Rift Valley fever (RVF) is an insect-borne disease affecting humans and animals. This is a major zoonosis in several African countries, particularly in Mauritania.
The presence of RVF virus in Mauritania was first suspected in 1984 as a result of serological investigations carried out by the national breeding and veterinary research centre (CNERV). Investigations concluded that the circulation of the virus was enzootic in the country and that the disease was likely to break out any time. This finally happened during the 1987 rainy season in the Trarza region (southwestern Mauritania). More than 200 human deaths were registered. Massive abortions (up to 100 percent within herds), stillbirth and high mortality rate in young animals were observed among domestic animals. Several viral strains were isolated from human samples, while only serological evidence of the disease (high IgM and IgG prevalence rate - up to 80 percent) was found in animals. Symptomatology in humans was fever, myo-arthralgy, with neurological and haemorrhagic signs, which explains why it was so difficult to decide on a differential diagnosis with other haemorrhagic fevers (particularly yellow fever and true neurological form of malaria).During the next rainy season, another human outbreak of RVF was diagnosed in the city of Aïoun el Atrouss (Hodh el Gharbi region) with viral isolation.
Serological investigations carried out by CNERV from 1989 to 1992 showed a decreasing prevalence rate among animals, followed by an increase in several places from 1994 to 1997. This evolution is the sign of an occult viral circulation, with small confined outbreaks. Aware of this situation and of the danger of RVF in humans, the veterinary service decided to include this disease in the Mauritanian epidemiological survey network (REMEMA), created in 1998.
Ten years after the major 1987 epidemic, RVF reappeared in the Hodh el Gharbi region during the 1998 rainy season. The disease was responsible for more than ten human deaths and, according to the livestock owners, for a high abortion rate and some stillbirth among animals. Several human and animal viral strains were isolated. It was the first time that RVF virus was isolated from animals in Mauritania. Because of this new outbreak, Mauritania, Senegal and Mali requested FAO's assistance. After a joint FAO/OIE/WHO evaluation mission, FAO granted a national TCP project to Mauritania, and a regional TCP project to Mauritania, Senegal and Mali.
In the meantime, with French Cooperation assistance and the participation of IRD Dakar and the Pasteur Institute in Dakar, the Mauritanian veterinary services carried out several field missions to assess the sanitary human and animal situation, and to investigate the potential vectors. The information collected during these missions contributed to a better understanding of the human and veterinary epidemiological feature of the disease. Furthermore, in the framework of the National Disease Surveillance System (REMEMA), nine sentinel herds (sheep and goats) were established in five regions of the country in order to monitor RVF.
IgG-positive animals were found in all herds, and a small number of IgM-positive animals were found in six different sentinel herds scattered in five regions, thus proving the persistence of RVF virus circulation on a large scale in the country.
Three outbreaks were diagnosed during the 1999 rainy season through REMEMA's activities and the TCP investigation missions in the Tagant, Guidimakha and Trarza regions. Three suspected human deaths (IgM-positive and influenza-like syndrome, sometimes associated with bleeding), massive abortion in small ruminants (more than 30 percent) and stillbirth were registered in the Tagant outbreak. In Guidimakha and Trarza, RVF was suspected in small ruminants because of massive abortion and stillbirth rates, and was confirmed by IgM- and IgG-positive serology.
Finally, RVF seems to have established itself in the main livestock husbandry areas of Mauritania. Its increase during the past two years has led Mauritanian sanitary authorities to take appropriate decisions (investigations, survey, communication on the disease, international and regional cooperation). As a result, the Veterinary Services have a better understanding of RVF epidemiology in Mauritania. Furthermore, a sensitization campaign concerning the disease was organized to inform the population (veterinary and health services, breeders and breeding associations, sponsors, etc.) about the risks of transmission and the possibility of preventing the disease from spreading.
All past studies show how necessary it is to keep on surveying for RVF and to carry on research activities. Future research-related activities will focus on the symptomatology of the disease (beyond abortion and stillbirth), the identification of the vectors and reservoirs responsible for the low circulation and maintenance of the virus between epizootic periods.
(Contributed by: Dr Yaghouba Kane, Infectious Disease Service, CNERV, BP 167, Nouakchott, Mauritania; e-mail: [email protected])
AN EPIDEMIOLOGICAL SURVEILLANCE NETWORK FOR MAURITANIA
An epidemiological surveillance network for animal diseases, REMEMA, was created in Mauritania at the end of 1998. Its official recognition through a ministerial decision should demonstrate its sustainability.
The elaboration of an epidemiological surveillance network was becoming necessary after the cessation of rinderpest vaccination in 1998. It was also an excellent opportunity to restructure the disease information collecting system related to major livestock epidemic diseases in Mauritania.
REMEMA links the different stakeholders of the animal health surveillance system: the national veterinary administration, the national veterinary laboratory (CNERV), private veterinarians and livestock owners. Although the Mauritanian Government is fully responsible for the network, since its creation it has been financially and technically supported by the European Union, through the PARC project, and French Cooperation and its technical assistance (one post of a veterinarian, specialist in epidemiology).
REMEMA's activities take place in the major animal husbandry areas of the country, that is the eight southern provinces. It was decided that REMEMA should focus on four main diseases: rinderpest (RP), contagious bovine pleuropneumonia (CBPP), Rift Valley fever (RVF) and foot-and-mouth disease (FMD). The encouraging development of the network allowed the inclusion of two other diseases - rabies and peste des petits ruminants (PPR).
The surveillance network relies mostly on passive surveillance: field agents send standardized reporting forms and samples to the Central Unit when they suspect an outbreak. Active surveillance through field investigations is not possible at present owing to limited resources. However, on occasion, REMEMA carries out specific investigations: for example, during the 1999 rainy season nine sentinel herds were regularly visited by REMEMA's field agents in the framework of RVF surveillance. A quarterly bulletin, REMEMA info, is the network's main communication tool. Two issues have already been published, and a third is now being processed.
Thanks to the strong motivation of the field agents, we can be reasonably optimistic about REMEMA's future.
REMEMA in figures
Number of field agents: 54
Number of survey posts: 38
Results from October 1998 to December 1999
Number of monthly reports:111
Number of RP cases suspected: 1 (confirmed: 0)
Number of CBPP cases suspected: 3 (confirmed: 0)
Number of RVF cases suspected: 7 (confirmed: 3)
Number of FMD cases suspected: 3 (confirmed: 1)
In addition to the project which was implemented in Mauritania, a regional project covering Mali, Mauritania and Senegal was initiated in April. One of its main activities is to establish a common surveillance approach for RVF among the three countries, by monitoring a sentinel herds network and strengthening disease surveillance activities. It will also be an excellent opportunity to assess the real impact of Rift Valley fever in Mali, which is not known at present, although it is obvious that the virus circulates in the livestock population.
At the end of the project, the various partners involved in RVF surveillance- and research-related activities will meet together at a workshop which will be held in Dakar in November 2000 to define a common control strategy