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NASA scientists use satellite images to help track a disease and keep it under surveillance (NASA Press Release 99-81).

Using weather satellites to spot the early signs of an El Niño, 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 and animals. NASA and United States Department of Defense researchers have determined that rising sea-surface temperatures in the western equatorial Indian Ocean, combined with an El Niño in the Pacific, can lead to abnormally heavy rains in East Africa. These rains create a favourable habitat for the mosquitoes that carry the Rift Valley fever virus, spreading it to humans and animals. Researchers at NASA's Goddard Space Flight Center, Greenbelt, Maryland, 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 16 July issue of 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." Linthicum consulted with Assaf Anyamba, a Goddard geographer who uses satellites to study the effects of El Niño, a phenomenon that occurs when sea-surface temperatures rise in the eastern Pacific Ocean. They found that some El Niño episodes over the past five decades led to large Rift Valley fever outbreaks. During an El Niño, East Africa often receives more rain than normal, but El Niño alone does not ensure an outbreak.

According to Anyamba, the decisive factor is the warming of the Indian Ocean along with the Pacific, which has occurred in two out of five El Niños over the last 17 years. "When the western equatorial Indian Ocean is similar to the eastern Pacific Ocean in sea-surface temperature, there is likely to be a large-scale outbreak of Rift Valley fever following heavy rainfall over large areas of East Africa," he said. "What is interesting 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 more than 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. This 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 reflooded and feed on local livestock. A second kind of mosquito, the genus 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 when both oceans are warmer than normal. The virus causes death in livestock populations and produces influenza-like symptoms that can be fatal to humans. Linthicum suggests that insecticides placed into the soil months before the mosquito season will stop production of Aedes mosquitoes. "If you know when the outbreak is going to happen, you can treat areas near domestic animals and human populations," he said. According to Linthicum, there are safe ways to treat the soil to prevent the mosquitoes from hatching. There are also vaccines for livestock.

NASA's research into the El Niño phenomenon and the subsequent study of Rift Valley fever are part of the NASA Earth Science Enterprise, Washington, DC, which is dedicated to the long-term study of how human-induced and natural changes affect the global environmental system. Scientists first successfully predicted El Niño from satellite data in 1997 and helped save the United States Government billions of dollars by giving officials advance warning of the potential for severe weather.

What is the El Niño Southern Oscillation?

The El Niño Southern Oscillation is the result of a cyclic warming and cooling of the surface ocean of the central and eastern Pacific. This region of the ocean is normally colder than its equatorial location would suggest, mainly owing to the influence of the northeasterly trade winds, a cold ocean current flowing up the coast of Chile and the upwelling of cold deep water off the coast of Peru.

At times, the influence of these cold water sources wanes, causing the surface of the eastern and central Pacific to warm up under the tropical sun - this is an EL NIÑO event. This results in heavy rainfall in South America, but severe droughts in eastern Australia. The more intense the El Niño, the more intense and extensive the Australian droughts.

At other times, the injection of cold water becomes more intense than usual, causing the surface of the eastern Pacific to cool - this is a LA NIÑA event. This results in droughts in South America and heavy rainfall, even floods, in eastern Australia. In this way, Australia experiences its characteristic cycle of droughts and floods - all caused by the El Niño/La Niña cycle described above.

Why are "El Niño" and "La Niña" so named?

"El Niño" is named after a Peruvian Christmas festival where the warming of the waters off Peru is said to occur near the birthday of "The Boy" (El Niño), or the Christ child. Meteorologists thus named the phenomenon the "El Niño Southern Oscillation", or ENSO for short. The reverse phenomenon, the cooling of the eastern Pacific waters, was at first called "Anti-El Niño", until it was realized that this literally meant the Anti-Christ! To avoid this unfortunate connotation, it was renamed "La Niña" (or "The Girl").


Under project TCP/RAF/8821 (Emergency analysis and control of Rift Valley fever and other vector-borne diseases in eastern Africa), a workshop gathering CVOs of Ethiopia, Kenya, Rwanda, the Sudan, the United Republic of Tanzania and Uganda was held in Kampala from 30 August to 3 September 1999.

It provided a most valuable opportunity to gauge the current status of transboundary animal diseases (TADs), especially those which are flood-related. The CVOs participated enthusiastically in presenting and discussing the many problems that beset them, the progress made in resolving them and future challenges. They described the serious weaknesses, which they are seeking to rectify, in TAD control in the region and in preparedness for animal disease emergencies. In common with many meetings concerning the control of transboundary animal diseases in developing countries, the main preoccupations were: insufficient recognition of the major contribution that livestock makes to national economies, food security and the welfare of rural communities, and the severe impact that transboundary animal diseases have on them; and the attrition of central veterinary executive authorities by structural adjustment programmes which impose unqualified decentralization of veterinary services. This is particularly acute in eastern Africa at present. The CVOs endorsed the desirability and inevitability of the restructuring process, involving decentralization and privatization of veterinary services. However, they stressed the need to differentiate between clinical service delivery with related activities, which are appropriate for privatization and decentralization, and the statutory responsibilities of national veterinary services, including the surveillance and control of transboundary animal diseases and disease emergencies, which are essential core national veterinary service functions, as is practised in industrialized countries. They expressed their grave concern that the statutory/regulatory element, and especially the surveillance and progressive control of TADs, has not been appropriately addressed in national policies and some national and regional projects that are being implemented by international agencies and donors.

FAO was requested to take the lead in bringing these concerns to the attention of the World Bank, regional development banks and other international agencies involved in livestock development, by convening a specific meeting for this purpose and by employing the services of FAO country representatives. In relation to animal disease emergencies, CVOs recommended that national governments should incorporate such eventualities in their national disaster planning and national veterinary services should undertake animal disease emergency preparedness planning as a core function. National veterinary services were recommended to develop the disciplines of risk analysis and economic appraisal to demonstrate the importance to national economies of livestock and transboundary animal diseases. FAO was requested to provide support through the provision of guidelines and training in these disciplines.

The International Bureau for Animal Resources (IBAR) of the Organization of African Unity (OAU) and FAO were requested to assist in establishing and implementing an Eastern African Commission for the Control of Transboundary Animal Diseases to assist in developing harmonized regional policies, expertise and infrastructure and in the progressive control of transboundary animal diseases. With regard to Rift Valley fever (RVF), national governments were advised to strengthen national and regional capacity for its diagnosis, surveillance and control, including the establishment of a regional early warning system. The importance of support from FAO and donors was stressed.

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