Rift Valley Fever: Zoonotic Transmission, Clinical Features, Diagnosis and Control
Clinical Medicine Year 3 • a One Health viral zoonosis affecting people, livestock, mosquitoes and food systems
Why Rift Valley fever matters
Rift Valley fever is a mosquito-borne viral zoonosis that mainly affects livestock but can infect humans. It is clinically important for two reasons: it can cause explosive abortion and neonatal death in sheep, goats and cattle, and it can produce human disease ranging from a short febrile illness to retinitis, meningoencephalitis, haemorrhagic fever, liver failure and death.
In Uganda and other African settings, a cluster of livestock abortions, sudden animal deaths and human fever after animal contact should trigger a One Health investigation. The “animal story” is often the clue that distinguishes RVF from malaria, dengue or ordinary viral hepatitis.
Learning outcomes
- Explain RVF virology, vector ecology, animal reservoirs and human exposure routes.
- Recognise the clinical spectrum from uncomplicated fever to ocular, neurologic and haemorrhagic disease.
- Construct a safe diagnostic and differential-diagnosis plan.
- Describe supportive treatment, infection prevention and outbreak control.
- Apply the One Health relationship between animal vaccination, mosquito control and human protection.
Definition and virology
RVF virus is an enveloped, segmented, negative-sense RNA virus in the genus Phlebovirus. Modern taxonomy places it in the family Phenuiviridae; older teaching materials may describe it under the former family Bunyaviridae. Its segmented genome allows reassortment, and its ecology is closely linked to mosquitoes, rainfall and livestock amplification.
The disease was first recognised in Kenya’s Rift Valley in 1931. It is established in sub-Saharan Africa and has also caused outbreaks in Egypt, the Arabian Peninsula and other regions. The virus primarily circulates between mosquitoes and animals, with humans infected as incidental hosts.
Animal hosts and transmission cycle
| Source/vector | How infection occurs | Clinical implication |
|---|---|---|
| Floodwater Aedes mosquitoes | Eggs can survive dry periods and hatch after heavy rain/flooding | Explains sudden outbreaks after unusual rainfall |
| Other mosquitoes | Anopheles, Culex, Mansonia and other genera can amplify and spread virus | Outbreak control cannot rely on one mosquito species only |
| Infected livestock | Sheep, goats, cattle and camels develop viraemia; pregnant animals may abort | Handling blood, placenta, foetus, meat or milk is a major occupational risk |
| Humans | Usually a dead-end host; infection follows mosquito bites or animal exposure | Human cases can signal an ongoing animal outbreak |
Routes of human infection
- Contact with blood, tissues, organs, placenta, aborted foetuses or vaginal secretions of infected animals.
- Inoculation through cuts, abrasions or contaminated needles.
- Aerosol exposure during slaughtering, necropsy, butchering or laboratory manipulation.
- Consumption of raw or inadequately pasteurised milk and possibly undercooked animal products.
- Bites from infected mosquitoes.
There is no evidence of sustained ordinary person-to-person transmission. Nevertheless, blood and tissues from a viraemic patient may expose healthcare workers, laboratory staff and caregivers, so standard precautions and outbreak-specific PPE are essential.
Incubation and pathogenesis
The human incubation period is commonly about 2–6 days. Virus first replicates at the entry site and lymphoid tissue, followed by viraemia. The liver is a major target, and hepatic necrosis, thrombocytopenia, endothelial injury and inflammatory responses explain fever, jaundice, bleeding and organ failure. In some patients the virus or immune response affects the retina or central nervous system.
- Exposure through mosquito or infected animal material.
- Viraemia and systemic inflammatory response.
- Most patients develop a self-limited influenza-like illness.
- A small minority develop localised ocular disease, meningoencephalitis or haemorrhagic/hepatic disease.
Clinical presentation in humans
1. Uncomplicated febrile illness
Many infections are asymptomatic or mild. Symptomatic patients may develop abrupt fever, severe headache, malaise, weakness, myalgia, arthralgia, backache, nausea, vomiting, photophobia and dizziness. Conjunctival injection and mild hepatitis can occur.
2. Ocular disease
Retinal vasculitis, macular lesions or retinitis may appear after the systemic illness. Blurred vision, scotomata, photophobia, floaters and reduced visual acuity require urgent ophthalmologic assessment. Visual loss may be permanent even when the fever resolves.
3. Meningoencephalitis
Neurologic disease may develop later, with severe headache, neck stiffness, photophobia, confusion, drowsiness, seizures, focal deficits, weakness or coma. Consider RVF in a patient with encephalitis and a compatible livestock/mosquito exposure, while urgently excluding bacterial meningitis, malaria, HSV, TB and other viral infections.
4. Haemorrhagic and hepatic disease
- Jaundice, hepatomegaly and marked transaminase elevation.
- Bleeding from gums, nose, gastrointestinal tract or injection sites.
- Easy bruising, petechiae, haematuria and disseminated intravascular coagulation.
- Shock, renal failure, hypoglycaemia, encephalopathy and multiorgan failure.
Severe-disease alert
Persistent vomiting, jaundice, altered mental state, visual symptoms, seizures, active bleeding, hypotension, oliguria, severe thrombocytopenia or rapidly worsening liver tests require admission and specialist escalation.
Animal clues that support human RVF
During an outbreak, veterinarians may observe sudden large numbers of abortions, high neonatal mortality, fever, weakness, diarrhoea and hepatic necrosis in young livestock. Pregnant ewes and goats can abort at very high rates. These findings are epidemiologically important even if human patients have only mild fever.
