Richard A Lovett
We listen to satellite warnings about the likely movement of cyclones and storms. When danger comes, ships and aeroplanes change their courses to avoid catastrophe, and vulnerable communities are evacuated to keep safe.
Now possible disease outbreaks, from malaria to West Nile virus, can also be predicted by orbiting oracles. The diseases that can be foreseen in this way are vector-borne, meaning they are transmitted by mosquitoes, ticks or other animals whose latest breeding zones can be monitored from space by mapping temperatures, recent rainfall patterns, soil moisture, vegetation and land use. Based on these measurements, public health researchers can predict when and where conditions will most likely be right for the next population explosion of disease-carrying pests. The use of satellites in public health was discussed at a meeting of the American Association for the Advancement of Science in California last month.
Why use satellites and not ground-based instruments? Having eyes in the sky has many benefits. Satellites such as NASA’s soil moisture active passive (SMAP) observatory, which launched in January, can measure the moisture in the top five centimetres of the soil around the world every two to three days. Other satellites map sea-surface temperatures, air temperature, rainfall, vegetation changes and flooding far more comprehensively – and quickly – than can be done by monitors on land.
If a predicted population explosion of disease-carrying insects overlaps with a town or village, infections can potentially be prevented before they begin. “Once you identify a hotspot, you can provide information for a spray team to mobilise,” says Archie Clements, a researcher from the Australian National University who has studied malaria outbreaks in Melanesia.
Something similar can be done with schistosomiasis, a snail-borne parasite that affects parts of the tropics and is a particular threat to children, says Uriel Kitron, a specialist in vector-borne diseases from Emory University in Atlanta.
Kitron says his team used satellite images of soil, water and weather conditions to map where snails are most likely to be found among the villages of coastal Kenya. “We tried to relate where you lived and where you might go swimming or do laundry to the chance you may become infected,” he says. Local public health agencies can then target likely snail-rich spots.
In America, Kitron’s team has used data from Chicago to see how patterns of rainfall and temperature affect the spread of West Nile virus. The illness – which generally causes flu-like symptoms, but in some people can cause life-threatening complications such as encephalitis or meningitis – first appeared in the US in New York City in 1999, and has spread quickly. “We mapped trees, houses, temperature data, [and] rainfall data to identify the locations were people are at highest risk of contact with mosquitoes that transmit the disease,” he says.
The result was a computer model that could predict where West Nile disease was likely to strike about two weeks in advance (the breeding and development cycle for many mosquitoes is about two weeks). Ideally, Kitron’s team would like to be able to predict mosquito-hatching conditions even further in advance. “But two weeks’ warning is not bad,” says Kitron, who is already working with health authorities on the idea.
Kenneth Linthicum, director of the US Department of Agriculture’s Centre for Medical Agriculture and Veterinary Entomology in Gainesville, Florida, has successfully predicted outbreaks of Rift Valley fever, a mosquito-borne plague affecting livestock and humans in parts of East Africa. Primarily a livestock disease, it can cost affected countries hundreds of millions of dollars in losses due to trade bans. But it can also spread to humans – through contact with infected animal blood or other tissues – producing thousands of debilitating illnesses and, in severe cases, death. The disease tends to crop up in the aftermath of heavy rainfall and floods, which expand the habitat – and the breeding opportunities – for disease carrying mosquitoes.
Linthicum’s team has been able to predict Rift Valley fever outbreaks as much as several months in advance – far enough out, he says, that it’s possible to use vaccines to protect livestock (and their human handlers) from infection. Difficulties can arise if an outbreak is in a remote region where vaccines aren’t available.
“When we see evidence of potential increased risk we send alerts to the World Health Organisation, as well as the US Food and Agriculture Organisation,” says Linthicum. “They in turn send out their own warning to the countries that are at risk.”
Even if no vaccines can be obtained in time, he says, it’s possible to prevent “tremendous losses” simply by reinforcing the message people should quarantine sick animals and avoid contact with them.
Tick-borne diseases such as Lyme disease, which is prevalent in the US, can also be monitored from space, using satellites that keep track of the dampness of an area’s soil. “Soil moisture is good for ticks,” Kitron says. And now satellites can help find them.
