Rift Valley Fever is an acute, or peracute, mosquito- borne viral disease affecting domestic ruminants - cattle, sheep, goats, camels and domestic buffaloes- and man. It occurs mainly in East and Southern Africa and more recently in Saudi Arabia and Yemen. During epidemics the occurrence of numerous abortions, deaths in young animals (and adults) - and acute symptoms in humans tends to be characteristic.

The virus is widely distributed in Africa, but major epidemic episodes in animals and humans are relatively rare, occurring in 5 to 20 year cycles. The virus survives in interepidemic periods in mosquito eggs, laid on vegetation in dambos - which are shallow depressions in forest edges. Only when these are flooded do the eggs hatch. This only occurs when the water table rises following prolonged heavy rain. The eggs then hatch and a new population of infected mosquitoes emerges.

Epidemics in domesticated animals are initiated by the bites of infected mosquitoes. After this infected aerosols generated by virus-infected aborted fluids spread the disease rapidly through the flock or herd. Man is usually infected by the aerosol route by handling infected animals or tissues. Until recently the disease in man was considered to be a non-serious influenza type illness, but a fatal haemorrhagic form has emerged and now RVF in man is considered to be one of the most dangerous diseases known.

The virus may be spread by windborne mosquitoes or by the introduction of viremic animals. There is a remarkable age - related innate resistance to RVF virus - the case mortality in lambs less than 1 week old exceeds 90% whereas the rate in lambs over 1 week drops to 20%. Formerly, apart from abortion, most cases in adult animals were subacute, but latterly a haemorrhagic form of the disease has emerged with rapid death in mature animals, including cattle.

The incubation period in lambs is 12 to 36 hours. A biphasic fever of up to 41C (106F) may develop. Peracute infections occur in newborn lambs which die within hours of infection. Acute reactions occur in older lambs and calves and occasionally in adult sheep. In very severe infection in calves, death may occur in 2 days after infection without their showing any clinical signs.

A haemorrhagic syndrome was observed during the last outbreak in Kenya, affecting adult cattle. In its severe form, calves will develop high fever, and may vomit. Some nasal discharge may also be seen followed by prostration and mortality may reach up to 70%.
In mature animals most, if not all, infected pregnant sheep, cattle and camels abort affected foetuses.

Subacute reactions occur in adult sheep, cattle and camels. There is a low-grade fever, partial inappetence and general weakness. Jaundice is prominent. The main post mortem lesion is focal necrosis of the liver but in young animals the foci coalesce to form a diffuse necrotic lesion. The liver is a bright yellow and studded with subcapsular haemorrhages. It is not enlarged. In adult affected sheep and cattle the focal necrosis of the liver is discrete. In addition to the hepatitis there are haemorrhages in most other organs and tissues.

In humans, there is lack of appetite, nausea, severe headache, joint pains, dizziness and nose bleeding. Encephalitis, retinitis, photophobia, loss of central vision, irritation, stupor and coma can occur. Haemorrhagic RVF is characterised by a very acute febrile illness, accompanied by jaundice. Widespread haemorrhages develop within 2 - 4 days and death usually occurs within another 3 - 6 days. Patients who are immunosupressed or malnourished are at particular risk. Recovery from RVF is followed by lifelong immunity.

Outbreaks of Rift Valley Fever generally occur only after periods of prolonged heavy rain. Such being the case livestock owners should be aware that if long rains are excessively heavy and continue for an excessively long period of time, an outbreak of Rift Valley Fever is likely to occur and action in the form of vaccination is required.

Control of vectors, movement of stock to higher altitudes and confinement of stock to insect-proof stables are usually not practical, instituted too late and of little value. Immunisation remains the only effective way to protect livestock.

Vaccination of animals with suitable vaccines should be practiced. Pregnant cows should be vaccinated with killed vaccines to avoid the risk of abortion while humans at particular risk should be vaccinated with formalin killed tissue culture vaccine.

There is no known medical treatment of Rift Valley Fever