Rift Valley Fever (RVF)

Rift Valley Fever (RVF) is a viral zoonotic disease primarily affecting domestic animals such as cattle, sheep, goats, and camels, and occasionally infecting humans. The disease is caused by the Rift Valley fever virus (RVFV), a member of the genus Phlebovirus within the family Phenuiviridae. It is transmitted mainly through mosquito bites and contact with the blood or organs of infected animals. RVF has significant implications for public health, livestock production, and international trade in affected regions, particularly in Africa and the Arabian Peninsula.

Historical Background and Discovery

Rift Valley Fever was first identified in 1931 during an outbreak among sheep on a farm in Kenya’s Rift Valley. The virus was isolated and described following high mortality among lambs and evidence of febrile illness in humans exposed to infected animals. Since then, numerous outbreaks have occurred across sub-Saharan Africa, Egypt, and parts of the Arabian Peninsula, often linked to heavy rainfall and flooding, which create ideal conditions for mosquito breeding.
Major epidemics have been recorded in countries such as Egypt (1977–1978), Kenya (1997–1998), Saudi Arabia and Yemen (2000–2001), and Sudan (2007–2008). These events highlighted RVF’s ability to spread beyond its traditional ecological range and underscored the importance of environmental monitoring and cross-border disease surveillance.

Causative Agent and Transmission

The Rift Valley fever virus is an RNA virus transmitted primarily by mosquitoes of the Aedes and Culex genera. These mosquitoes act both as vectors and reservoirs; Aedes mosquitoes can transmit the virus vertically through their eggs, allowing it to survive long dry seasons.
Modes of transmission include:

• Mosquito bites: The most common means of spread to animals and humans.
• Direct contact: Exposure to the blood, bodily fluids, or tissues of infected animals during slaughtering, birthing, or veterinary procedures.
• Aerosol transmission: Inhalation of infectious droplets, particularly in laboratory or abattoir settings.
• Consumption of unpasteurised milk or undercooked meat: Rare but possible route of infection.

Human-to-human transmission has not been documented, distinguishing RVF from many other viral haemorrhagic fevers.

Epidemiology and Geographic Distribution

RVF is endemic in many regions of sub-Saharan Africa, including Kenya, Tanzania, South Africa, Sudan, and Madagascar. Periodic outbreaks are closely associated with periods of heavy rainfall and flooding that promote mosquito population surges. Environmental and climatic factors such as the El Niño–Southern Oscillation (ENSO) phenomenon have been linked to major epidemics.
The disease spread beyond Africa for the first time in 2000 when outbreaks occurred in Saudi Arabia and Yemen. Concerns persist about the potential for RVF to spread further due to increased livestock trade, human movement, and changing climatic conditions.

Clinical Features in Animals

RVF causes severe disease in livestock, particularly in sheep, goats, and cattle. The severity of illness varies with species, age, and virus strain.
Common clinical signs include:

• High fever and weakness.
• High rates of abortion in pregnant animals (often termed “abortion storms”).
• Death of newborn animals, with mortality rates reaching up to 90% among young lambs.
• Jaundice and haemorrhages in severe cases.

Economic losses arise from animal deaths, trade restrictions, and reduced productivity, making RVF a disease of major veterinary importance.

Clinical Features in Humans

In humans, RVF infection presents with a wide range of symptoms. Most cases are mild and self-limiting, but severe forms can occur.
Typical symptoms:

• Fever, muscle pain, joint pain, and headache.
• Nausea and general malaise lasting several days.

Severe complications (in <2% of cases):

• Ocular disease: Retinitis, which may cause temporary or permanent blindness.
• Encephalitis: Inflammation of the brain leading to confusion, seizures, or coma.
• Haemorrhagic fever: Bleeding disorders, jaundice, and organ failure, often with a high fatality rate.

The incubation period in humans is generally two to six days after exposure.

Diagnosis and Laboratory Detection

Diagnosis of RVF relies on laboratory testing due to the similarity of its symptoms with other febrile illnesses. Diagnostic methods include:

• Serological tests: Detection of specific IgM and IgG antibodies using enzyme-linked immunosorbent assays (ELISA).
• Virus isolation: Cultivation of the virus in specialised laboratories under biosafety conditions.
• Molecular techniques: Reverse transcription polymerase chain reaction (RT-PCR) for detecting viral RNA in blood or tissue samples.

Laboratory handling requires biosafety level 3 (BSL-3) containment due to the risk of aerosol infection.

Prevention and Control

Prevention of RVF focuses on breaking the transmission cycle between mosquitoes, animals, and humans.
Key control measures include:

• Vector control: Use of insecticides, removal of mosquito breeding sites, and use of protective clothing.
• Animal vaccination: Live attenuated and inactivated vaccines are available for livestock, though their use is regulated to avoid risks during pregnancy or in uninfected regions.
• Public awareness: Educating farmers, abattoir workers, and veterinarians about safe handling of animal products and carcasses.
• Surveillance and early warning: Monitoring rainfall patterns, mosquito populations, and livestock health to predict outbreaks.

No specific antiviral treatment exists for RVF. Human cases are managed through supportive care, including rehydration and treatment of complications.

Economic and Public Health Impact

RVF outbreaks result in heavy economic losses due to livestock deaths, abortions, and restrictions on meat and livestock trade. In addition, human infections cause significant healthcare burdens in rural and pastoral communities. The disease can also affect tourism and export markets, making it a priority disease under the World Organisation for Animal Health (WOAH) and the World Health Organization (WHO) joint frameworks for zoonotic disease control.

Research and Future Prospects

Scientific research into RVF focuses on developing safer vaccines for both humans and animals, improving diagnostic techniques, and enhancing surveillance using satellite-based climate prediction models. Understanding the ecological relationships between mosquitoes, livestock, and humans is essential for anticipating future outbreaks in the context of global climate change.