Rift Valley Fever

Unique identifier / Notation

BI0311

Synonyms

Fièvre de la Vallée du Rift (fr) Fiebre del Valle del Rift (es)

Rift Valley fever (RVF) is an acute haemorrhagic viral disease, affecting small and large ruminants and camels. RVF virus is a member of the Phlebovirus genus. The disease causes high mortality, especially in newborns and mass abortions in pregnant animals. Humans become infected from contact with tissues/blood of infected animals including abortive material and through mosquito bites. Disease in humans presents as influenza-like illness, haemorrhagic fever, encephalitis and occasionally death (adapted from FAO, 2003; WHO, 2018; OIE 2020).

Primary reference(s)

FAO, 2003. Recognizing Rift Valley fever. Animal Health Manual No. 17. Food and Agriculture Organization of the United Nations (FAO). Accessed 1 January 2025.

WHO, 2024. Newsroom Fact Sheet Rift Valley fever. World Health Organization (WHO). Accessed 1 January 2025.

WOAH, no date. Rift Valley Fever (RVF). World Organisation for Animal Health (WOAH). Accessed 1 January 2025.

Annotations

Additional scientific description

Human

Rift Valley fever (RVF) is a viral zoonosis that primarily affects animals but also has the capacity to infect humans. Infection can cause severe disease in both animals and humans (WHO, 2018).
Rift Valley fever outbreaks in domesticated animals are often accompanied by human disease. The majority of human infections result from contact with the blood, other body fluids or organs of infected animals. Human infections have also resulted from the bites of infected mosquitoes. To date, no human-to-human transmission of the RVF virus has been documented (WHO, 2018).
Rift Valley fever (RVF) is a viral zoonosis that primarily affects animals but also has the capacity to infect humans. Infection can cause severe disease in both animals and humans (WHO, 2018).
Rift Valley fever outbreaks in domesticated animals are often accompanied by human disease. The majority of human infections result from contact with the blood or organs of infected animals. Human infections have also resulted from the bites of infected mosquitoes. To date, no human-to-human transmission of the RVF virus has been documented (WHO, 2018). RVF is an economically important disease and affects different species of animals and humans. Immunisation and vector control are the main strategies to reduce the incidence of RVF (WHO, 2018; no date).
RVF virus is a member of the Phlebovirus genus. The virus was first identified in 1931 during an investigation into an epidemic among sheep on a farm in the Rift Valley of Kenya (WHO, 2018). Since then, outbreaks have been reported in sub-Saharan Africa. In 1977 an explosive outbreak was reported in Egypt, the RVF virus was introduced to Egypt via infected livestock trade along the Nile irrigation system. In 1997-1998, a major outbreak occurred in Kenya, Somalia and the United Republic of Tanzania following an El Niño event and extensive flooding. After infected livestock trade from the horn of Africa, RVF spread in September 2000 to Saudi Arabia and Yemen, marking the first reported occurrence of the disease outside the African continent and raising concerns that it could extend to other parts of Asia and Europe (WHO, 2018).

Animal

RVF is caused by RVF virus (RVFV), an arbovirus, transmitted by mosquitoes. Outbreaks of RVF in animals can be prevented by a sustained programme of animal vaccination. There are three commercially available vaccines: the inactivated RVF vaccine, produced in South Africa and Egypt, the live attenuated vaccine, based on the Smithburn virus strain, produced in South Africa and Kenya, and the more recently registered RVF Clone 13, produced by Onderstepoort Biological Products (OBP) in South Africa and by MCI Santé Animale in Morocco (Dungu, & Bouloy, 2021). The live attenuated Smithburn RVF vaccine is safe for use in all breeds of cattle, sheep and goats, but it may cause foetal abnormalities or abortion in pregnant animals. Inactivated RVF vaccine is also used in outbreak situations, and in pregnant animals, as the attenuated Smithburn vaccine is not suitable for this group (WOAH, 2024a). Most countries maintain a list of approved veterinary vaccines for use. For specific regulations, consult the country's veterinary authorities.
RVF virus is able to infect many species of animals causing severe disease in domesticated animals including cattle, sheep, camels and goats (WHO, 2024). The disease occurs in climatic conditions favouring the breeding of mosquito vectors and is characterised by abortion, neonatal mortality and liver damage. The disease is most severe in sheep, goats and cattle. Older non-pregnant animals, although susceptible to infection, are more resistant to clinical disease. Camels usually have an inapparent infection with RVF virus (RVFV), but sudden mortality, neonatal mortality and abortion occur, and abortion rates can be as high as in cattle (WOAH, 2024a).
Aedes mosquitoes acquire the virus from feeding on infected animals, and may potentially vertically transmit the virus, so that new generations of infected mosquitoes may hatch from their eggs. This provides a potential mechanism for maintaining the virus in nature, as the eggs of these mosquitoes may survive for periods of up to several years in dry conditions. Once livestock is infected, a wide variety of mosquito species may act as vectors for transmission of RVFV and can spread the disease (WOAH, 2024a). Thus, RVF epidemic/epizootic has been associated with periods of flooding following heavy rainfall especially after a long drought. These floods create pools of water conducive for hatching/emergence of Aedes mosquitoes, which may be infected with the RVFV and can transmit the virus to ruminants or camels when they drink from or feed close to the water pools (FAO, 2003).
RVF is one of the World Organisation for Animal Health (WOAH) listed diseases (WOAH, 2024b)

