Yellow Fever

Yellow fever is an acute viral haemorrhagic disease caused by the yellow fever virus, transmitted to humans through the bites of infected Aedes and Haemagogus mosquitoes. Nigeria is endemic for yellow fever and has experienced significant outbreaks in recent years, including major epidemics in 2017–2018 that affected multiple states. The disease gets its name from the jaundice (yellowing of the skin and eyes) that affects some patients. While most people recover from initial infection, approximately 15% progress to a severe and often fatal second phase of illness. A highly effective vaccine exists and is the cornerstone of yellow fever prevention. Nigeria requires proof of yellow fever vaccination for international travel.

Yellow fever proceeds in two phases: Acute phase (3–4 days after infection): • Sudden onset fever • Headache, backache, and muscle aches • Nausea and vomiting • Loss of appetite • Chills • Weakness Most people recover at this stage. In approximately 15% of patients, the illness enters the toxic phase after a brief remission of 24 hours: Toxic (haemorrhagic) phase — severe and life-threatening: • Return of high fever • Jaundice: yellowing of the skin and eyes (from liver damage) • Dark yellow or orange urine • Abdominal pain and vomiting • Bleeding from the gums, nose, stomach, eyes, or skin • Vomiting blood or blood in stools • Kidney failure: reduced or absent urine output • Multi-organ failure • Shock and delirium • Death in 20–50% of toxic-phase patients

Yellow fever is caused by the yellow fever virus (a Flavivirus), transmitted exclusively by mosquito bites — it is NOT spread from person to person. Transmission cycles: • Sylvatic (jungle) cycle: infected mosquitoes bite monkeys; humans entering forest areas are incidentally bitten • Urban cycle: Aedes aegypti mosquitoes transmit the virus between infected humans in urban areas: the basis of major urban outbreaks • Intermediate cycle (savannah): transmission between humans and animals; the most common form in Africa The same Aedes aegypti mosquito that transmits dengue and Zika also transmits yellow fever.

• Living in or travelling to yellow fever endemic areas (tropical regions of Nigeria, other West African states) • Unvaccinated status: the most important risk factor for severe disease • Working or visiting forested or rural areas (hunters, forest workers, farmers) • Working outdoors in areas with high mosquito density • Infants and young children: in endemic areas with low vaccination coverage • Men: more frequently exposed in occupational settings

• Hepatitis: liver inflammation and jaundice • Kidney failure • Coagulopathy: abnormal blood clotting leading to severe haemorrhage • Multi-organ failure • Shock and cardiovascular collapse • Death: the toxic phase carries a 20–50% fatality rate; overall case fatality is approximately 5–10% of all clinical cases • Survivors of yellow fever are immune for life and do not need booster vaccination

Vaccination — the only effective preventive measure: • A single dose of yellow fever vaccine provides lifelong protection for over 99% of recipients • Yellow fever vaccine is part of Nigeria's routine childhood immunisation schedule at 9 months • Adults who have never been vaccinated should receive one dose • Yellow fever vaccination is required by the Nigerian government for all travellers departing to and from high-risk countries • The vaccine should be administered at least 10 days before travel to an endemic area Mosquito control: • Use insect repellent (DEET-containing) on exposed skin • Wear long-sleeved clothing, especially at dawn and dusk • Use mosquito nets: particularly important for infants • Eliminate mosquito breeding sites around the home (stagnant water) Surveillance and response: • Rapid vaccination campaigns during outbreaks can contain spread • Report suspected yellow fever cases to health authorities immediately