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February 5, 2002
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.
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Contact:
Cynthia M. O'Carroll
cocarrol@pop100.gsfc.nasa.gov
Goddard Space Flight Center
(Phone:301/614-5563)
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A Satellite Derived
Image Used to Track Potential for Rift Valley
Fever in January 1998 - 2.7 MB TIFF
This Normalized Difference Vegetation Index
(NDVI) anomaly map depicts the percent
deviation or change from mean vegetation
greenness for January 1998. Areas in the
savanna lands of East Africa had an increase
in vegetation vigor greater than 70 % above
normal, due to persistent above normal
rainfall, flooding dambos areas, that is,
shallow depressions in savannah areas where
mosquito eggs containing the virus are found.
These are good habitats for the breeding of
Aedes and Culex mosquito species, which
serve as vectors for transmission Rift Valley
Fever.
Mapping the Risk of Rift Valley Fever - 120 KB JPEG
This is a Rift Valley Fever (RVF) Outbreak risk
map for the period
December 1997 - February 1998. Areas
shown in red (East Africa)
represent areas where there was an outbreak
of RVF during this
period. Areas shown in green represent the
savanna areas of Africa,
where RVF is generally endemic and has
occurred in the past. The
outbreak of RVF during this period was
associated with above
normal normal and widespread flooding during
the warm ENSO Event of
1997/98 in the East Africa region.
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