Rift Valley Fever (RVF)

RVF is a mosquito-borne zoonotic viral disease affecting many animals, especially domestic livestock, and humans. It is most common in eastern and southern Africa. Sheep, in particular exotic, introduced breeds, are most susceptible.

It can cause fever, haemorrhage and abortion in pregnant animals. More than 90% of infected lambs die; mortality of adult sheep is as low as 10%. Humans can contract RVF from mosquitoes and possibly other bloodsucking insects.

RVF epidemics have a serious impact on the economy due to death of livestock, infection of humans and bans on livestock trade from RVF-infected areas.

Rift Valley Fever  - symptoms and prevention

Is an infectious zoonotic disease affecting sheep, goats, and cattle.

Rift Valley Fever (infectious enzootic hepatitis) and humans are susceptible to the  disease.

Rift valley disease is a viral disease of sub-Saharan Africa. The virus attacks the liver and causes symptoms ranging from fevers and listlessness to hemorrhage and abortion rates approaching 100% in pregnant sheep. It is transmitted by mosquitos. RVF is a notifiable disease and it is thus important for farmers to inform the State Vet and Animal Health Technician when they suspect the disease.

Vaccination of animals against RVF has been used to prevent disease in endemic areas and to control epizootics. Rift Valley fever is more deadly than West Nile virus. Animals should be moving away from standing water and moved to higher altitude areas. Caution must be exercised when handling infected tissues by wearing gloves, masks, goggles and by using viricidal disinfectant. The incubation period is 1 – 3 days.

Symptoms in animals:

Abortions, mortalities in young animals, jaundice, lagging behind, weakness and exhaustion, bloody diarrhea, bleeding from the nose and fever.

Symptoms in humans:

Influenza-like symptoms – headaches, muscle pain, joint pain, abdominal pain and nausea.

Prevention

Vaccinate sheep older than 6 months. Inactivated vaccine can be used in pregnant ewes. Animals must be given a booster within 3-4 weeks after initial vaccinations and then must be done annually.

Movement restrictions are recommended. No movement without notifying the Provincial State Vet.The disease is caused by the Rift Valley Fever (RVF) virus, a member of the genus Phlebovirus in the family Bunyaviridae and the disease is transmitted by mosquitoes.Limited to Africa in earlier years, it causes the enormous waste of livestock, especially in wet conditions

For decades Rift Valley fever has caused the illness and death of large numbers of livestock in Kenya and in much of sub-Saharan Africa. Increasing evidence linking the disease to human deaths as well has led epidemiologists to include Rift Valley fever on the list of emerging viruses (including HIV and Ebola) that infect thousands of people each year.

Clinical Signs

In young lambs the incubation period varies from 20 to 72 hours. Some lambs die suddenly without showing signs of this disease. Usually, however, affected lambs develop fever, refuse food, physically weaken, recline and die after a course of 24 hours. Mortality often reaches 95%.

In adult sheep the most common clinical finding is abortion. Most affected sheep show fever of 41 to 42 °C, abortion and vomiting. During fever, severe leukopenia, especially of neutrophils, forms.                

Controlling RVF in animals

•           Outbreaks of RVF in animals can be prevented by a sustained programme of animal vaccination. Both modified live attenuated virus and inactivated virus vaccines have been developed for veterinary use. Only one dose of the live vaccine is required to provide long-term immunity but the vaccine that is currently in use may result in spontaneous abortion if given to pregnant animals. The inactivated virus vaccine does not have this side effect, but multiple doses are required in order to provide protection which may prove problematic in endemic areas.

•           Animal immunization must be implemented prior to an outbreak if an epizootic is to be prevented. Once an outbreak has occurred animal vaccination should NOT be implemented because there is a high risk of intensifying the outbreak. During mass animal vaccination campaigns, animal health workers may, inadvertently, transmit the virus through the use of multi-dose vials and the re-use of needles and syringes. If some of the animals in the herd are already infected and viraemic (although not yet displaying obvious signs of illness), the virus will be transmitted among the herd, and the outbreak will be amplified.

•           Restricting or banning the movement of livestock may be effective in slowing the expansion of the virus from infected to uninfected areas.

•           As outbreaks of RVF in animals precede human cases, the establishment of an active animal health surveillance system to detect new cases is essential in providing early warning for veterinary and human public health authorities.

RVF FORESCASTING AND CLIMATIC MODELS

Forecasting can predict climatic conditions that are frequently associated with an increased risk of outbreaks, and may improve disease control. In Africa, Saudi Arabia and Yemen RVF outbreaks are closely associated with periods of above-average rainfall. The response of vegetation to increased levels of rainfall can be easily measured and monitored by Remote Sensing Satellite Imagery. In addition RVF outbreaks in East Africa are closely associated with the heavy rainfall that occurs during the warm phase of the El Niño/Southern Oscillation (ENSO) phenomenon.