What is Rift Valley Fever ?
Rift valley Fever (RVF) is an infectious disease caused by an arbovirus of the genus Phlebovirus of the family Bunyaviridae. Primarily spread amongst animals (mainly bites from infected mosquitoes), the RVF virus can infect humans through contact with blood, body fluids or organs during care or slaughter of infected animals. The virus may be transmitted by other biting insects (see Weiss, 1957 ; Meegan and Bailey, 1988 ; IEMVT-CIRAD, 1990 ; Prehaud and Bouloy, 1997 ; Lefevre, 1997 ; Saluzzo et al., 1997 ; Glyn Davies and Martin, 2003 ; Dubois, 2001 ; OMS, 2000 ; Sabatier and al, 2000).
(More informations about arbovirus on Pasteur Institute web site)
Virus : It has a single-stranded RNA genome and its serology defines a very large host range (Meegan and al., 1988 ; Le Guenno). Considering the large number of potential vectors, the RVF virus can put up with various bioclimatic conditions such as wet and tropical areas (Gambia), hot and arid areas (Saudi Arabia, Yemen, Chad) or flooded areas (Nile Delta, Senegal River valley), ... wherever there are vectors (Prehaud and Bouloy, 1997).
Vectors : Many arthropods have been found naturally infected with RVF virus. They belong to different species : Aedes, Neomelaniconion, Ochlerotatus, Stegomyia, Anopheles, Culex, Eunelanomyia, Eretmapodites, Coquillettidia, Mansonia, Mansoniodes, Culicoides, Simulum (Meegan and al., 1988).
In Senegal, Aedes vexans, Aedes dalzieli, Aedes ochraceus and Culex poicilipes (Fontenille and al., 1995 ; Zeller and al., 1997 ; Fontenille and al., 1998 ; Mondet, 1999 ; Diallo and al., 2000 ; Mondet and al., 2005) have been identified as potential vectors. The Aedes species is also a reservoir as the infected female which can transmit the virus to offspring via eggs which can lay dormant for a long time waiting for suitable conditions to hatch.
- Aedes vexans mosquitoe (photo : Bruce Marlin Cirrus Digital Imaging 2003)
Symptoms : There are sometimes no symptoms associated with RVF infection. Clinical signs appear after an incubation period of two to six days. They are feverish symptoms and neurological ones. Feverish symptoms are influenza-like illness, fever, headache, myalgia, backache, ... ; neurological symptoms are liver and kidney disorder, neck stiffness, photophobia and vomiting. Severe cases lead to retinitis with visual impairment or risk of permanent vision loss (1%-10% of convalescents), extreme weight loss, liver necrosis with haemorrhaging or meningoencephalitis. In extreme cases, there are haemorrhages, vascular collapse, followed by shock and death. Otherwise, after four to seven days, the symptoms disappear (and so does viraemia) as the immune response becomes detectable (antibodies IgM and IgG).
RVF animal cases (sheep, goats, cattle, ...) of RVF are associated with up to 100% abortion in pregnant females and a neonatal mortality rate of 80 to 100% among lambs under ten days of age. Sheep are the most affected amongst domestic animals.
RVF-like phenomena has been observed in other animals during RVF <a
href="http://www.websters-online-dictionary.org/definition/english/Ep/Epizootic.html" target="_blank">epizooty . Cases of abortion and neonatal deaths has been noted among camel, without any clinical signs in adult animals. Other animals, mainly wild animals, can develop RVF viraemia without symptoms of infection.
More information about :
href="http://www.cdc.gov/ncidod/dvrd/spb/mnpages/dispages/rvf.htm" target="_blank">CDC website
href="http://www.who.int/csr/don/archive/disease/rift_valley_fever/en/" target="_blank">OMS / WHO website
href="http://www.oie.int/eng/maladies/fiches/a_A080.htm" target="_blank">OIE website