Rift Valley fever virus infection among French troops in Chad - Letters - Letter to the Editor

Emerging Infectious Diseases, June, 2003 by Jean Paul Durand, Michele Bouloy, Laurent Richecoeur, Christophe Nicolas Peyrefitte, Hugues Tolou

To the Editor: During the rainy season every year, outbreaks of self-limiting nonmalarious febrile syndromes have occurred in French military troops on duty in Chad. To determine the cause of these syndromes, the Tropical Medicine Institute of the French Army Medical Corps implemented an arbo-virus surveillance program in Marseille.

During summer 2001, we collected samples from 50 soldiers who had a febrile illness. All blood spot samples tested negative by enzyme-linked immunosorbent assay (ELISA) for certain antigens (i.e., dengue virus, West Nile virus, Chikungunya virus, and Wesselsbron virus). However, after co-culture of 31 peripheral blood lymphocyte samples with C6/36 and Vero cell lines collected in NDjamena, Chad, in August to September 2001, two strains of Rift Valley fever virus (RVFV) were isolated and identified by using indirect immunofluorescence with a specific mouse ascitic fluid and by using reverse transcriptase-polymerase chain reaction (RT-PCR) and sequencing. In retrospective testing, we found that all serum specimens tested by ELISA for RVFV-specific immunoglobulin (Ig) M and IgG were negative. The second serum samples from the two case-patients with these strains, collected 2 months later, were strongly positive (IgM 1/200,000; IgG 1/5,000).

Rift Valley fever, a febrile disease that affects livestock and humans, is transmitted by mosquitoes and caused by a virus (genus: Phlebovirus, family: Bunyaviridae) that can persist in nature in contaminated eggs. The virus was first isolated in Kenya in 1930 (1) and is endemic in the region. In Chad, the disease was first reported in 1967 at the same time as in Cameroon (2); no strain was isolated at that time. Since 1977, RVFV infection resulted in 600 deaths in Egypt (3), 300 in Mauritania in 1987 (4), and 200 in Saudi Arabia and Yemen (5,6) in 2000 to 2001.

To characterize these RVFV strains, parts of the three genome segments (L, M, and S) were amplified by using RT-PCR and sequenced as described (7,8). The figure shows the phylogenic tree constructed from the sequence of the region coding for NSs in the S segment, by using the neighbor-joining method implemented in Clustal W (version 1.6; available from: URL: http://www-igbmc.ustrasbg.fr/BioInfo/ClustalW/clustalw. html). The two strains identified in Chad are quite similar. They are located within the East/Central lineage established previously (6,7), which contains the virus that circulated in Madagascar (1991), Kenya (1997-1998), and Yemen and Saudi Arabia (2000-2001) (9,10). Sequencing of the region in the M and L segments led to the same clustering (not shown), suggesting that this virus did not evolve by reassortment. Deter-mining the origin of the virus is difficult, but its genetic properties suggest that this strain has a Kenyan origin. Before this isolation, no RVFV strains from Chad had been genetically characterized. This strain may be endemic in this region of Central Africa, or the RVFV strain circulating in the Eastern countries may have been transported outside of the territory (which was likely the case in Yemen and Saudi Arabia in 2000) (9,10). Of the two case-patients, one soldier did not leave NDjamena during his 3-month tour of duty, whereas the other had been in contact with livestock in a flooded area before onset of symptoms. Contamination may have occurred through infected animals or mosquitoes, although sheep living in the area did not show any sign of disease (i.e., spontaneous abortions, deaths). The two cases we describe were self-limiting; however, deaths from this illness have been reported in nonepidemic settings in Central African Republic (11). Our data emphasize that healthcare providers should systematically consider Rift Valley fever as a diagnosis for febrile syndromes in persons returning from Africa, even in nonepidemic settings (12).

References

(1.) Daubney R, Hudson JR, Graham PC. Enzootic hepatitis of Rift Valley fever, an undescribed virus disease of sheep, cattle and man from East Africa. Journal of Pathology and Bacteriology 1931;34: 545-79.

(2.) Maurice Y. Premieres constatations serologiques sur l'incidence de la maladie de Wesselsbron et de la Fievre de la Vallee du Rift chez les ovins et les ruminants sauvages du Tchad et du Cameroun. Rev Elev Med Vet Pays Trop Revue d'Elevage et de Medecine Veterinaire des Pays Tropicaux 1967;20:395-405.

(3.) Meegan JM, Hoogstraal H, Moussa MI. An epizootic of Rift Valley fever in Egypt in 1977. Vet Rec 1979;105:124-5.

(4.) Jouan A, Le Guenno B, Digoutte JP, Philippe B, Riou O, Adam F. An RVF epidemic in southern Mauritania. Ann Inst Pasteur Virol 1988; 139:307-8.

(5.) Nasher AAW, Shiban AK, Eriyani MA, Aly Bourgy A, Al Kohlani AH, Benbrake M, et al. Outbreak of Rift Valley fever, Yemen, August-October 2000. Wkly Epidemiol Rec 2000;75:392-5.

(6.) Arishi H, Ageel A, Abdu Rahman M, Al Hazmi A, Arishi AR, Ayoola B, et al. Outbreak of Rift Valley fever, Saudi Arabia, August-November 2000. MMWR Morb Mortal Wkly Rep 2000;49:982-5.