Metrics and numeric limits

Until 1975, RVF was regarded as an African, animal disease. Human cases were rare and with mild clinical manifestations. Severe outbreaks of haemorrhagic fever cases and fatalities in humans were reported in South Africa in 1975, Egypt in 1977 and Mauritania in 1987. One of the most noticeable outbreaks occurred in East Africa in December 1997, when unexplained human deaths were reported in the northeastern province of Kenya and southern Somalia. This epidemic was considered the most devastating in the region. In September 2000, RVF was detected for the first time outside the African continent, in Saudi Arabia and Yemen, leading to human deaths and major losses in livestock populations. In 2006–07 an outbreak was declared in Kenya. Tanzania and Somalia were affected later. Madagascar and South Africa were hit in 2007 and 2008 (ECDC, no date).

Key relevant UN convention / multilateral treaty

International Health Regulations (2005), 3rd ed. (WHO, 2016).

WTO Agreement on the Application of Sanitary and Phytosanitary Measures (SPS Agreement) (WTO, 1994).

United Nations. 2023. UN Recommendations on the Transport of Dangerous Goods - UN Model Regulations Model Regulations. https://unece.org/transport/dangerous-goods/un-model-regulations-rev-23 Accessed 1 January 2025.

Drivers

Human

Several different species of mosquito can act as vectors for transmission of the RVF virus. The dominant vector species varies between different regions and different species can play different roles in sustaining the transmission of the virus. During major outbreaks in primary foci, the disease can spread to secondary foci through livestock movement or passive mosquito dispersal and amplifies in naïve ruminants via local competent mosquitoes like Culex, Mansonia and Anopheles that act as mechanical vectors. Irrigation schemes, where populations of mosquitoes are abundant during long periods of the year, are highly favourable places for secondary disease transmission (WHO, 2018).

Animal

RVF-infected mosquitoes; flash floods caused by unusual rainfall in an area where RVF occurred in the past; lack of biosecurity in handling aborted foetuses/slaughtered carcasses.

Impacts

Human

The incubation period (the interval from infection to onset of symptoms) for RVF varies from 2 to 6 days. The most common form of the disease in humans is a self-limiting, flu-like illness. Complications in a minority of cases include ocular disease, neurological signs, kidney dysfunction and a life-threatening haemorrhagic syndrome with hepatic dysfunction. Although overall case fatality rates are thought to be low (≤2%), there may be a very large number of cases during some epidemics, resulting in hundreds of serious cases and significant numbers of deaths (Spickler, 2015; WHO, no date).

Animal

Abortion and/or death of infected animals and humans, spread of infection, trade ban.

Multi-hazard context

Bites of infected mosquitoes; flash floods caused by unusual rainfall in an area where RVF occurred in the past; and lack of biosecurity in handling aborted foetuses/slaughtered carcasses are the main drivers in humans. 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. RVF is considered an occupational disease of livestock handlers, dairy farmers, abattoir workers and veterinarians. The direct socio-economic impact of RVF is on livestock producers due to high levels of mortality and morbidity in animals (WHO, 2018; no date).

Risk Management

During an outbreak of RVF, close contact with animals, particularly with their body fluids, either directly or via aerosols, has been identified as the most significant risk factor for RVF virus infection. Raising awareness of the risk factors of RVF infection as well as the protective measures individuals can take to prevent mosquito bites is the only way to reduce human infection and deaths. For animal risk management, FAO recommends vaccination programmes, movement control, improved biosecurity and waterbody management. Restricting or banning the movement of livestock may be effective in slowing the expansion of the virus from infected to uninfected areas. (WHO, 2018).

