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Letter
Rift Valley Fever Virus Infection
among French Troops in Chad
Jean Paul Durand,* Michèle Bouloy,† Laurent Richecoeur,‡ Christophe
Nicolas Peyrefitte,* and Hugues Tolou*
*Tropical Medicine Institute of the French Army Medical Corps (IMTSSA),
Marseille, France; †Institut Pasteur, Paris, France; and ‡3ème Régiment
d’Infanterie de Marine (RIMa), Vannes Cedex, France
Suggested citation for this article: Durand JP,
Bouloy M, Richecoeur L, Peyrefitte CN, Tolou H. Rift Valley fever virus
infection among French troops in Chad. Emerg Infect Dis [serial online]
2003 Jun [date cited]. Available from: URL: http://www.cdc.gov/ncidod/EID/vol9no6/02-0647.htm
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 arbovirus 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.u-strasbg.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. Determining
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
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