Using Satellites Monitoring to Identify Ecological Niches Conducive to Disease Outbreaks

Accurate predictions of epidemics are still years away. However, in the short term, satellite monitoring could benefit public health in developing countries where resources to combat disease are limited. It is generally not feasible to send health workers everywhere, but a knowledge of where outbreaks are likely will help target those areas. Efforts can be focused where they are needed.

Locating these vulnerable areas requires the use of satellites such as the NASA Terra satellite, to monitor vegetation on the ground. Because green vegetation cover varies with rainfall, it is a good indicator of climate variability, and therefore of conditions necessary for disease outbreaks. So far, areas of Africa that are at risk for Rift Valley fever (RVF) outbreaks have been mapped with satellite-derived information (Figure 11). A more

Carte Pid Miologie Borr Liose

January 1998

Figure 11 Left: satellite-derived normalized difference vegetation index (NDVI) showing percent deviation from mean vegetation for January 1998. Areas in the savanna lands of East Africa had an increase in vegetation vigor greater than 70% above normal, due to persistent above-normal rainfall, flooding dambo areas, that is, shallow depressions in savanna areas where mosquito eggs containing the virus are found. These are good habitats for the breeding of Aedes and Culex mosquito species, which serve as vectors for transmission of RVF. Right: this is an RVF outbreak risk map for the period December1997-February 1998. Areas shown in red (East Africa) represent areas where there was an outbreak of RVF during this period. Areas shown in green represent the savanna areas of Africa, where RVF is generally endemic and has occurred in the past. The outbreak of RVF during this period was associated with above-normal and widespread flooding during the warm El Nino-Southern Oscillation (ENSO) event of 1997-98 in the East Africa region. Credit: Compton Tucker and Assaf Anyamba, NASAGoddard Space Flight Center.

January 1998

Figure 11 Left: satellite-derived normalized difference vegetation index (NDVI) showing percent deviation from mean vegetation for January 1998. Areas in the savanna lands of East Africa had an increase in vegetation vigor greater than 70% above normal, due to persistent above-normal rainfall, flooding dambo areas, that is, shallow depressions in savanna areas where mosquito eggs containing the virus are found. These are good habitats for the breeding of Aedes and Culex mosquito species, which serve as vectors for transmission of RVF. Right: this is an RVF outbreak risk map for the period December1997-February 1998. Areas shown in red (East Africa) represent areas where there was an outbreak of RVF during this period. Areas shown in green represent the savanna areas of Africa, where RVF is generally endemic and has occurred in the past. The outbreak of RVF during this period was associated with above-normal and widespread flooding during the warm El Nino-Southern Oscillation (ENSO) event of 1997-98 in the East Africa region. Credit: Compton Tucker and Assaf Anyamba, NASAGoddard Space Flight Center.

detailed description than provided here and below is contained in http://eospso.gsfc.nasa.gov/newsroom/ viewStory.php?id=231.

RVF outbreaks are linked to abnormally high and persistent rainfall in semiarid Africa. Ensuing flooding creates conditions necessary for breeding of mosquitoes that transmit the virus, first to domestic cattle and frequently to people as well. Though RVF causes relatively low mortality among humans (c. 1% of cases), it is often fatal to livestock, which can have devastating economic impacts on the countries affected.

In East Africa, animal husbandry is a major part of economy. Arab countries purchase a great deal of their meat products from East Africa. During the last RVF outbreak, many Arab nations stopped imports from the region completely, which was catastrophic to the local economy, especially in the semiarid and arid regions of East Africa. In the late 1990s, a 'Climate and Disease Connections: Rift Valley Fever Monitor' was developed jointly by NASA/ GSFC (Goddard Space Flight Center) and DoD/GEIS. The monitor, which includes climate- and satellite-derived vegetation anomalies that are associated with RVF outbreaks, is widely disseminated; it is internationally available with monthly updates at http://www.geis.fhp.osd.mil/ GEIS/SurveillanceActivities/RVFWeb/indexRVF.asp.

Another example is Ebola hemorrhagic fever. Ebola is encountered in the tropical forest areas of Africa, but despite its notoriety as a highly fatal disease it remains a mystery in many respects. Though the first known Ebola epidemic occurred in Sudan in 1976, scientists have still not identified how the virus is transmitted or what animals might host it. In an effort to identify conditions under which the virus appears, scientists examined satellite data of tropical areas of Gabon and the Congo afflicted in 1994-96. They noted a sharp change from persistent dry conditions to wetter conditions over a 1-2 month period prior to the outbreaks, suggesting these dry to wet changes might be a 'trigger event'. However, it is cautioned that additional work is needed to verify the existence of the climatic trigger for Ebola. To quote Tucker (http://eosp-so.gsfc.nasa.gov/newsroom/viewStory.php?id=231), ''It's fortunate for those affected by Ebola that we have so few outbreaks to study, but it makes [the job of associating outbreaks with specific antecedent conditions] more difficult. Drawing conclusions from a small sample is risky.''

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