Rift Valley Fever: The Virus That Kills Livestock and Blinds Humans

When unusually heavy rain falls over East Africa, something else often follows. Pregnant sheep begin aborting in large numbers, young lambs die within days, and people who handled the carcasses start running fevers. Rift Valley fever (RVF) is a viral disease of livestock that spills into people through mosquito bites and contact with infected animal blood. Most human cases are mild. A small share cause blindness, brain inflammation, or fatal bleeding. This article explains how the virus persists between outbreaks, why rainfall triggers them, and what is still missing from the toolbox.

Educational content, not medical advice. Talk to a doctor about any symptoms, treatment, or travel plans. Information here reflects published research and public-health agency guidance, but every person's situation differs.

RVF is a useful case study because it sits where climate, animals, insects, and human behavior overlap. Understanding it means following a virus that can wait in dry soil for years, then reappear within weeks of a flood.

What is Rift Valley fever?

Rift Valley fever is caused by Rift Valley fever virus (RVFV), an RNA virus in the genus Phlebovirus. Its genome comes in three pieces, called segments, and a virus with a split genome can mix segments when two strains infect the same cell, a mechanism for generating new variants. For how viruses enter cells and copy themselves, see how viruses work from infection to replication.

The World Health Organization (WHO) says the virus was first identified in 1931 during an epidemic among sheep on a farm in Kenya's Rift Valley, which is where the name comes from (WHO fact sheet, December 20, 2024). Since then, outbreaks have been reported across sub-Saharan Africa and, as of 2000, on the Arabian Peninsula. The WHO also lists RVF among its priority diseases for research and development, a list of illnesses with epidemic potential and too few countermeasures (WHO R&D prioritization).

RVF is classed as a zoonosis, a disease that passes between animals and humans. The reservoir in practice is a combination of livestock, mosquitoes, and possibly wild animals, which is why public-health agencies handle it as a One Health problem involving human, animal, and environmental sectors. For the general mechanics of species-jumping, see how viruses jump from animals to humans.

Two routes of transmission: what makes RVF unusual

Most mosquito-borne viruses reach humans almost exclusively through the bite of an infected insect. RVF travels by two parallel pathways, which makes it harder to predict and harder to contain.

Mosquito-borne transmission

According to WHO, Aedes mosquitoes are the main vector among animals, and they can pass the virus to their offspring through eggs. Other mosquitoes, including Culex, Mansonia, and Anopheles, act as mechanical vectors that carry the virus to new livestock in secondary areas. The World Organisation for Animal Health (WOAH) adds that Aedes eggs can survive dry periods for years (WOAH).

Picture the eggs as seeds lying dormant in a dry pond bed. In this picture, the seeds are already carrying the virus. When flooding refills the pond, many hatch together, and a swarm of infected insects emerges at the same moment. Livestock bitten by them become infected, develop high levels of virus in the blood, and become a source for other biting insects.

Irrigation can create the same conditions without a storm. WHO notes that irrigation schemes are highly favorable for secondary transmission, and the large 1977 outbreak in Egypt spread through livestock trade along the Nile irrigation system.

Direct contact transmission

The second route is the one that most separates RVF from typical arboviruses (viruses carried by arthropods). WHO states that most human infections come from contact with the blood or organs of infected animals during slaughter, butchering, animal births, veterinary procedures, and the disposal of carcasses or fetuses. Herders, farmers, slaughterhouse workers, and veterinarians therefore carry higher risk.

The US Centers for Disease Control and Prevention (CDC) adds that eating or drinking raw or undercooked animal products can spread the virus, and that laboratory workers have been infected accidentally during testing (CDC). WHO notes some evidence of infection through unpasteurized or uncooked milk.

Route	Source	Groups at higher risk
Mosquito bite	Infected Aedes and other mosquitoes	People near flooded pastures or irrigation
Contact with animal blood or tissue	Slaughter, births, necropsies	Herders, farmers, abattoir workers, veterinarians
Raw animal products	Unpasteurized milk, undercooked meat	Consumers in affected regions
Laboratory exposure	Samples and cultures	Laboratory staff
Person to person	Never documented	None established

Both WHO and CDC state that human-to-human transmission has not been documented, and WHO reports no transmission to health workers when standard infection-control precautions were used. That fact has limited the virus's potential for pandemic spread so far, but it depends on the virus staying as it is.

