Overview of Rift Valley Fever

General Information Rift Valley Fever (RVF) is an infectious zoonotic disease (transmitted from animals to humans) and vector-borne (spread by vectors such as insects) affecting both ruminants and humans. Its causative agent is a virus belonging to the Bunyaviridae family and the Phlebovirus genus. It was first described in Kenya by Daubney in 1931 in the region of Lake Naivasha (Daubney et al. 1931).

Initially, the disease was considered relatively harmless to humans. However, during the epidemic in Egypt in 1977, it was observed that the disease had spread to the regions of major river valleys and, for the first time, had a significant impact on humans. Since then, the disease has been recognized as a zoonosis of real concern, spreading to almost all of sub-Saharan Africa and manifesting in various forms.

It then appeared in the form of epizootic-epidemic in 1987 and 1998, and in Egypt in 1993 and 1997, and in Kenya in 1997-98. Subsequently, Mauritania experienced several epidemic episodes between 2010 and 2022. In 2000, Rift Valley Fever (RVF) was reported for the first time outside the African continent, precisely in Yemen and Saudi Arabia, where it resulted in numerous fatalities. RVF is predominantly present in sub-Saharan African countries and Madagascar, as depicted in the map below.

Conditions of Occurrence

Rift Valley Fever (RVF) epidemics are often associated with risk factors such as hydraulic developments (Egypt 1977, Mauritania 1987), biological adaptations and changes, international traffic and trade, demographics, human behaviors, and heavy rainfall following a period of drought.

Affected Species

The disease affects major domestic species such as sheep, goats, buffaloes, cattle, and camels. These species are considered susceptible, meaning they develop clinical signs of the disease with the virus detectable in laboratory tests. Generally, exotic cattle breeds are more susceptible to infection than local breeds. Other susceptible animal species have been described, including antelopes, Cape buffaloes, monkeys, cats, dogs, and rodents. Similarly, the equine species is also receptive, as the virus was isolated from the blood of horses during the Egyptian epidemic in 1977.

Ecology of Rift Valley Fever and Vector Mosquitoes

The diversity and abundance of potential vector species for transmitting the Rift Valley Fever (RVF) virus vary from one geographical area to another. However, the main genera of identified vectors are Aedes (Ae. vexans) and Culex (Cx. poicilipes). These vectors sometimes share roles in ensuring transmission. The Aedes genus typically plays an initial role by populating puddles and water bodies. Subsequently, the relay is taken over by the Culex genus. During their withdrawal, female Aedes mosquitoes are also capable of transmitting the virus directly to their eggs. The new generations of mosquitoes are then already infected at the time of hatching.

Irrigation networks, where mosquito populations are abundant for a significant part of the year, are particularly conducive to the secondary transmission of the disease.

Sources and Transmission of Infection

Generally, the disease begins with an amplification cycle in livestock. Vector mosquitoes feed on sick animals and then transmit the virus to healthy animals. The massive proliferation of mosquitoes, combined with a high density of livestock, leads to significant amplification of the virus within the livestock. This viral amplification is responsible for waves of abortions in pregnant females. Many mosquito genera, such as Aedes, Anopheles, Culex, Eretmapodites, and Mansonia, are known as confirmed vectors of the Rift Valley Fever (RVF) virus.

Human cases most often result from an epizootic, meaning the spread of the disease among livestock. In the majority of cases, human infection arises from direct or indirect contact with blood or organs of contaminated animals. The virus can be transmitted to humans during the handling of animal tissues during slaughter or butchering, during childbirth and veterinary interventions, or during the disposal of carcasses or fetuses.

Certain professional groups, such as farmers, agricultural workers, slaughterhouse employees, and veterinarians, are therefore more exposed to the risk of infection.

The virus enters humans through inoculation, for example, in the case of an injury with a contaminated knife or contact with damaged skin, or through inhalation of aerosols produced during the slaughter of infected animals. Humans can also be infected by ingesting raw or unpasteurized milk or undercooked meat from infected animals. Human infections have also been observed following mosquito bites, most commonly from the Aedes and Culex genera.

There has never been reported human-to-human transmission when proper hygiene measures are observed.

Clinical Signs of the Disease

The Rift Valley Fever (RVF) virus has an incubation period (the time between infection and the onset of the first symptoms) ranging from 2 to 6 days.

In livestock, the signs are variable. Rift Valley Fever is often associated with waves of abortions, especially in ewes, goats, and cows. In camels, the disease manifests with hemorrhagic signs.

