Rift Valley fever (RVF) is an acute arthropod-borne virus infection with a wide range of vertebrate disease hosts. This is a zoonotic disease problem. Until relatively recently, the range of RVF had been confined to the Ethiopian faunal region of Africa, but the disease was identified in Egypt in 1977 and in the Arabian Peninsula in 2000. The virus is a member of the Phlebovirus genus of the Bunyaviridae family. It is an RNA virus, which is related to some other members of the group by haemagglutination or by indirect fluorescent antibody tests at low titres and from which it may be readily distinguished by virus-serum neutralization tests. The virus is transmitted by mosquitoes of at least six genera and probably over 30 different species. It is transmitted transovarially by some of the Aedes spp. of the Neomelaniconium group of mosquitoes. These are floodwater breeding species, which emerge in enormous numbers in floodplains and other habitats where they oviposit.
The disease was first recognized and characterized in the Great Rift Valley in Kenya in 1931 - hence its name - although it may have occurred earlier.
The Rift Valley
Several thousand ewes aborted and there was 90 percent mortality in young lambs. While the problem was first identified in sheep, cattle were also affected, although the abortion rates and mortality in calves were much lower. The affected animals were of breeds that had been imported from Europe or elsewhere into Africa to improve livestock production. Indigenous breeds on adjacent farms were unaffected. A further point of some interest was that humans associated with the affected animals suffered an influenza-like disease with fever, headaches and muscle and joint pains. The investigations carried out during this outbreak showed that the disease was being transmitted by mosquitoes. The movement of livestock to the top of the escarpment above the Rift Valley resulted in a cessation of transmission and no new cases of the disease since the location is at a much greater altitude with few insect vectors present.
Subsequently the disease has been recognized in an enzootic or epizootic form in many tropical and subtropical African countries, and in Madagascar. However, the absence of any livestock disease problems has resulted in few searches for RVF virus activity in many other countries in Africa. In some of these countries, clinical RVF has not been encountered, either in humans or animals, and yet there is evidence of the existence of a cryptic cycle for RVF virus maintenance involving mosquitoes and various vertebrate hosts. Sporadic human cases have sometimes been encountered.
Subsequent RVF epizootics were experienced in South Africa in the 1950s, when huge losses were sustained in the wool sheep populations, and subsequently in most of the neighbouring southern African countries. Epizootic RVF has been seen in the Sudan, at the Gezira irrigation scheme in 1973-74 and in Egypt in 1977-79 and 1993-94, where hundreds of thousands of human cases were reported, with at least 600 deaths. A highly fatal haemorrhagic syndrome was described in Egypt and also an ocular syndrome with macular degeneration, in some cases leading to blindness. There were enormous losses in sheep, goat and cattle populations, with abortion in camels. In Mauritania and Senegal, there was an epizootic associated with the creation of a barrage on the Senegal River in 1987-88. Both humans and animals were affected and, in this instance, neurological signs were detected in humans. In 1997-98, a major epizootic of RVF occurred, which affected countries in the Horn of Africa. There were hundreds of human cases, with some mortality, and abortion storms and neonatal deaths in domestic animals, including camels. This was probably the most dramatic episode of RVF ever encountered.
Saudi Arabia: typical swamp area susceptible to mosquito breeding during the epidemic
In 2000, RVF was recognized in humans and animals in the Arabian Peninsula. Both Yemen and Saudi Arabia simultaneously experienced an epizootic, which principally involved the Tihama region of the country, adjacent to the Red Sea. The Red Sea forms the floor of the Rift Valley in this region and the Tihama is the floor of the Rift in the east with an escarpment running from north to south 40-70 km inland. There were some 100 human fatalities and many thousands of deaths and abortions in domestic animals.
RVF is one of the most significant zoonotic disease problems in Africa. The occurrence of the highly fatal haemorrhagic human disease syndrome, similar to Ebola and other haemorrhagic fevers, generates a degree of panic among the human populations at risk. RVF is highly contagious for humans if animals are viraemic at the time of slaughtering. However, one of RVFs greatest impacts is upon trade in livestock. Even if the disease tends to disappear after epizootics, livestock bans may last for several years, severely affecting the livelihood of pastoralists. Indeed, viraemic animals constitute a serious hazard during epizootic periods and all trade in livestock from the affected countries and their neighbours ceases. In the pastoral regions of eastern Africa, incomes are almost entirely derived from the sale of mature male sheep and goats for the religious festivals in Mecca. Cessation of this trade has had disastrous effects upon the livelihood of highly vulnerable population groups.
Trade of livestock between the Horn of Africa and the Arabian Peninsula (Port of Berbera, northern Somalia)
RVF also has the potential to extend its range to other receptive regions to the north and northeast outside Africa, such as the Tigris/Euphrates Delta zone, which would be receptive for RVF virus transmission. Delta areas such as the Indus in the Indian subcontinent are also at risk. Aerial transport of vectors and increased animal movements facilitate the introduction of the disease.