The livestock catastrophe: abortion storms

In animals, RVF is primarily a disease of sheep, cattle, goats, camels, and buffalo. The clinical picture differs sharply from the human one, and it is the animal disease that usually signals an outbreak is underway.

WOAH reports that pregnant sheep and cattle almost always abort, at rates of 80 to 100 percent, and that young lambs and goat kids suffer the highest mortality, at 70 to 100 percent. Sheep and calves beyond the newborn stage are also highly susceptible, with mortality of about 20 to 70 percent (WOAH words this group slightly differently in different sections), while goats, buffalo, and camels are moderately susceptible, with mortality below 10 percent. Some species can be infected without showing clinical signs.

Animal group	Reported impact (WOAH)
Pregnant sheep and cattle	Abortion in 80 to 100 percent
Newborn lambs and kids	Mortality of 70 to 100 percent
Sheep and calves (older animals)	Mortality of about 20 to 70 percent
Goats, buffalo, camels (adults)	Mortality below 10 percent

WHO emphasizes that a wave of unexplained abortions in livestock may signal the start of an epidemic, and that animal outbreaks precede human cases. For that reason, active veterinary surveillance acts as an early warning system for human health. A herder who notices a flock aborting is, in effect, the first epidemiologist on the scene.

The economic consequences fall hard on communities that depend on their herds. Livestock trade bans, which are imposed to slow the spread, compound the loss. WOAH lists RVF as a notifiable disease, meaning clinical cases must be reported immediately upon detection under its Terrestrial Animal Health Code.

Rainfall, El Niño, and the outbreak clock

RVF outbreaks do not strike at random. WHO states that outbreaks in Africa, Saudi Arabia, and Yemen are closely linked to above-average rainfall, and that in East Africa they are tied to heavy rain during the warm phase of the El Niño-Southern Oscillation (ENSO), a pattern of ocean-temperature swings in the Pacific that shifts rainfall worldwide.

The chain runs like a row of dominoes. Warm ocean anomalies bring heavy rain, flooded low-lying ground triggers mosquito hatching, infected mosquitoes bite livestock, livestock abort, and people who handle those animals fall ill. Because each domino can be measured, the sequence is partly predictable.

That predictability has been tested in the field. A 2009 study in PNAS combined satellite data on sea-surface temperature, rainfall, and the normalized difference vegetation index (NDVI, a satellite measure of how green the land is) into an RVF risk model. The model predicted outbreaks in humans and animals in the Horn of Africa from December 2006 to May 2007 with a lead time of 2 to 6 weeks, and field investigations later confirmed the predicted areas. The authors described it as the first prospective prediction of an RVF outbreak (Anyamba et al., PNAS, 2009). WHO says satellite and climate data can be used to trigger early detection of animal cases.

Outbreak history in numbers

The history of RVF is a series of episodes linked to floods, trade, and irrigation. The table below lists the events that agencies describe most consistently.

Year	Place	What happened	Source
1931	Kenya, Rift Valley	Virus first identified during an epidemic among sheep	WHO
1977	Egypt	Explosive outbreak spread through livestock trade along Nile irrigation	WHO
1997 to 1998	Kenya, Somalia, Tanzania	Major outbreak after El Niño flooding	WHO
2000 to 2001	Saudi Arabia and Yemen	First reported occurrence outside Africa	WHO, CDC, Madani et al.
2006 to 2007	Kenya, Somalia, Tanzania	Large outbreak after exceptional rainfall	CDC MMWR, Anyamba et al.

The 2000 outbreak on the Arabian Peninsula

In September 2000, RVF was reported in Saudi Arabia and Yemen. WHO states that it spread through livestock trade from the Horn of Africa, and that this was the first reported occurrence outside Africa. An early report from the CDC's Morbidity and Mortality Weekly Report counted 316 suspected severe cases as of October 9, 2000, all hospitalized, with at least 66 deaths (21 percent). Of those, 304 cases (96 percent) came from Jizan Province and the neighboring Asir and Al Quenfadah health regions, and the cases investigated so far had lived in or visited floodplains of seasonal riverbeds (CDC MMWR, 2000).

