Use Satellites to Help Track and Control a Disease
David E. Steitz
Headquarters, Washington, DC
Goddard Space Flight Center, Greenbelt, MD
USE SATELLITES TO HELP TRACK A DISEASE AND KEEP IT UNDER CONTROL
satellites to spot the early signs of an El Nino, scientists may
be able to help save East Africans and their livestock from Rift
Valley Fever, a mosquito-borne disease that can be fatal to humans
NASA and Department
of Defense researchers have determined that rising sea-surface temperatures
in the western equatorial Indian Ocean, combined with an El Nino
in the Pacific, can lead to abnormally heavy rains in East Africa.
These rains create a favorable habitat for the mosquitoes that carry
the Rift Valley Fever virus, spreading it to humans and animals.
at NASA's Goddard Space Flight Center, Greenbelt, MD, and the Department
of Defense-Global Disease Infections System, Walter Reed Army Institute
of Research, Washington, DC, studied nearly five decades of data
to produce these findings. According to their report in the July
16 issue of the journal Science, satellite data can help predict
Rift Valley Fever outbreaks up to six months in advance.
"In the early
1980s, we discovered a cycle of Rift Valley Fever outbreaks that
appeared to depend on rainfall," said Kenneth Linthicum, a Walter
Reed entomologist. "There were large outbreaks every seven to ten
years, but the virus apparently disappears between outbreaks," he
consulted with Assaf Anyamba, a Goddard geographer who uses satellites
to study the effects of El Nino, a phenomenon that occurs when sea-surface
temperatures rise in the eastern Pacific Ocean. They found that
some El Nino episodes over the past five decades led to large Rift
Valley Fever outbreaks. During an El Nino, East Africa often receives
more rain than normal, but El Nino alone does not ensure an outbreak.
Anyamba, the decisive factor is the warming of the Indian Ocean
along with the Pacific, which occurred in two of five El Ninos over
the last 17 years. "When the western equatorial Indian Ocean is
similar to the East Pacific Ocean in xsea-surface temperature, there
will likely be a large-scale outbreak of Rift Valley Fever following
heavy rainfall over large areas of East Africa," he said.
here is that satellite data can provide advance warning of conditions
suitable for Rift Valley Fever outbreaks and then identify the actual
areas affected," said Compton Tucker, a Goddard biologist who has
used satellite data to study vegetation in Africa for over 20 years.
Satellites provide synchronous measurements of ocean temperature
and vegetation conditions. The close relationship between ocean
temperature, rainfall and land vegetation helps scientists determine
which areas received the most rain and are greener than normal,
making them likely habitats for the mosquitoes that carry the Rift
Valley Fever virus.
The Rift Valley
Fever virus is passed into the eggs of Aedes mosquitoes. The mosquitoes
lay their eggs in moist soil when floodwaters recede. The young
insects hatch when the area is re- flooded and feed on local livestock.
A second kind of mosquito, the Culex, then causes the large outbreaks
by contracting the virus from infected livestock and spreading it
rapidly. Culex mosquitoes are only prevalent when there are excessive
rains. Heavy rains typically hit the area over Eastern Africa only
when both oceans are warmer than normal. The virus causes death
in livestock populations and produces flu-like symptoms that can
be fatal to humans.
that insecticides placed into the soil months before
the mosquito season will stop production of the Aedes mosquitoes.
"If you know when the outbreak is going to happen, you can treat
areas near domestic animals and human populations," he said. Linthicum
said that there are safe ways to treat the soil to prevent the mosquitoes
from hatching. There also are vaccines for livestock.
into the El Nino phenomena and the subsequent study of Rift Valley
Fever are part of NASA's Earth Science Enterprise, Washington, DC,
dedicated to the long-term study of how human-induced and natural
changes affect the global environmental system. Scientists first
successfully predicted El Nino from satellite data in 1997 and helped
save the U.S. government billions of dollars by giving officials
advance warning of the potential for severe weather.
Science and Technology