Rift Valley fever (RVF) is an acute viral hemorrhagic fever that is most commonly seen in domesticated animals (such as cattle, buffalo, sheep, goats, and camels) and can also cause illness in people. The disease is caused by RVF virus (RVFV), a member of the genus Phlebovirus in the order Bunyavirales. Some related Bunyavirales viruses can also cause illness in people, such as hantaviruses and Crimean-Congo hemorrhagic fever (CCHF) virus.

RVF was first reported in livestock by veterinary officers in Kenya’s Rift Valley in the early 1910s. It is generally found in regions of eastern and southern Africa where sheep and cattle are raised, but exists in most of sub-Saharan Africa, including West Africa and Madagascar. In September 2000, an outbreak of RVF was reported in Saudi Arabia. It was then also found in Yemen. These were the first cases of Rift Valley fever identified outside of Africa.

Outbreaks of RVF can have major societal impacts, including significant economic losses and trade reductions. The disease most commonly affects livestock, causing severe illness and abortion in domesticated animals, an important income source for many. Outbreaks of disease in animal populations are called “epizootics.” The most notable RVF epizootic occurred in Kenya in 1950-1951, resulting in the death of an estimated 100,000 sheep.

Epizootic outbreaks of RVF also increase the likelihood of contact between diseased animals and humans, which can lead to outbreaks of RVF in people. For instance, in 1977 RVF was found in Egypt (possibly imported from infected domestic animals from Sudan) and caused a large outbreak among both animals and people that resulted in over 600 human deaths. Another example occurred in West Africa in 1987 and was linked to construction of the Senegal River Project. The project caused flooding in the lower Senegal River area, which changed both ecological conditions and interactions between animals and people, resulting in a large RVF outbreak in both animals and humans.

People usually get Rift Valley fever through contact with blood, body fluids, or tissues of infected animals, mainly livestock such as cattle, sheep, goats, buffalo, and camels. This direct contact can occur during slaughter or butchering, while caring for sick animals, during veterinary procedures like assisting an animal with giving birth, and when consuming raw or undercooked animal products.

People can also get RVF through bites from infected mosquitoes and, rarely, from other biting insects. Infection with the RVF virus (RVFV) has occurred in laboratories when someone has inhaled virus that was in the air (known as aerosol transmission). Spread from person to person has not been documented, and no transmission of RVF to health care workers has been reported when standard infection control precautions have been put in place.

The transmission cycle of RVFV can look like this:

• The virus can be spread from female mosquitos to their offspring through the eggs (vertical transmission).
• In the eggs, the virus remains viable (infectious) for several years during dry conditions.
• Excessive rainfall allows more mosquito eggs to hatch.
• As mosquito populations increase, the potential for the virus to spread to animals and people increases.
• RVFV outbreaks in animals, most commonly livestock, lead to increased handling of infected animals, which then increases risk of exposure to the virus for people.

Several mosquito species can spread RVFV, most commonly the Aedes and Culex mosquitoes, and these vary by region. Environmental conditions, particularly rainfall, are an important risk factor for outbreaks in both animals and people. RVF outbreaks are most often linked to years of unusually heavy rainfall and flooding, because mosquitoes spread the disease and heavy rainfall allows more mosquito eggs to hatch.

Spending time in rural areas and sleeping outdoors at night in regions where outbreaks of RVF occur could be risk factors for exposure to mosquitoes and other insect vectors. People who work with, or butcher/handle raw meat from, potentially-infected animals in RVF-endemic areas (areas where the virus is known to occur) have an increased risk for infection. This could include animal herdsmen and farmers, abattoir (slaughterhouse) workers, veterinarians, and other people who work with animals and animal products. Laboratory workers who may be exposed to the RVF virus may also be at risk.

International travelers increase their chances of exposure to the virus when they visit RVF-endemic locations during periods when sporadic cases or outbreaks are occurring.

RVFV has an incubation period of 2-6 days following exposure to the virus, and can cause several different disease syndromes if symptoms do appear. Most commonly, people with RVF have either no symptoms or a mild illness that includes fever, weakness, back pain, and dizziness at the onset of illness. Typically, patients recover within two days to one week after symptoms start.