The RVF virus has an incubation period of 2 to 6 days following contamination and can cause several syndromes. People infected with RVF typically either show no symptoms or experience mild discomfort accompanied by fever and liver dysfunction. At the onset of the disease, patients may suffer from fever, generalized fatigue, back pain, and dizziness.

However, the disease can take severe forms such as hemorrhagic fever. Symptoms of hemorrhagic fever occur two to four days after the onset of the disease, initially manifesting with signs of severe liver involvement, such as jaundice. Hemorrhagic phenomena then appear: vomiting blood, blood in the stool, purpura or ecchymoses (resulting from internal skin bleeding), nosebleeds or gum bleeding, menorrhagia, and bleeding at venipuncture sites. The fatality rate for this hemorrhagic syndrome is high, around 50%. Death usually occurs three to six days after the onset of symptoms.

Diagnostic

As the symptoms of Rift Valley Fever are variable and non-specific, clinical diagnosis is often challenging, especially in the early stages of the disease. It is difficult to distinguish RVF from other viral hemorrhagic fevers and numerous other diseases that cause fever.

Confirmation diagnosis is easily achieved in all laboratories equipped with essential facilities and trained technicians, particularly using ELISA and PCR techniques. Blood samples are usually sufficient to establish the diagnosis, especially when taken early:

Sample collection must be done with optimal biosafety and protection precautions. The transport of samples should be secure until reaching the laboratory.

Socio-Economic Impact

Rift Valley Fever (RVF) has a significant impact both economically and in terms of public health.

In animals, the disease leads to a high rate of abortion, reduced production, and mortality in young animals, not to mention the costs associated with prophylactic measures (vaccination, biosecurity measures). Substantial losses due to RVF can reach up to 70% of the livestock. In Southern Africa in 1950, RVF caused 100,000 deaths and 500,000 abortions in sheep. The Egyptian epidemic in 1977 resulted in a high rate of abortions and mortality in the livestock, affecting a million people and causing 2000 clinical cases, including 600 deaths. The 1997/98 epidemic, starting in Kenya and spreading to Somalia and Tanzania, claimed several hundred lives. In Kenya alone, 27,000 infections were reported, resulting in over 200 deaths. The 1987 epidemic in Mauritania affected several thousand people and led to several hundred deaths. In addition to these economic losses and human casualties, other economic repercussions (exceptional health measures, export embargoes, etc.) are also considerable. The 2000 epidemic in Saudi Arabia had 882 confirmed cases with 124 deaths. It also caused 1087 cases, including 121 deaths in Yemen. The epidemics in Mauritania in 2010, 2012, 2020, and 2022 resulted in 13, 19, 27, and 24 deaths respectively, along with hundreds of cases, in addition to high livestock mortality and abortion rates.

Prevention and Control

Outbreaks of Rift Valley Fever (RVF) in animals can be prevented by implementing a sustainable vaccination program. Both a live attenuated vaccine and an inactivated vaccine have been developed for veterinary use. A single dose of the live attenuated vaccine is sufficient to provide long-term immunity, but its administration to pregnant females can cause spontaneous abortion. The inactivated vaccine does not have this side effect, but several doses are needed to achieve the desired protection, which can be challenging in endemic areas.

To prevent an epizootic, animal vaccination should be implemented before an outbreak occurs. Once an outbreak is declared, vaccination should NOT be done, as it could significantly intensify the outbreak.

Restricting or prohibiting the movement of livestock can be an effective means of slowing the spread of the virus from an infected area to unaffected areas.

Health Education and Risk Reduction: One Health Approach

Rift Valley Fever (RVF) is a good example where the One Health approach can be applied to significantly improve the prevention and management of the epidemics it causes. Managing RVF epidemics involves the triad of Public Health, Animal Health, and Environmental Health.

Awareness of risk factors and individual measures for protection against mosquito bites is the only way to decrease the number of infections and deaths in humans.

Public health messages related to risk reduction focus on the following points:

  • Reducing the risk of transmission from animals to humans resulting from unsafe farming and slaughtering practices. This involves following hand hygiene rules and wearing gloves and other appropriate protective equipment when handling sick animals or their tissues, as well as during slaughter.
  • Reducing the risk of transmission from animals to humans resulting from the consumption of raw milk or undercooked meat.

Individual and community protection against mosquito bites: using insecticide-treated mosquito nets and available personal repellents, wearing light-colored clothing (long-sleeved shirts and pants), and avoiding outdoor activities during peak mosquito activity hours.

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