Illustration by David Simonds

IN DECEMBER 1997 large numbers of cattle, goats and sheep began dying in the Garissa district of north-eastern Kenya. A month later people started dying, too. It was, at the time, the biggest recorded outbreak of Rift Valley fever in east Africa. Some 100,000 stock animals succumbed and about 90,000 people were infected—hundreds fatally—in five countries.

In December 2007 the same thing happened. Or, rather, it started to happen but was stopped in its tracks. The difference was that the second time around there was warning. In September researchers at the Goddard Space Flight Centre in Greenbelt, Maryland, part of America's space agency, NASA, told the authorities in Kenya that they had a problem. They told them again in October. And again in November. By the time the epidemic emerged, the Kenyan health ministry had dispatched teams to the area to distribute mosquito nets and urge village leaders and religious authorities to stop people slaughtering and eating animals. Though the outbreak still killed 300 people in Kenya, Somalia and Tanzania, it could have been a lot worse. According to Kenneth Linthicum of America's Department of Agriculture, the number of deaths would probably have been more than twice as high without the warning.

The warning itself was possible because of a model of how disease spreads that Dr Linthicum helped design. And the data that were plugged into that model came from satellites.

What the researchers at Goddard had noticed at the time of the first outbreak was that in the months preceding it, surface temperatures in the equatorial part of the Indian Ocean had risen by half a degree. These higher temperatures brought heavy and sustained rains, cloud cover and warmer air to much of the Horn of Africa. Mosquitoes multiplied wildly—and lived long enough for the virus that causes the fever to develop to the point where it is easily transmissible. In September 2007 the researchers saw the same thing happening in the ocean, and suspected the same consequences would follow.

Vein hopes

Attempts to foresee epidemics such as these have traditionally relied on fieldwork on the ground. This is often slow and expensive. Crunching data from satellites is much less costly. Satellites transmit copious information on temperatures, precipitation, vegetation cover and even the health, moisture content and chlorophyll-production of plants.

As David Rogers, an expert in ecology and disease at Oxford University, observes, this is an enormously useful combination of variables. His research has linked levels of photosynthesis, detected from satellites, to the size of a vein in the wings of west African tsetse flies. This vein-measurement indicates the health and size of fly populations—and suggests the likelihood of epidemics of sleeping sickness, a tsetse-borne parasitic disease that kills tens of thousands of Africans a year. That means satellite data can be used to predict epidemics without having to collect any flies on the ground.

Jacques-André Ndione, a researcher at the Centre de Suivi Ecologique, a government public-health agency in Dakar, Senegal, is also impressed by the power of satellite monitoring. He cites one study which showed that in west Africa malaria tends to spread faster in suburban neighbourhoods than in cities and slums. The reason, revealed by satellite, is that the suburbs have more backyard ponds and puddles. Indeed, satellites can not only count such small bodies of water, they can measure their longevity, salinity and mud content—and thus how mosquito-friendly they are.

And it is not only Africa that is benefiting. Satellite research indicates a “significant risk” that dengue fever, malaria and Rift Valley fever will enter Europe, according to Renaud Lancelot, the head of the EDEN project, a group of laboratories and public-health agencies in 24 European and African countries. Indeed, chikungunya, a mosquito-borne virus endemic to tropical Africa and Asia, has already arrived in Albania and Italy. A harbinger of things to come, perhaps?