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Special Report
An Outstanding Example of Remote Epidemiology Cooperation in Senegal

http://www.bulletins-electroniques.com/actualites/59799.htm

RVF or Rift Valley Fever that mainly affects domestic animals, but that may also contaminate humans, causes major socioeconomic problems in African livestock farming areas. This is why the research conducted by the Dakar Centre de Suivi Ecologique (CSE, Ecological Monitoring Center) in Senegal, working with Toulouse-based CNES (French Space Agency) is so important. Satellite images track the spatial and temporal variations of small temporary ponds in the Ferlo Region, Senegal. The ponds are larvae deposits for Aedes vexans and Culex poicilipes, the main RVF-vector mosquitoes. Scientists can also use the images to draw up maps of the high-risk areas highlighting Zones Potentially Occupied by Mosquitoes (ZPOM). This major step forward should lead to the development of an early warning system.

The RVF virus belongs to the Phlebovirus strain, one of the five in the Bunyaviridae family. RVF was identified for the first time in 1931 during an investigation into an epidemic affecting the sheep on a farm in the Rift Valley, Kenya. Since then, epidemic outbursts have been reported throughout the African continent, from the southern Sahara to Madagascar, including South and East Africa. In 1997-1998, RVF hit Kenya, Somalia, and Tanzania hard. In September 2000, cases were confirmed in Saudi Arabia and Yemen. However, Senegal had its first epidemic in the Senegal River Valley, in 1987. Since then, the situation has been alarming, and an outbreak of viral activity at any time is very possible.

This was the situation when in the early 2000's, the CNES working with the Dakar CSE (Senegal) started research on RVF, a climate-dependent disease. Antonio Guell, Head of the Enhancement and Applications Department was Research Director, and Murielle Lafaye, the Coordinator for the Development of Downstream Applications and Manager of the Environment-Climate-Health Program, was in charge of the research. "We had a twofold challenge to meet. First, we had to understand the emergence mechanisms of the fever where climate and environmental conditions are decisive. Then we had to develop spatial products tailored to our approach," summarized climatologist Jacques-André Ndione, who is in charge of different CSE programs on Environment, Climate and Health issues.

A Multidisciplinary Approach

Of course, Senegal did not wait until the 2000's to start tackling RVF. After the 1987 epidemic, the Dakar Pasteur Institute working with partners took the lead in setting up an active animal tracking and entomological program in several biogeographic zones in the country. For the program, different species of mosquitoes, specifically Aedes and Culex, were identified as potential RVF vectors, with each mosquito species playing a specific part in disease epidemiology. For instance, Aedes mosquitoes are involved in starting the disease cycle whereas the Culex species play a part in viral amplification. Although the vectors are different than the ones conveying the disease in East or South Africa, they do deposit their eggs in the same type of locations, viz., inundable areas and ponds. As the ponds are full at the start of the rainy season, they are watering places for livestock, but in places pond water provides drinking water for the population, which use it for other purposes, as well. However, the epidemiological schema considered for Kenya and, more widely for East Africa, does not explain the RVF outbreaks in Senegal. "So we asked ourselves whether the Sahelian climate had a particular feature. Could the rainfall distribution be associated with the emergence of the RVF virus? Accordingly, we had to gain further knowledge and a better grasp on the climate and environmental conditions linked to RVF emergence on the one hand, and understand how the temporary ponds, which are the larvae deposits of the main RVF vectors, worked," explained the Senegalese climatologist.

Satellites and satellite imaging are obviously the ideal tool for this type of study. The CSE is already well familiar with the tool as it has two receiving antennas, one from US NOOA (National Oceanic Atmospheric Administration) and another called DDS (Data Dissemination System), which was acquired during a project rolled out with ESA (European Space Agency). "We receive a lot of Earth observation optic or radar data. CSE is tasked to collect, input, process, analyze, and disseminate data and information on the territory and natural resources, using space technologies, with a view to improving resource and environmental management." The researchers use very high resolution satellite images (SPOT-5) to study the ponds in the Ferlo Region (that takes it name from a stream) in the sylvopastoral area of Senegal . This is because the ponds are very small, and their distribution complex. Five multispectral images comprising four spectral channels, i.e., green, red, near-infrared, and medium infrared (MIR) from the Spot-5 satellite (10 meter resolution) were used to track the detailed spatial and temporal variations of the ponds. The images date back to August 26, 2003, 17 and 27 October, and 18 November, 2003, and 19 January 2004, respectively.

Thanks to satellite imaging, the researchers were able to characterize the plant coverage and turbidity of the ponds, which enabled them to define some indexes. Using the MIR and green channels of the SPOT image, the Normalized Difference Pond Index highlighted the water area and made it possible to differentiate the vegetation inside and outside the ponds. Another index based on using the red and green channels of the SPOT image was used to assess the degree of pond turbidity. The Dakar Pasteur Institute entomologists took several measurements in the field, and were able to determine the range of one mosquito around a pond, i.e., about 550 meters for Culex poicilipes, and 620 meters for Aedes vexans. "The multidisciplinary approach that brings together entomologists, epidemiologists, climatologists, and meteorologists led to the development of the first high-risk maps, which in turn led to what we call Zones Potentially Occupied by Mosquitoes or ZPOM. To delimit the ZPOM, we applied a 500-meter buffer zone around the detected ponds," explained Jacques-André Ndione.

Extrapolating the Approach to Other Pathologies

The work accomplished so far around the 1,345 ponds that were identified around Barkédji is undoubtedly a major step forward. This should eventually lead to the development of an early warning system mainly based on the satellite tool. "The approach involving mapping the ponds based on satellite data is not a Senegal exclusive. However, using the high-resolution data for Environment-Health issues combined this highly original approach leading to results obtained thanks to new indexes that still cannot be found anywhere else is a pioneering approach in this area, thanks to support from CNES," Jacques-André Ndione pointed out. Admittedly, the current model has to be optimized. So, the researchers will have to find the answers to other very important questions. For instance, why does a given mosquito prefer one pond to another? The answer to this question may eventually mean only monitoring a limited number of ponds.

Furthermore, the model should also incorporate socioeconomic data. For instance, some settlements of Peul livestock farmers are near the Zone Potentially Occupied by Mosquitoes while others are far from it. Why is this? Have the populations of the remote settlements empirically realized that there is a risk factor linked to the water? "Projects are underway, or will soon get off the ground. Several stakeholders are involved, such as CSE, CNES, the Dakar Pasteur Institute, the Siméon Fongang Laboratory of Atmosphere and Ocean Physics, and the Senegal Department of Livestock." Generally speaking the researchers would like to apply the approach developed for Rift Valley Fever to other vector-borne pathologies, such as malaria or dengue. "Drawing on our experience, we have begun to address the issue of applying the approach to malaria. Within three to four years, we would like to publish the first results of the research," the CSE climatologist said.

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CSE - Jacques-Andre Ndione - email: jacques-andre.ndione@cse.sn

Code brève
ADIT : 59799

ADIT - Jean-François Desessard - email: jfd@adit.fr

eTech France numéro 228 (2/07/2009) - ADIT / ADIT - http://www.bulletins-electroniques.com/actualites/59799.htm