Following the outbreak of Rift Valley fever in Mauritania in September 1998 (see EMPRES Bulletin No. 9, p. 6), a joint FAO-OIE mission visited Mali, Mauritania and Senegal to assess the situation in the region and help those countries control the disease more effectively.
In accordance with the mission's recommendations, FAO decided to finance an emergency TCP project in Mauritania, starting in May 1999 (TCP/MAU/8923 - Mise en place d'un système d'alerte et de contrôle de la fièvre de la Vallée du Rift et de certaines maladies transfrontalières). The main objective of the project is to strengthen the capability of the Veterinary Services to detect the early signs of the disease in order to react promptly and implement appropriate control measures.
All project activities are being implemented within the framework of the newly established network of animal diseases surveillance, REMEMA (Réseau Mauritanien d'Épidémiosurveillance des Maladies Animales), and in close collaboration with institutions involved in Rift Valley fever surveillance and research-related activities such as the Institut Pasteur in Dakar, the Institute of Research for Development (IRD), IAEA and French Cooperation.
Three international consultants' missions were carried out between July and November 1999:
Pierre Formenty, an epidemiologist at WHO, participated actively in the elaboration of a sensitization campaign against Rift Valley fever, carried out a field mission to visit sentinel herds in high-risk areas and finally assessed the feasibility of implementing a vaccination trial with vaccine strains currently available.
Yaya Thiongane, a virologist and Head of the Virology Section of the Central Veterinary Laboratory of Senegal (LNERV), undertook the training of laboratory staff in the various diagnostic techniques of RVF.
Bernard Mondet, an entomologist at IRD, Montpellier, France, assessed the relevance of using vector control to reduce viral amplification in domestic animals and consequently prevent the spread of the disease to humans.
These consultancies were successful in obtaining a better understanding of the current situation and highlighted the major constraints of Rift Valley surveillance in Mauritania. Indeed, even if the surveillance of the disease can be considerably improved through the support of REMEMA activities (staff training in disease recognition, sampling procedures, disease reporting and database management), defining a control strategy remains a major problem with the means currently available.
In fact, the relatively low safety of the live Smithburn vaccine, which can cause abortions in pregnant animals, does not allow the establishment of a massive vaccination campaign in rural areas where the individual status of each animal is not known precisely. The Smithburn strain of RVF modified live virus vaccine (MLVV) has been modified in virulence by serial passage in infant mouse brain and is widely used as a vaccine for African livestock. The vaccine is protective against disease and abortion in cattle, sheep and goats, it is extremely cheap to produce and protection is attained within a few days of vaccination. It has the disadvantage, however, that it can cause foetal death and abnormalities following vaccination of pregnant, susceptible breeds of sheep. This may be an acceptable hazard to livestock owners, who have experienced the total loss of neonatal lambs and abortions which follow infection with RVF virus. Many sheep owners routinely vaccinate breeding ewes after weaning to avoid this problem. The vaccine has not been shown to revert to virulence in the field or to be associated with any RVF virusactivity when used in interepidemic periods. However, this possibility cannot be totally discounted and vaccine should not be used in livestock to be exported to an RVF-free country. Several candidate vaccine strains have been tested for use as alternatives to the Smithburn strain but so far they appear to be poorly immunogenic or suffer other disadvantages.
Furthermore, validated predictive epidemiology could allow early warning so that mosquito control and vaccination of humans or livestock could be carried out in advance of RVF epidemics. However, the biology of the vector responsible for RVF transmission in Mauritania is not well enough known to establish a vector control strategy. Ongoing research activities carried out jointly by the Institut Pasteur in Dakar and IRD should considerably improve our knowledge in this field.
All of these issues will be more specifically addressed in a regional TCP project on Rift Valley fever that FAO has decided to finance (TCP/RAF/8931 - Mise en place d'un système de surveillance et de contrôle de la fièvre de la Vallée du Rift et des autres maladies transfrontalières). This project, to commence shortly, will focus mainly on coordinated activities to be carried out at the regional level and will try to establish a strategic plan for controlling the disease.