From a satellite orbiting 500 miles above the earth, the jungles of central Africa can look disarmingly innocent. To the untrained eye, their wrinkled swathes of green offer no hint of the potentially lethal infectious diseases lurking within. But for the past two years, African epidemiologists studying images from earth observation satellites have found that for those who know how to look, those images may contain clues about when and where such diseases might erupt.
At Gabon's International Centre for Medical Research, for example, Ghislain Moussavou has been using data from the European Space Agency's (ESA) Envisat satellite to help predict outbreaks of Ebola hemorrhagic fever. When Ebola emerges from the deep jungle to strike human populations, the effects can be devastating. In the absence of any effective treatments, Moussavou explains, "prediction, prevention, and fast control of epidemics must constitute a major priority in public health."
The trouble is, scientists have yet to pin down the virus' natural reservoir, making prediction and prevention difficult. So two years ago, Moussavou began looking at the Ebola problem from a different perspective. While others studied the infectious agent, or hunted for the viral reservoir in populations of bats or other animals, he and his colleagues focused on the medium through which the disease spreads - the environment. "Our work consists of following and characterizing the space-time dynamics of parameters of the environment such as relief, vegetation, hydrography, climate, and human and animal populations," he says.
The sheer biological diversity and geographic inaccessibility of the Central African rainforest makes that a difficult task, which is where the satellites come in. Through an ESA program dubbed Epidemio, Moussavou and his colleagues have had access to satellite images that they can link to epidemiologic and ecologic data. "We found that the epidemic area seems to present a particular environmental pattern linked to the dynamics of water, specifically dense evergreen vegetation due to permanent high humidity maintained by a flat relief," Moussavou explains. "We think that in this particular configuration, specific interactions occur between humans, animals, and biota, following the dynamics of seasons."
In particular, Moussavou and his colleagues found that especially dry periods seemed to trigger a disturbance in the jungle ecosystem, resulting in increased contact among animal populations and between animals and humans. "It's at this moment that the contamination should occur, resulting from a contact between the reservoir and a sensitive species such as gorillas or chimpanzees," he says. Determining more precise connections between humidity levels and outbreaks will allow officials in countries such as Gabon and Congo to tell villagers in at-risk areas that current conditions for transmission are high, Moussavou says, and that they need to take extra precautions.
Elsewhere in Africa, ESA satellite data have been used to study the epidemiology of malaria, meningitis, and schistosomiasis, says Kathrin Weise, the Epidemio project manager, from the German firm Jena-Optronik. For the past two years, Weise's firm and five other companies that won the contract to run the project have been developing specific packages of satellite information to meet the needs of the African epidemiologists. The future use of the satellite data is uncertain, however, because the project's funding has now come to an end, says Weise: "There's no budget now to take it forward."
Simon Pinnock, the technical officer responsible for the project within ESA, points out that Epidemio was intended as a demonstration project, not something that would go on indefinitely. "The idea is to try to convince people that there's value in this remote sensing data, and either they're convinced or they're not," he says. An international meeting on the subject took place in March, and Pinnock says that researchers in the field do seem to see potential value in it. "But really," he says, "the acid test for us now will be to see whether it is supported by other funding mechanisms."