Yellow Fever

Acute viral hemorrhagic fever caused by yellow fever virus (Flavivirus, same family as dengue/Zika), transmitted by Aedes (urban cycle) and Haemagogus/Sabethes (sylvatic/jungle cycle) mosquitoes. Endemic to tropical regions of sub-Saharan Africa and South America (notably absent from Asia despite presence of competent vectors; reasons not fully understood). Significant public health importance given epidemic potential and high case-fatality rate in severe disease.

Transmission cycles:

• Sylvatic (jungle): between non-human primates and forest mosquitoes, occasional spillover to humans (forest workers/travelers)

• Intermediate: in savannah regions of Africa, mosquitoes transmit between monkeys and humans

• Urban: human-to-human transmission via Aedes aegypti; epidemic potential, particularly concerning in areas with low vaccination coverage

Biphasic illness pattern:

• Period of infection (acute phase, 3-4 days): sudden onset fever, chills, severe headache, myalgia/lumbosacral pain, nausea/vomiting, relative bradycardia despite fever (Faget's sign ; classic but not universally present), conjunctival injection, flushed face

• Period of remission: fever and symptoms improve for 24-48 hours in most cases ;majority recover here without progressing further (mild/self-limiting illness in ~85% of symptomatic cases)

• Period of intoxication (severe disease, ~15% of cases): fever recurs, with multi-organ involvement:

• Jaundice (hepatic involvement; gives the disease its name)

• Hemorrhagic manifestations: epistaxis, gum bleeding, hematemesis ("black vomit"; coffee-ground appearance from upper GI bleeding), melena, petechiae

• Renal impairment: oliguria, progressing to acute kidney injury

• Hypotension, shock

• Encephalopathy, confusion

• Case fatality rate in severe disease: 20-50%

• Serology: IgM detection via ELISA (from ~day 5 of illness), IgG for past infection/immunity — cross-reactivity with other flaviviruses (dengue, Zika) can complicate interpretation, especially in endemic-overlap regions; plaque reduction neutralization test (PRNT) can help distinguish

• PCR/RT-PCR: detects viral RNA, most useful in first few days of illness before antibody response develops

• Viral isolation: reference laboratories, early illness

• Labs supporting diagnosis: leukopenia early, thrombocytopenia, elevated transaminases (can be markedly elevated, AST often higher than ALT), elevated bilirubin, prolonged PT/INR, elevated creatinine in severe disease

• Vaccination and travel/exposure history critical ; unvaccinated status plus travel to/residence in endemic area with compatible illness

Differentials: other viral hemorrhagic fevers (Lassa, Ebola, Marburg, Crimean-Congo), malaria, dengue, leptospirosis, viral hepatitis, typhoid fever ; differentiation often requires specific serology/PCR given overlapping clinical presentation, especially in endemic regions with multiple co-circulating pathogens.

No specific antiviral treatment exists ; management is entirely supportive.

Supportive care:

• Fluid and electrolyte management ; careful monitoring given risk of both dehydration and fluid overload/hemorrhagic complications

• Blood product support: fresh frozen plasma, platelets, packed red cells as needed for significant bleeding/coagulopathy

• Renal support: monitor renal function closely, renal replacement therapy if acute kidney injury develops

• Avoid NSAIDs/aspirin (bleeding risk, and aspirin specifically also carries Reye's syndrome concern in relevant age groups)

• Antipyretics: paracetamol for fever/pain

• Monitor closely for progression to severe disease during the critical period following apparent remission; do not discharge prematurely

• ICU-level supportive care for severe disease: hemodynamic support, correction of coagulopathy, management of hepatic/renal failure

Prevention (primary strategy, given no specific treatment):

• Yellow fever vaccine (live-attenuated, 17D strain): highly effective, single dose provides lifelong immunity per current WHO recommendation (previously boosters advised, now considered unnecessary for most); mandatory for entry to/from certain endemic countries per International Health Regulations (requires International Certificate of Vaccination)

• Contraindications: infants <9 months (age-dependent encephalitis risk), pregnancy/breastfeeding (relative contraindication, risk-benefit assessment if travel to high-risk area unavoidable), severe immunosuppression, severe egg allergy (vaccine egg-based), thymus disorder/thymectomy history

• Rare but serious adverse events: yellow fever vaccine-associated viscerotropic disease (YEL-AVD) and neurotropic disease (YEL-AND) ; very rare, higher risk in elderly and first-time vaccinees

• Vector control: eliminate Aedes breeding sites, insecticide use, personal protection (repellents, protective clothing) especially in endemic/outbreak areas

• Case reporting and outbreak response: yellow fever is subject to International Health Regulations, requires notification; outbreak response includes emergency mass vaccination campaigns and vector control

Public health note: no specific antiviral exists, making prevention through vaccination the cornerstone of yellow fever control ; this should be emphasized in any management discussion given the disease's high severe-case fatality rate and complete absence of curative treatment.