History and examination
| History/exam focus | Questions and findings | Why it matters |
|---|---|---|
| Animal contact | Slaughtering, butchering, assisting births, handling abortions, veterinary work, farming | Direct tissue exposure is a major route |
| Animal outbreak | Abortion storms, neonatal deaths, livestock illness or unexplained animal deaths | Strongly raises suspicion and guides public-health response |
| Mosquito/rainfall exposure | Flooding, heavy rains, outdoor work, mosquito abundance | Supports vector-borne acquisition |
| Eyes | Visual loss, floaters, scotoma, photophobia | May indicate retinal disease |
| Neurologic state | Headache, neck stiffness, seizures, weakness, confusion | Identifies meningoencephalitis |
| Bleeding/liver/kidney | Jaundice, bleeding, abdominal pain, urine output, bruising | Identifies severe haemorrhagic/hepatic disease |
Investigations and specimen safety
Routine assessment
- Full blood count and platelets.
- AST, ALT, bilirubin, albumin, glucose, urea, creatinine and electrolytes.
- PT/INR, aPTT, fibrinogen and lactate where available.
- Malaria test, blood cultures and other febrile-illness investigations.
- CSF studies and brain imaging only when safe and when meningitis/encephalitis is suspected.
- Ophthalmologic examination for any visual complaint.
Specific diagnosis
- RT-PCR: detects viral RNA early in blood or tissue and is particularly useful during acute viraemia.
- Antigen detection: immunoassay or other reference-laboratory methods may detect viral antigen.
- Serology: IgM supports recent infection; IgG seroconversion or a fourfold rise in paired samples supports acute infection.
- Virus isolation: reserved for specialised high-containment laboratories because of occupational risk.
- Animal/tissue specimens: blood, liver, spleen, lymph node or aborted foetal tissues may be tested by veterinary reference laboratories. Do not collect or transport these casually.
Notify the laboratory before dispatching specimens. Label and package according to national dangerous-pathogen regulations. A negative result from a late or poorly handled specimen does not automatically exclude RVF.
Differential diagnosis
| Condition | Why it resembles RVF | Useful discriminators |
|---|---|---|
| Malaria | Fever, thrombocytopenia, jaundice, renal failure, coma | Parasite test, treatment response, epidemiology; coinfection possible |
| Yellow fever | Arbovirus with hepatic injury and bleeding | Vaccination/travel/outbreak data and flavivirus testing |
| Dengue | Fever, thrombocytopenia, capillary leak and bleeding | Aedes exposure, NS1/PCR/serology, serial haematocrit pattern |
| Ebola/Marburg/CCHF | Viral haemorrhagic fever with occupational exposure | Outbreak contact, testing and strict VHF pathway |
| Leptospirosis | Fever, jaundice, renal failure and animal/water exposure | Conjunctival suffusion, renal pattern and serology/PCR |
| Brucellosis/Q fever | Livestock exposure and fever | More chronic pattern, focal symptoms and bacterial serology/culture |
Management
Supportive treatment
- ABC assessment, oxygen for hypoxaemia and frequent monitoring of vital signs, urine output, mental state and glucose.
- Careful isotonic fluid treatment for dehydration or shock, reassessing for pulmonary oedema and capillary leak.
- Paracetamol for fever within safe liver-adjusted limits; avoid NSAIDs and aspirin if thrombocytopenia or bleeding is possible.
- Correct hypoglycaemia, electrolyte disturbance, acidosis and hypoxaemia.
- Use blood products for clinically significant haemorrhage or coagulopathy with specialist guidance.
- Manage acute kidney injury with fluid balance, electrolyte treatment and dialysis when indicated.
- Manage encephalitis, seizures, respiratory failure, hepatic failure and shock in a high-dependency or intensive-care setting.
- Retinal disease requires urgent ophthalmology; document visual acuity and fundus findings.
Infection prevention in healthcare
- Use standard precautions for all care and gloves/gowns/eye protection when blood or body fluids may splash.
- Use safe sharps practice, appropriate disinfection, waste handling and specimen packaging.
- Isolate according to the national viral-haemorrhagic-fever risk assessment; avoid unnecessary invasive procedures.
- Trace and assess occupational exposures promptly. There is no need to stigmatise or isolate family members who have had only ordinary social contact, but direct blood/body-fluid exposure requires assessment.
Prevention and One Health control
Protect people
- Wear gloves, boots, eye protection and protective clothing when handling animals, aborted products, carcasses or raw meat.
- Do not slaughter, butcher or consume animals that are sick or found dead without veterinary/public-health clearance.
- Pasteurise milk and cook meat thoroughly.
- Use repellents, long clothing, bed nets and screens; remove standing water and support larval/vector control.
- Report suspected human cases, animal abortion clusters and unusual livestock deaths immediately.
Protect livestock and interrupt amplification
- Veterinary surveillance and rapid laboratory confirmation help identify outbreaks.
- Vaccination of susceptible livestock can reduce amplification, but vaccine choice and timing must follow veterinary authorities; vaccination during active outbreaks requires careful programme planning.
- Discourage animal movement from affected areas to unaffected areas.
- Provide safe abattoir procedures and train farmers, butchers, veterinarians and laboratory staff.
- Use coordinated human, animal and mosquito surveillance rather than treating each case as an isolated fever.
Exam-focused pearls
- RVF is a zoonotic phlebovirus strongly associated with livestock abortions and neonatal deaths.
- Humans acquire infection through mosquito bites or contact with blood, tissues, aborted foetuses, raw milk or meat.
- The incubation period is usually a few days; most disease is mild but ocular, neurologic and haemorrhagic forms are dangerous.
- An animal outbreak can be the earliest warning of human risk.
- Diagnosis requires safe reference-laboratory testing; routine informal handling of specimens is hazardous.
- Prevention is One Health: animal vaccination where appropriate, mosquito control, safe animal handling and rapid reporting.
