Rift Valley fever

What is Rift Valley Fever (RVF)?

Rift Valley fever (RVF) is an acute arthropod-borne viral disease that can cause severe disease in domestic animals, such as buffalo, camels, cattle, goats, and sheep.

Rift Valley fever is also an important zoonosis that can cause severe disease in humans.

Disease in susceptible animals can vary in severity and is characterized by fever, listlessness, anorexia, disinclination to move, abortions, and high morbidity and mortality rates in neonatal animals.

The virus which causes RVF belongs to the genus Phlebovirus, order of Bunyavirales, family Phenuiviridae.

RVF is a WOAH-listed disease and must be reported to the WOAH as indicated in the WOAH Terrestrial Animal Health Code Chapters 1.1 and 8.15.  Because of its long inter-epizootic intervals, it is also regarded as a re-emerging disease.

Vector transmission and spread

Several different species of mosquitoes are competent vectors for the RVF virus, and rises in incidence of RVF most commonly occur after periods of heavy rainfall which lead to an abundance of mosquitoes.

Competent mosquito vectors transmit virus from infected animals to other animals (and to humans).

Some species of mosquito (Aedes, for example) are capable of transmitting the virus from infected female mosquitoes to offspring via their eggs. This contributes to the survival of the virus in the environment. Mosquito eggs may survive during prolonged periods (up to several years) in dry conditions. During periods of high rainfall and/or flooding the eggs hatch and there is an increase in the infection of animals on which these mosquitoes feed.

Public health risk (see WHO for further information)

Rift Valley fever (RVF) is a viral zoonosis that primarily affects animals but also has the capacity to infect humans.

The majority of human infections result from direct or indirect contact with the blood or organs of infected animals. The virus can be transmitted to humans through the handling of animal tissue during slaughtering or butchering, assisting with animal births, conducting veterinary procedures, or from the disposal of carcasses or foetuses. Certain occupational groups such as herders, farmers, slaughterhouse workers, and veterinarians are therefore at higher risk of infection.

The virus infects humans through inoculation, for example via a wound from a virus-contaminated knife or contact with broken skin, or inhalation of aerosols produced during the slaughter of infected animals.

• Human infections have also resulted from the bites of infected mosquitoes, most commonly the Aedes and Culex mosquitoes.
• To date, no human-to-human transmission of RVF has been documented, and no transmission of RVF to health care workers has been reported when standard infection control precautions have been put in place.
• There has been no evidence of human outbreaks of RVF in urban areas.

For more information on public health risk and clinical signs in humans http://www.who.int/news-room/fact-sheets/detail/rift-valley-fever

Clinical signs in animals

Clinical signs vary depending on the species of animal affected, age, and whether the animal is pregnant. 

Young lambs and goats (kids) are considered to be the most susceptible with mortality rates of 70-100%. Sheep and calves are highly susceptible with mortality rates of 20%-70%. Moderately susceptible species (with mortality rates of less than 10%) include goats, African buffalo, domestic buffalo, Asian monkeys and humans. Other species can be infected but are resistant and do not show any clinical signs.

Signs of the disease tend to be non-specific, rendering it difficult to recognise individual cases during epizootics. The occurrence of numerous abortions and mortalities among young animals, together with disease in humans, is characteristic. Pregnant sheep and cattle affected by this disease will almost always abort (80-100%).

Diagnostics

In areas where the disease is known to occur, RVF may be suspected based on clinical signs, insect activity, concurrent disease in animals and humans, rapid spread of the disease and concurrent contributing environmental factors. Laboratory tests are required to confirm Rift Valley fever infections (WOAH Manual of Diagnostic Tests and Vaccines for Terrestrial Animals).

Prevention and control

RVF is a regional problem and cooperation between neighbouring countries is essential for surveillance, prevention and control.

Essential elements for prevention and control of RVF include systematic ongoing surveillance in sentinel animals to monitor RVF infections in susceptible animals; immediate notification of clinical cases upon detection (in line with WOAH reporting requirements); and implementation of sanitary measures to prevent spread (as described in the WOAH Terrestrial Animal Health Code Chapter 8.15.).

Coordinated sharing of information between animal health and public health services is essential for effective implementation of public and animal health interventions.

Controlling the vector (mosquito) population through spraying and management of mosquito breeding grounds has also been effective, especially during heavy flooding. However, its effectiveness remains limited because of the high cost and geographical extent of the endemic area.

Systems to monitor variations in climatic conditions (often using remotely sensed data) may sometimes provide advance warning of conditions that favour the flourishing of mosquito populations and signal the need to implement enhanced control measures.

Vaccination is the primary option available for prevention of RVF infections in animals in areas where the disease is endemic, but this is hampered by uncertainties on when and where outbreaks are most likely to occur and the time it may take to produce the vaccines. Therefore, public education, livestock quarantine, and slaughter bans are perhaps the most effective measures against disease spread during the pre-outbreak and outbreak phases.

There are several types of vaccines for RVF and they can be broadly classified as follows:

• Conventional vaccines: These are produced by standard methods (non-recombinant DNA techniques). They include inactivated and live attenuated vaccines. These vaccines have been used extensively in controlling RVF but lack important properties such as the ability to differentiate infected from vaccinated animals (DIVA). Live attenuated vaccines can produce long lived immunity but they carry a certain level of risk since they are associated with increased risk of abortion and carry the risk of re-assortment with wild virus strains. Inactivated vaccines require multiple doses to maintain immunity in vaccinated animals.
• Novel vaccines: The formulations are based on recombinant DNA techniques. A number of recombinant vaccines are under development but are not yet licensed for use in endemic countries.

Geographical distribution

For the most recent information on the reported distribution of Rift Valley fever in animals visit the WOAH’s World Animal Health Information Database.

To date, RVF is endemic in many sub-Saharan African countries (including Madagascar) and may involve several countries in the region at the same time or progressively expand geographically over the course of a few years. In addition to sub-Saharan Africa, large outbreaks have been observed in Egypt, the Arabian Peninsula (South-west), and in some Indian Ocean Islands.

Regional activities in Africa

• Rift Valley fever: new options for trade, prevention, and control (2015)
• Risk-based decision-support framework for prevention and control of Rift Valley fever epidemics in Eastern Africa (ILRI, 2015)

References

• The Center for Food Security and Public Health, Iowa State University
• Merck Veterinary Manual
• Rift Valley fever surveillance (FAO, 2018)