Rift Valley fever has occurred in:
the sub-Saharan region of Africa
the Arabian Peninsula - Yemen and Saudi Arabia
The natural distribution of RVF is throughout the Ethiopian faunal region of sub-Saharan Africa. In Saudi Arabia and Yemen, the ecology of the RVF-affected Tihama regions (eastern Rift Valley zone) is identical with those of the west floor of the Rift Valley in Ethiopia and Eritrea, across the Red Sea. The occurrence of RVF in Egypt in 1977 was thought to result from movement of the infection from RVF enzootic areas in Africa to the south. This may or may not be the case. There are descriptions in the Bible of a plague in Egypt, which clinically appears very similar to RVF.
ECOLOGICAL CONSIDERATIONS - THE WADI BIOTOPE IN THE ARABIAN PENINSULA
The Tihama describes the whole of the coastal plain in the Arabian Peninsula in the west and southwest of Saudi Arabia and Yemen. It consists of low hillocks bordering the north-south mountain chain. Wadis or river valleys occur in the Tihama and constitute the alluvial floodplains of the rivers as they emerge from the mountains. These soils are sands, loamy silts and clays, of very low salt content, with little humus or nitrogen. The first ecozone is of Panicum and Cyperus grasslands, which are seasonally flooded. In many areas, terraced catchments have improved the utilization of water from the rivers at these points. The second and larger zone borders the rivers as they proceed to the sea, and consists of Acacia zizyphispina and Dobera spp., with some grasses and bare earth. The riverine zones are characterized by the presence of Dactyloctenium grasses and are altered in some wadis by lateral canal systems extending north and south of the rivers. The rivers are seasonal in flow, largely fed by the mountain catchment areas, and are dry for much of the year. Towards the sea is a belt of land with a high salt content, planted with Salsola spp.
The Tihama biotope has been the focus of extensive agricultural development over the last 20 to 30 years as greater use is made of the available water resources for cultivation. These changes have a direct impact upon the environment, creating a more extensive habitat for RVF virus mosquito vectors. In Yemen, for example, the largest wadi (Wadi Mawr) covers some 18 000 hectares watered by the canal systems.
The methods used for the utilization of the spate flow in the wadis are very similar in Saudi Arabia and Yemen. Agriculture is practised in the riverine alluvial deposits and surrounding sandy soils. Water flow is directed by channel systems into field units, and new areas are flooded sequentially. Together with the rainfall, this results in many large and small water pools suitable as breeding sites for certain mosquito species.
The changes in the wadi systems, which have been made to ensure more effective use of the available water, are also those that favour the development of more extensive breeding sites for the mosquito species that are believed to be of greatest importance in amplifying and transmitting the RVF virus.
Additional ecozones where primary RVF virus amplification may take place following the emergence of Aedes mosquitoes are in the wet highland plateau grasslands planted with Acacia combretum and allied species. These are found in Thaiz and Ibb Governorates and possibly also in Sadah. The virus may also be expected to occur along the wadi river beds far up into the mountain zones, especially where these broaden out into alluvial plains with pockets of clay soil.
Throughout much of its range, RVF virus infection remains cryptic. Evidence of its existence may be found in sporadic cases of human disease, virus isolation from mosquito pools or serum antibody prevalence to RVF of 2-15 percent or more in domestic animals. Such virus activity takes place with no manifestation of clinical disease in these animals.
Typical ecosystem close to the mountains where RVF was prevalent in Saudi Arabia in 2000
RVF manifests itself as a serious disease problem for livestock production systems in those countries where exotic breeds of sheep, goats and cattle have been imported. Such highly susceptible animals then act as an indicator host system for RVF virus activity, since they show obvious signs of disease. Regions such as East Africa (Kenya, the United Republic of Tanzania and Uganda) and southern Africa (Zambia, Zimbabwe and South Africa) have introduced such bloodlines and have been most severely affected. Many other neighbouring countries have also imported exotic ruminants at different times and have experienced RVF problems.
Epizootic RVF disease also occurs in domestic animals in the arid and semi-arid Sahelian zones in the north and south of the continent; they appear to be more susceptible than those in bushed and wooded grasslands and forest zones. Animals, particularly sheep, goats and camels have been affected in the Sudan, Mauritania and Senegal. They suffer abortion storms but at lower levels than the exotic breeds - usually 5-40 percent abortion rates in small ruminants and low levels or no abortions in cattle. Some neonatal mortality may also occur.
Camels are probably the best animal indicator host system for RVF in the arid and semi-arid zones of Africa. Virtually all pregnant females are likely to abort if there is a high level of RVF virus amplification associated with extensive flooding in the riverine floodplains.
Surveillance for RVF should be established in areas to the north of the existing epizootic range for the disease. Areas considered to be at high risk for the extension of RVF are the Tigris/Euphrates Delta in Iraq/Islamic Republic of Iran and all irrigation project zones in Arabian countries, whether wellhead, oasis or catchment systems for water conservation are utilized. Further east, the riverine delta systems in Pakistan and India may be considered potential extension zones for RVF, although at a lower level of risk than those mentioned above because of their greater distance from the enzootic areas and the prevailing wind currents and trade in animals.