The epidemic grew beyond that early snapshot. A later clinical analysis of the Saudi epidemic, which ran from August 26, 2000, to September 22, 2001, reported 886 cases, of which 81.9 percent of those with laboratory results were laboratory-confirmed, and a case fatality of 13.9 percent. Hemorrhagic manifestations occurred in 7.1 percent of patients, neurological manifestations in 17.1 percent, and vision loss or scotomas (blind spots) in 1.5 percent. About four in five patients were male, with a mean age near 47, and bleeding, neurological signs, and jaundice were independently associated with higher mortality (Madani et al., Clinical Infectious Diseases, 2003).

The episode changed how health authorities think about RVF. It showed that the virus could leave Africa when infected animals moved and when the destination had suitable mosquitoes and dense livestock.

The 2006 to 2007 outbreak in East Africa

Exceptional rainfall in late 2006 produced the largest recent outbreak in the Horn of Africa. A CDC report with data as of January 25, 2007, counted 404 cases in Kenya, 115 of them laboratory-confirmed and 109 probable among the rest, with 118 deaths, a case fatality of 29 percent. Cases clustered in Garissa (175), Ijara (125), and Wajir (26) districts of North Eastern Province. The outbreak peaked on December 24, 2006, and rainfall in eastern Kenya from October to December 2006 was about three times the average for that period over the previous eight years (CDC MMWR, 2007).

Those figures are a snapshot taken while the outbreak was still being counted, and they cover Kenya only. The same report noted livestock and human illness consistent with RVF across the border in Somalia, where security concerns had hampered assessment. The PNAS study described the outbreak as spreading from southern Somalia through much of Kenya and then reaching northern Tanzania. Final tallies for the whole region vary by source, so this article uses the dated, sourced snapshot rather than a single regional total.

Human disease: mostly mild, occasionally devastating

Symptoms begin two to six days after exposure, according to both WHO and CDC. Many people have no symptoms or a mild illness with fever, weakness, back pain, muscle and joint pain, and headache, which usually resolves within a week. Early illness is hard to tell apart from malaria, typhoid, and other viral hemorrhagic fevers, so laboratory testing is needed. For another virus where severity is concentrated in a small share of cases, see Ebola virus explained.

How common is severe disease? Sources give different figures, partly because they define "severe" differently. CDC states that 8 to 10 percent of people develop severe symptoms and that about 1 percent of infected people die overall. WHO breaks the severe forms down by syndrome and gives smaller shares for each. A 2026 review of countermeasures reported that 2 to 3 percent of infected humans develop severe disease (PLoS NTD, 2026). The spread across sources is a reminder that case counts depend on who gets tested.

Severe form	Share of patients (WHO)	Timing	Outcome
Ocular (retinal lesions)	0.5 to 2 percent	1 to 3 weeks after symptoms begin	About 50 percent permanent vision loss when the macula is affected
Meningoencephalitis	Less than 1 percent	1 to 4 weeks after symptoms begin	Death rate low; neurological deficits common
Hemorrhagic fever	Less than 1 percent	2 to 4 days after illness begins	Case fatality about 50 percent, usually 3 to 6 days after onset

Hemorrhagic fever

The hemorrhagic form begins early, with severe liver impairment followed by bleeding. WHO puts case fatality at approximately 50 percent, with death usually occurring three to six days after symptom onset. It resembles other viral hemorrhagic fevers in presentation; for a comparison, see Crimean-Congo hemorrhagic fever explained.

Encephalitis

Meningoencephalitis, inflammation of the brain and its coverings, appears one to four weeks after symptoms begin. WHO notes that the death rate is low, but residual neurological deficits are common and complications can appear two months or more later.

Eye disease

Retinal lesions appear one to three weeks after symptoms begin. In many cases the lesions resolve within 10 to 12 weeks, but when the macula (the central part of the retina used for sharp vision) is affected, 50 percent of patients lose vision permanently, according to WHO. CDC similarly states that about half of patients with sores at the center of the retina have permanent vision loss. This ocular complication is the reason the virus appears in the title of this article.

RVF also illustrates a wider pattern: for several viral diseases, the average case is mild while a minority of cases carry the risk. The same logic appears in hantavirus infections, where severity varies sharply between patients.

Why there is no human vaccine yet

WHO states that a human inactivated vaccine has been developed but is not licensed or commercially available; it has been used experimentally for high-risk veterinary and laboratory personnel. CDC says no vaccine for people is currently available and that several vaccines exist for animals.

