.
Experts at the WHO say they are now working with Snow's team to
refine their own estimates of malaria's incidence. The WHO's new
figures, which are due to be published later this year, lie be-
tween 350 and 500 million and largely overlap with Snow's, says
Eline Korenromp, who works for the organization's malaria-
monitoring unit in Geneva, Switzerland.
References
1. Snow R. W., Guerra C. A., Noor A. M., Myint H. Y. & Hay S. I.
Nature doi:10.1038/nature03342 (2005)