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. Other ways in which to control the spread of RVF involve control of the vector and protection against their bites. As an example of a coordinated response, in the 2016 Niger outbreak, the World Health Organization (WHO) sent a multisectoral national rapid response team, including members from the Ministry of Health, veterinary services and Centre de Recherche Médicale et Sanitaire (CERMES). The unit was deployed for field investigation on 31 August 2016. In Niger, the WHO Country Office provides technical and financial support for surveillance, outbreak investigation, technical guidelines regarding case definition, case management, shipment of samples, and risk communication. The Food and Agriculture Organization of the United Nations (FAO), the World Organisation for Animal Health (OIE), and the WHO are coordinating animal and human health and providing additional support to Niger for the outbreak response (WHO, 2018).

An inactivated vaccine has been developed for human use. However, this vaccine is not licensed and is not commercially available. It has been used experimentally to protect veterinary and laboratory personnel at high risk of exposure to RVF. Other candidate vaccines are under investigation (WHO, 2018).

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. The association of RVF outbreaks with the heavy rainfall that occurs during the warm phase of the El Niño–Southern Oscillation (ENSO) phenomenon have been documented in East Africa. These findings have enabled the successful development of forecasting models and early warning systems for RVF using satellite images and weather/climate forecasting data. Early warning systems, such as these, could be used to trigger the detection of animal cases at an early stage of an outbreak, enabling authorities to implement measures to avert impending epidemics. Within the framework of the new International Health Regulations (2005), the forecasting and early detection of RVF outbreaks, together with a comprehensive assessment of the risk of diffusion to new areas, are essential to enabling the implementation of effective and timely control measures (WHO, 2018).

Rift Valley fever (RVF) Early Warning Decision Support Tool (RVF DST): The Food and Agriculture Organization of the United Nations (FAO) has developed a web-based Rift Valley fever (RVF) Early Warning Decision Support Tool (RVF DST), which integrates near real-time RVF risk maps with geospatial data, RVF historical and current disease events from EMPRES Global Animal Disease Information System (EMPRES-i) and expert knowledge on RVF eco-epidemiology.

The widening geographic range of disease is associated with climatic variations, with the likely impact of wider dispersal of the virus to new areas of endemicity and future epidemics. Climatic variables including rainfall, temperature, humidity and normalised difference vegetation index (NDVI), livestock production system, and land-use change are influencing this widening range (Situma et al., 2024).

Monitoring

The table below offers an overview of monitoring for Rift Valley Fever. This information can be used for forecasting within a national early warning system (EWS). Since EWS capacities and processes differ across countries, the most current and specific information regarding EWS should be obtained from the appropriate national or regional agency/authority responsible for disaster management.

Which institution(s) produce(s) Disaster Risk Data/Information?

Ministry of Agriculture, Ministry of Livestock, Ministry of Health, FAO Reference Centres, WOAH Reference Centres

How is the Hazard Observed/Monitored/Forecast?

Through its global early warning system, FAO has been supporting Members with risk monitoring, assessment and forecasting for animal health threats to enhance preparedness and response to animal health threats:

The Global Animal Disease Information System, EMPRES-i+, plays a crucial role in supporting countries with early warning and early detection of animal disease outbreaks. By providing a comprehensive platform for real-time disease tracking, EMPRES-i+, enables monitoring and analysis of animal health threats effectively, collaborating closely with countries and regions on disease information sharing. This supports the dissemination of risk assessments and alerts for evidence-based decision-making.
FAO develops and maintains decision support tools that provide risk maps of existing and predicted risks of animal disease emergence, as well as other threats to the agri-food chain.
Additionally, FAO collaborates closely with international organizations like WHO and WOAH to verify disease information through integrated mechanisms such as the Epidemic Intelligence from Open Sources (EIOS), the Global Early Warning System (GLEWS+), and the World Animal Health Information System (WAHIS) for WOAH-listed diseases. These collaborations allow for the dissemination of early warning alerts and risk assessments to Members, to enable timely responses to potential outbreaks.

FAO empres-i+ https://empres-i.apps.fao.org/diseases

Rift Valley fever (RVF) Early Warning Decision Support Tool (RVF DST)

WOAH WAHIS https://wahis.woah.org/#/event-management

WHO supports countries to conduct all-hazards strategic risk assessment in the contexts of health emergencies and disasters, which results in the development of a country risk profile. Empowered with the country's risk profile, inclusive of a seasonal risk calendar, countries can anticipate potential emergencies before they occur to trigger early alerts and inform early actions (WHO, 2021).

WHO's Early Warning, Alert and Response System (EWARS) has been designed to improve disease outbreak detection in emergency settings, such as in countries in conflict or following a natural disaster. It is a simple and cost-effective way to rapidly set up a disease surveillance system. EWARS is deployed during an emergency as an adjunct to the national disease surveillance system. WHO works with Ministries of Health and health sector partners to train local health workers to use the system. After the emergency, EWARSs should re-integrate back into the national system (WHO, no date a).