Candidate human vaccines are now in clinical testing. A first-in-human phase 1 trial of a ChAdOx1 RVF vaccine, built on a non-replicating vector derived from a monkey adenovirus, enrolled 15 healthy adults in Oxford between June 2021 and January 2022. A single dose was safe, well tolerated, and produced neutralizing antibodies in all groups, according to the authors (Lancet Infectious Diseases, 2023). A phase 1 trial in 30 healthy adults in Masaka, Uganda, run from May to September 2022, detected neutralizing antibodies by day 14 in 75 to 90 percent of participants, depending on the dose group (Lancet Infectious Diseases, 2026). Phase 1 trials test safety and immune response, not protection against disease, so neither result means a vaccine is near licensure.

Animal vaccines are further along, and they involve trade-offs. WOAH says vaccination is the main prevention tool in endemic areas, but the timing and location of outbreaks are hard to predict. Live attenuated vaccines give long-lasting immunity but can cause abortion or fetal abnormalities in pregnant animals. Inactivated vaccines need multiple doses. Recombinant vaccines are in development but not yet licensed for endemic countries.

WHO makes one counterintuitive point: a sustained vaccination program must be in place before an outbreak, because starting vaccination once an outbreak has occurred risks intensifying it through multi-dose vials and needle reuse. Vaccines for people or animals work by training the immune system ahead of time; for the human version of that story, see how vaccines actually work.

Treatment and prevention

Most human cases are mild and short, and CDC says over-the-counter medicines can relieve fever and body aches. WHO states that severe cases need early intensive supportive care, including fluid management and treatment of symptoms. No specific antiviral treatment is established, and this article does not suggest any medication.

Prevention targets each route in turn, based on WHO and CDC guidance:

• Animal contact: avoid contact with blood, body fluids, or tissues of potentially infected animals, and wear gloves, boots, long sleeves, and face protection when handling them.
• Food: cook animal products thoroughly, including meat, milk, and blood, in affected regions.
• Mosquitoes: use repellent, bed nets, and protective clothing, and avoid outdoor activity at peak biting times.
• Movement control: restricting or banning livestock movement may slow spread.
• Surveillance: monitor animal health so that abortion waves trigger an early response.
• Vector control: larviciding works only at clearly identified, limited breeding sites and is usually not feasible during flooding.

Diagnosis in practice

Confirmation depends on laboratory methods. WHO lists reverse transcription polymerase chain reaction (RT-PCR, which detects viral RNA), IgG and IgM antibody tests, and virus isolation by cell culture. Unprocessed samples are an extreme biohazard and require maximum containment and triple packaging for transport.

Because early symptoms mimic malaria and other fevers, RVF can be missed where laboratory capacity is limited. That delay matters, because a recognized animal outbreak can prompt protective steps for people before the first human case is confirmed. CDC advises that US clinicians who suspect RVF contact their health department immediately.

Climate, mosquitoes, and where the virus could go

If rainfall drives RVF, shifts in rainfall patterns could change where the virus appears. A 2026 study in Proceedings of the Royal Society B on interepidemic RVF in East Africa estimated that about 49 million people have historically been exposed and projected that more than 90 million could be by 2061 to 2080 under global change, an increase the authors attribute largely to population growth, with disease risk itself projected to decline (Proc R Soc B, 2026). These are model projections that depend on assumptions about climate, land use, and population.

Europe is a different question. A 2026 narrative review of 69 sources concluded that continental Europe has no established RVFV transmission and recommended differential diagnosis of livestock abortions and neonatal deaths, trigger-based sampling, and predefined control decisions (Veterinary Sciences, 2026). Mosquitoes capable of carrying related viruses exist in many regions, but the presence of a vector is only one link in the chain. The virus must also arrive, find susceptible animals, and meet conditions that sustain a cycle.

The virus is also still moving within Africa. Sequencing of livestock samples in Rwanda from 2022 to 2025 indicated sustained local circulation of lineage C rather than repeated introductions (One Health, 2026). For other mosquito-borne diseases tied to warming and water, compare dengue virus and chikungunya virus.

Common misunderstandings

"RVF is a human disease." It is primarily a livestock disease. People are a side branch, and the animal epidemic is the main event.

"Mosquitoes are the only route." Contact with infected animal tissue is a major route for humans, which is why abattoir workers and herders face higher risk.

"It spreads from person to person." Neither WHO nor CDC reports documented person-to-person transmission.

