Rift Valley fever (RVF) is an acute, fever-causing viral disease that affects domestic animals (such as cattle, buffalo, sheep, goats, and camels) and humans. RVF is most commonly associated with mosquito-borne epidemics during years of unusually heavy rainfall. The disease is caused by the RVF virus, a member of the genus Phlebovirus in the family Bunyaviridae. It is a zoonotic, arthropod-borne viral disease important in domesticated ruminants.
This disease is characterized by high mortality rates in young animals and abortions in pregnant ruminants. Rift Valley fever is endemic in sub-Saharan Africa. Epidemics occur in this region when heavy rainfalls cause infected mosquito eggs to hatch, and large numbers of susceptible animals are present. The most common form of the disease is a self-limiting, flu-like illness; however, ocular disease and rare cases of fatal hemorrhagic fever also occur.
Rift Valley fever can affect many species of animals including sheep, cattle, goats, buffalo, camels, and monkeys, as well as gray squirrels and other rodents. The primary amplifying hosts are sheep and cattle. Viremia without severe disease may be seen in adult cats, dogs, horses and some monkeys.
An epizootic of RVF is generally observed during years in which unusually heavy rainfall and localized flooding occur. The excessive rainfall allows mosquito eggs, usually of the genus Aedes, to hatch. The mosquito eggs are naturally infected with the RVF virus, and the resulting mosquitos’ transfer the virus to the livestock on which they feed.
Once the livestock is infected, other species of mosquitoes can become infected from the animals and can spread the disease.
Once it has been amplified in animals, the RVF virus can also be transmitted by other vectors, including many mosquito species and possibly other biting insects such as ticks and midges.
The virus can be transmitted in utero to the fetus. It has also been found in semen and raw milk.
Both animals and humans theoretically have the potential to introduce Rift Valley fever into new areas by infecting mosquitoes.
The clinical signs vary with the age, species and breed of the animal.
In endemic regions, epidemics of Rift Valley fever can be recognized by high mortality rates in newborn animals and abortions in adults.
Rift Valley fever is usually most severe in young animals. In lambs, a biphasic fever, anorexia and lymphadenopathy may be followed by weakness and death within 36 hours. Hemorrhagic diarrhea or abdominal pain can also be seen. The youngest animals are most severely affected; in neonates, the mortality rate may reach 90% to 100%. Similar symptoms occur in young calves: fever, anorexia and depression are typical, with mortality rates of 10% to 70%.
Abortions are the most characteristic sign in adult sheep and cattle. Other symptoms that may occur in adult sheep include fever, weakness, a mucopurulent nasal discharge (sometimes bloodstained), melena, hemorrhagic or foul-smelling diarrhea, and vomiting.
In adult cattle, fever, anorexia, weakness, excessive salivation, fetid diarrhea and decreased milk production have been reported. Icterus may also be seen, particularly in cattle.
Control and prevention
No specific treatment, other than supportive care, is available.
Vaccines for veterinary use are available, but they can cause birth defects and abortions in sheep and induce only low-level protection in cattle.
Surveillance (close monitoring for RVF infection in animal and human populations) is essential to learning more about how RVF virus infection is transmitted and to formulate effective measures for reducing the number of infections.
Vaccines are generally used to protect animals from Rift Valley fever in endemic regions. During epidemics, vaccination of susceptible animals can prevent amplification of the virus and protect people as well as animals. Attenuated and inactivated Rift Valley fever vaccines are both available. Attenuated vaccines produce better immunity; however, abortions and birth defects can occur in pregnant animals.
Preventative measures include vector controls, movement of stock to higher altitudes, and the confinement of stock in insect-proof stables. These control methods are often impractical, or are ineffective because they are instituted too late. The movement of animals from endemic areas to RVF-free regions can result in epidemics.