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SATELLITES
TRACKING CLIMATE CHANGES
AND LINKS TO DISEASE OUTBREAKS IN AFRICA By
observing climate variability from space with satellites, scientists are working
to determine where disease epidemics are likely to occur on Earth.
In
a pair of recent reports, NASA earth scientists have studied weather changes and
subsequent outbreaks of two viral hemorrhagic fevers prevalent in Africa: Rift
Valley Fever (RVF) and Ebola. The diseases are dissimilar -Ebola only afflicts
people in tropical forest areas, while RVF is deadly to livestock and occasionally
to people in semi-arid regions. But both are more likely to spread when the right
climatic conditions exist-conditions which can be observed by satellite months
in advance. "Satellite
data can be an important tool for public health disease surveillance," said
Dr. Assaf Anyamba, a research scientist at NASA's Goddard Earth Sciences Technology
Center. "Once we learn more, we could be able to predict climatically-linked
outbreaks before they occur." While the study on RVF was conclusive, the
Ebola study was limited by the small number of Ebola outbreaks which occurred
over the past 20 years.
The
two studies, which will appear in an upcoming special issue of Photogrammatic
Engineering on Remote Sensing and Human Health, will provide further evidence
that climate variability can affect disease patterns. They come fast on the heels
of recent NASA research that connects outbreaks of the South American disease
Bartonellosis with the appearance of the weather phenomenon El Niño. Accurate
prediction of epidemics is still years away. But in the short term, satellite
monitoring could still benefit public health in developing countries where resources
to combat disease are limited. "It
is not feasible to send health workers everywhere," Anyamba said. "But
if we know where outbreaks are likely, those areas can be targeted. We can focus
our efforts where they are needed." Locating
those areas requires the use of polar orbiting satellites, such as the Terra satellite,
which NASA scientists use to monitor vegetation on the ground. Since green vegetation
cover varies with rainfall, it is a good indicator of climate variability, and
therefore of conditions necessary for disease outbreaks. "So
far, our team has mapped areas of Africa at risk for RVF outbreaks," Anyamba
said. "Satellite mapping has identified where and when RVF outbreaks will
occur." Rift
Valley Fever outbreaks were linked to abnormally high and persistent rainfall
in semi-arid Africa. Ensuing flooding creates conditions necessary for breeding
of mosquitoes that transmit the virus, first to domestic cattle and frequently
to people as well. Though RVF causes relatively low mortality among humans (about
1% of cases), it is often fatal to livestock, which can have devastating economic
impacts on the countries affected. In
East Africa, animal husbandry is a major part of economy. Arab countries purchase
a great deal of their meat products from East Africa. During the last Rift Valley
Fever outbreak, many Arab nations stopped imports from the region completely,
which was catastrophic to the local economy, especially in the semi-arid and arid
regions of East Africa. Ebola
hemorrhagic fever is encountered in the tropical forest areas of Africa, but despite
its notoriety as a highly fatal disease, it remains a mystery in many respects.
Though the first known Ebola epidemic occurred in Sudan in 1976, scientists still
have not identified how the virus is transmitted or what animals might host it.
In an effort to identify conditions under which the virus appears, the Goddard
scientists examined satellite data of tropical areas of Gabon and the Congo afflicted
in 1994-1996. They noted a sharp change from persistent dry conditions to wetter
conditions over a 1-2 month period prior to the outbreaks, suggesting these dry
to wet changes might be a "trigger event." Dr.
Compton Tucker, lead author on the Ebola paper, cautions that additional work
is needed to verify the existence of the climatic trigger for Ebola. "It's
fortunate for those affected by Ebola that we have so few outbreaks to study,
but it makes our job more difficult," said Tucker. "Drawing conclusions
from a small sample is risky." The
work continues using satellite data of Earth beamed to receiving stations around
the clock by NASA satellites. Back
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