"It always causes severe illness." Most infections are mild or silent. The severe forms are a minority, and the exact share depends on the source and how severity is defined.

"Starting vaccination in the middle of an outbreak is always the answer." WHO warns that vaccinating during an outbreak can intensify it through multi-dose vials and needle reuse, which is why vaccination before an outbreak is stressed.

A mass-gathering scenario as a thought experiment

Public-health planners sometimes use a hypothetical to explain why surveillance matters on the Arabian Peninsula: suppose an animal epidemic were active while large numbers of travelers gathered and animals were slaughtered in a short window. In that scenario, many people would handle animal tissue and then return to countries where few clinicians would think to test for RVF. This is a hypothetical scenario, not a prediction, and the 2000 outbreak is the documented case that gives it plausibility: the virus did cross the Red Sea with livestock trade. What the scenario really argues for is the same list as above, namely animal surveillance, vaccination before outbreaks, and diagnostic capacity.

The takeaway

Rift Valley fever shows how a disease can be shaped as much by weather and trade as by the virus itself. The same flood that fills a dry pond wakes dormant mosquito eggs, and the same livestock that feed a community carry the virus to the people who tend them. Most human cases are mild, but the minority that cause blindness or fatal bleeding make early animal surveillance the most effective warning system available. With human vaccines still in phase 1 testing, the practical defenses remain watching herds, protecting people who handle animals, and using climate forecasts to act before the first abortion wave.

FAQs

Q: Can you get Rift Valley fever from eating meat or drinking milk? A: CDC says eating or drinking raw or undercooked animal products can spread the virus, and WHO notes some evidence of infection from unpasteurized or uncooked milk. Both advise thorough cooking of animal products, including meat, milk, and blood, in affected regions.

Q: Why don't we have a human RVF vaccine yet? A: An inactivated human vaccine has been developed but is not licensed or commercially available, according to WHO. Newer candidates such as ChAdOx1 RVF have completed phase 1 safety and immune-response trials, and larger trials would be needed to show protection.

Q: How is RVF diagnosed in humans? A: WHO lists RT-PCR, IgG and IgM antibody tests, and virus isolation. Early illness resembles malaria, typhoid, and other fevers, so laboratory confirmation is necessary.

Q: Is there a risk of RVF spreading to Europe or North America? A: A 2026 review found no established transmission in continental Europe. The virus would have to be introduced, for example with infected animals, and then find a suitable vector and susceptible hosts. The 2000 outbreak in Saudi Arabia and Yemen shows that movement outside Africa is possible.

Q: What makes abattoir workers high risk? A: WHO identifies slaughter, butchering, and handling of carcasses and fetuses as common routes of human infection, and lists slaughterhouse workers among higher-risk groups. Contact with blood and organs of infected animals is the main hazard.

Q: How long can the virus persist between outbreaks? A: WHO says Aedes mosquitoes can pass the virus to their offspring through eggs, and WOAH says those eggs can survive dry periods for years. That is why outbreaks can appear after heavy rain in places with no recent cases.

Q: How deadly is Rift Valley fever? A: CDC states that about 1 percent of infected people die overall, while WHO puts case fatality for the hemorrhagic form at approximately 50 percent. The 2006 to 2007 Kenyan snapshot reported 118 deaths among 404 cases, a figure that reflects who was tested and hospitalized.

Sources

• World Health Organization. Rift Valley fever fact sheet, December 20, 2024.
• Centers for Disease Control and Prevention. About Rift Valley Fever.
• World Organisation for Animal Health. Rift Valley fever.
• CDC MMWR. Outbreak of Rift Valley fever, Saudi Arabia, August to October 2000.
• CDC MMWR. Rift Valley fever outbreak, Kenya, November 2006 to January 2007.
• Madani TA, et al. Rift Valley fever epidemic in Saudi Arabia. Clinical Infectious Diseases, 2003. academic.oup.com
• Anyamba A, et al. Prediction of a Rift Valley fever outbreak. PNAS, 2009. doi:10.1073/pnas.0806490106
• Safety and immunogenicity of a ChAdOx1 vaccine against Rift Valley fever in UK adults. Lancet Infectious Diseases, 2023. PubMed 37060917
• Safety, tolerability, and immunogenicity of the ChAdOx1 RVF vaccine among healthy adults in Uganda. Lancet Infectious Diseases, 2026. PubMed 41237791

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