Satellites Help Track Disease Epidemics

By Cat Lazaroff

WASHINGTON, DC, February 6, 2002 (ENS) - Satellite images of droughts, floods and heat waves are now helping scientists track and predict the spread of disease in the U.S. and around the globe. Using climate and vegetation data, biologists are now gathering information that could aid health workers in preparing for and combating outbreaks of a host of deadly viruses.

Scientists and public health officials hope one day to use near real time maps to focus resources and stave off diseases more efficiently.

In a pair of recent reports, earth scientists at the National Aeronautics and Space Administration (NASA) have studied weather changes and subsequent outbreaks of two viral hemorrhagic fevers 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.


This satellite map from January 1998 shows vegetation changes associated with higher than average rainfall. In the savanna lands of East Africa, vegetation grew 70 percent more than normal, indicating wet conditions favorable to disease carrying mosquitoes (All maps courtesy NASA)
"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."

The two studies, which will appear in an upcoming special issue of "Photogrammatic Engineering on Remote Sensing and Human Health," provide further evidence that climate variability can affect disease patterns. They come on the heels of recent NASA research that connects outbreaks of the South American disease Bartonellosis with the appearance of the weather phenomenon El Niņo.

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 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."

risk map

A Rift Valley Fever outbreak risk map for the period December 1997 - February 1998 shows a high risk in the wetter than usual areas of east Africa
Rift Valley Fever outbreaks were linked to abnormally high and persistent rainfall in semi-arid Africa. The resulting flooding creates the conditions necessary for breeding of mosquitoes, which then transmit the virus, first to domestic cattle and frequently to people as well.

Though RVF is generally fatal in just one percent of human cases, it is often fatal to livestock, which can have devastating economic impacts on the countries affected.

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.

Ebola hemorrhagic fever, a highly fatal disease, is encountered in the tropical forest areas of Africa. 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 one to two 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."

Another NASA funded study uses temperature and vegetation data from satellites to help track and predict where West Nile Virus is spreading in North America.

West Nile

This composite image, created to help monitor and predict the spread of West Nile Virus in the U.S., shows mean land surface temperatures between 1997 and 2000
The disease, first reported in the U.S. in 1999, causes flu like symptoms that can lead to fatal encephalitis in people with compromised immune systems, like the elderly.

Though the theory has not been proven, scientists believe the West Nile Virus may be spread across the country by infected birds traveling along their migration routes. Mosquitoes carry the virus and pass it on when feeding on hosts like birds, livestock, other animals and people.

The satellite maps show nationwide temperatures, distributions of vegetation, bird migration routes and areas pinpointing reported cases. The combined data helps scientists predict disease outbreaks by showing where conditions are right for the insects to thrive and where the disease appears to be spreading.

"The images are derived from satellite data that capture a number of variables that are crucial for detecting whether a habitat is suitable for a vector, like a mosquito that carries West Nile Virus," said David Rogers, the lead author of the study. Rogers is Professor of Ecology at Oxford University in the United Kingdom and a member of the International Research Partnership for Infectious Diseases (INTREPID) group, based at NASA's Goddard Space Flight Center.

"It's not a single variable that tends to determine whether a disease will occur, but rather a combination of variables," Rogers concluded.