However, a small percentage (8-10%) of people infected with RVFV develop much more severe symptoms, including:

• Ocular disease (disease of the eye), which sometimes accompanies the mild symptoms described above. Lesions on the eyes may occur 1-3 weeks after onset of initial symptoms with patients reporting blurred and decreased vision. For many patients, lesions disappear after 10-12 weeks; however, for those with lesions occurring in the macula (the center of the retina), about half will have permanent vision loss.
• Encephalitis, or inflammation of the brain, which can lead to headaches, coma, or seizures. This occurs in less than 1% of patients and presents 1-4 weeks after first symptoms appear. Death from encephalitis in RVF patients is rare, but neurological deficits may be severe and long-lasting.
• Hemorrhagic fever, which occurs in less than 1% of all RVF patients. Symptoms of hemorrhaging may begin with jaundice and other signs of liver impairment, followed by vomiting blood, bloody stool, or bleeding from gums, skin, nose, and injection sites. These symptoms appear 2-4 days after onset of illness. Fatality for those who do develop symptoms of hemorrhagic fever is around 50% and death usually occurs 3-6 days after symptoms start.

RVF causes severe disease in animals that is characterized by fever, weakness, abortions (loss of pregnancy), and a high rate of severe illness and death, particularly among young animals. RVFV infection causes abortion in nearly 100% of livestock pregnancies and most young animals that are infected will die, whereas fatality among adult animals is significantly lower.

Because RVF symptoms can be mild and non-specific, clinical diagnosis is often difficult, especially early in the course of the disease. Definitive diagnosis of RVF requires laboratory testing of blood or other tissue samples. The virus can be detected in the blood (during illness) and in postmortem tissue by virus isolation in cell culture and by molecular techniques (reverse transcriptase polymerase chain reaction, or RT-PCR).

Antibody testing using enzyme-linked immunoassay (ELISA) can also be used to confirm infection with RVFV by showing the presence of IgM antibodies, which appear briefly as an early response to a recent infection, and IgG antibodies, which persist for several years. Both IgM and IgG antibodies are specific to RVF virus.

US-based clinicians should notify local health authorities immediately of any suspected cases of RVF or other viral hemorrhagic fevers occurring in patients residing in the United States. For laboratory testing requests, contact your local or state health department.

There are no FDA-approved treatments for Rift Valley Fever. Because most cases of RVF are mild and self-limiting, a specific treatment for RVF has not been established. Symptoms of mild illness such as fever and body aches can be managed with standard over-the-counter medications. Most of the time, people will get better within 2 days to 1 week after their illness starts. Treatment for more serious cases may require hospitalization and are generally limited to supportive care.

People living in or visiting areas with RVF can prevent infection with these steps:

• Avoid contact with blood, body fluids, or tissues of infected animals. People working with animals in RVF-endemic areas should wear appropriate protective equipment (such as gloves, boots, long sleeves, and a face shield) to avoid any exposure to blood or tissues of animals that may potentially be infected.
• Avoid unsafe animal products. All animal products (including meat, milk, and blood) should be thoroughly cooked before eating or drinking.
• Protect yourself against mosquitoes and other bloodsucking insects. Use insect repellents and bed nets, and wear long sleeved shirts and long pants to cover exposed skin.

No vaccines are currently available for vaccination in people.

A number of questions and challenges remain in the control and prevention of RVF. Scientists are still researching how the virus is maintained and spread within different mosquito species, and the risk factors associated with severe cases of RVF in people. Surveillance (close monitoring for RVF infection in animal and human populations) is essential to learning more about how RVF virus infection is spread and to formulating effective prevention measures. Establishing environmental monitoring and case surveillance systems may help to predict and control future RVF outbreaks.

Animal Vaccination

Different types of vaccines are available for use in animals. Inactivated, or killed, vaccines are not practical in routine animal field vaccination because of the need for multiple doses. A modified live vaccine (the Smithburn vaccine) is one of the oldest and most widely used vaccines for controlling RVF in Africa. This vaccine only requires a single dose, but is known to cause birth defects and abortions in pregnant livestock and may only provide cattle with limited protection from infection with RVF.

Several candidate vaccines are being developed and evaluated. The live-attenuated vaccine, MP-12, has shown promising results in laboratory trials in domesticated animals, but more research is needed before the vaccine can be used in the field. The live-attenuated clone 13 vaccine was recently registered and used in South Africa. Alternative vaccines using molecular recombinant constructs are in development and show promising results.