Yellow Fever.

Yellow fever is an acute viral hemorrhagic disease caused by a flavivirus transmitted by Aedes and Haemagogus mosquitoes in tropical Africa and South America. Most infections are mild or asymptomatic, but about 15% of symptomatic cases develop a toxic phase with jaundice, kidney failure, and bleeding, with 30-60% mortality.

Yellow fever virus is transmitted to humans by bites of infected female mosquitoes. Aedes aegypti drives urban epidemics, while Haemagogus and Sabethes mosquitoes drive sylvatic (jungle) transmission between primates and humans in South America. The disease is not spread directly person-to-person.

Symptoms begin 3-6 days after a bite with sudden fever, headache, muscle and back pain, nausea, and conjunctival injection. About 15% progress after a brief remission to the toxic phase with jaundice, severe vomiting (sometimes black vomit), bleeding, and acute kidney injury, with high mortality.

There is no specific antiviral therapy for yellow fever; treatment is entirely supportive. Most patients recover with rest, fluids, and observation. Severe disease requires intensive care with renal replacement therapy, blood product support, and hepatic supportive care. The most effective intervention is prevention through vaccination.

Yellow fever is endemic in 47 countries in tropical Africa and 13 in Central and South America. Over 90% of cases are reported from Africa, with Nigeria, Democratic Republic of Congo, Ethiopia, and Sudan accounting for the largest burdens. The disease is not found in Asia or Australia despite Aedes aegypti's presence.

Many countries require proof of yellow fever vaccination for entry of travellers arriving from endemic regions. WHO recommends vaccination for all travellers aged 9 months and older going to endemic countries. Vaccinate at least 10 days before travel. Check current country-specific requirements before your trip.

A single dose of 17D yellow fever vaccine produces protective antibody in over 95% of recipients within 10 days and provides lifelong protection per WHO 2013 position. Some special populations (HIV-positive, vaccinated during pregnancy, post-hematopoietic stem-cell transplant) may benefit from booster doses.

Most recipients experience only mild side effects: low-grade fever, headache, or injection-site soreness for a few days. Rare serious events include anaphylaxis (1 per 130,000-250,000) and vaccine-associated viscerotropic disease (1 per 100,000-400,000, mostly in adults over 60). Vaccinate at designated centres with pre-vaccination assessment.

Contraindications include severe egg allergy, severe immunosuppression (advanced HIV, chemotherapy, transplantation), thymic disorders, and infants under 6 months. Pregnancy is a relative contraindication — vaccinate only if travel to high-risk areas cannot be deferred. Adults over 60 require additional pre-vaccination evaluation.

The period of infection lasts 3-4 days. About 85% of symptomatic patients recover at this stage. The 15% who enter the toxic phase have a critical 7-10 day course with high mortality; survivors typically need 2-6 weeks for full convalescence. There is no chronic infection.

The toxic phase is a return of severe illness 24-48 hours after apparent recovery in roughly 15% of symptomatic patients. It features jaundice, severe vomiting (sometimes hematemesis), kidney failure, mucocutaneous bleeding, and hepatic encephalopathy. Mortality is 30-60% even with intensive care.

No, although yellow fever can cause acute viral hepatitis with jaundice and very high transaminases. Yellow fever is caused by a flavivirus transmitted by mosquitoes and is vaccine-preventable. Viral hepatitis (A, B, C, D, E) is caused by different unrelated viruses with different transmission routes and treatments.

No. Yellow fever is caused by a virus, so antibiotics have no direct effect on the disease. Empirical antibiotics are sometimes used to treat secondary bacterial infections during severe yellow fever, but treatment of the underlying viral disease remains supportive.

Diagnosis uses RT-PCR on blood within the first 7-10 days of illness, yellow fever IgM serology from day 5-7 onwards, and confirmatory plaque-reduction neutralization testing for definitive diagnosis. Post-mortem liver histology shows characteristic mid-zonal necrosis with Councilman bodies.

Yellow fever does not spread directly from person to person under normal circumstances. Transmission requires an infected mosquito vector. Rare laboratory-acquired cases and breast-milk transmission to infants have been documented. Standard infection-control measures are still applied to confirmed cases in hospitals.

The name 'yellow fever' refers to the jaundice that develops in the toxic phase, giving the skin and eyes a yellow color. Spanish colonial chroniclers used the term 'fiebre amarilla' from the 17th century. In Brazil and parts of Latin America the disease was historically called 'vomito negro' (black vomit) for its hemorrhagic feature.

Recovery from yellow fever produces lifelong immunity, so reinfection is essentially unheard of. The vaccine likewise produces lifelong protection in over 95% of recipients per WHO 2013 position. There is no chronic carrier state.

Yellow fever remains a significant global health threat. WHO estimates 84,000-170,000 severe cases per year. Recent outbreaks in Angola (2015-2016), DRC (2016), Brazil (2017-2019), and Nigeria (2017-2020) have triggered emergency vaccination campaigns. The WHO EYE strategy aims to protect 1 billion people by 2026.

Seek urgent medical care if you develop fever within 14 days of travel to an endemic region. Tell the clinician about your travel history immediately. Avoid aspirin and other NSAIDs. Do not delay care if you experience jaundice, vomiting, reduced urine output, or any bleeding — toxic-phase disease requires intensive care.

Yes, with appropriate vaccination at least 10 days before travel. Carry the International Certificate of Vaccination or Prophylaxis (yellow card). Use insect repellent, sleep in screened or air-conditioned rooms, and avoid daytime mosquito bites. Consult a travel medicine clinic 4-6 weeks before departure.

Yellow fever vaccine can be given to HIV-positive individuals with CD4 counts above 200 cells/µL and well-controlled HIV. Vaccine response may be weaker than in HIV-negative people, so booster doses are sometimes recommended. Severe immunosuppression (CD4 below 200 or active AIDS-defining illness) is a contraindication.