Yellow Fever
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Direct answer
Yellow fever is an acute viral haemorrhagic fever caused by a flavivirus transmitted by Aedes and Haemagogus mosquitoes, and it is endemic in parts of Africa and South America. Its hallmark is a biphasic illness in which a brief infection phase is followed, in some patients, by jaundice, haemorrhage and renal failure. Prevention depends on a single dose of the live attenuated 17D vaccine, while treatment is supportive.
What you must remember
- Causative agent: a flavivirus, an enveloped RNA virus, carried by Aedes aegypti in cities and by Haemagogus in forests.
- Incubation: three to six days, followed by fever, headache, myalgia and nausea in the infection phase.
- Faget sign: relative bradycardia, in which the pulse stays low despite a high fever.
- Toxic phase: jaundice, haemorrhage, hepatorenal failure and shock, carrying a high mortality.
- Pathology: midzonal hepatocellular necrosis with Councilman bodies.
- Vaccine: a live attenuated 17D strain; one dose is considered to give lifelong protection under current WHO recommendations.
- Certificate: valid from 10 days after vaccination under the International Health Regulations, and required for travellers from endemic areas.
- Diagnosis: RT-PCR in the early phase and IgM serology later; there is no specific antiviral treatment.
The course of an infection
A traveller returns from an endemic region with fever and headache, then develops jaundice several days later. The illness follows a predictable sequence, and each stage has a clinical signature that the examiner may ask about.
- Incubation: the virus replicates for three to six days after the mosquito bite, with no symptoms.
- Infection phase: abrupt fever, headache, myalgia, nausea and vomiting last about three to four days.
- Remission: the fever settles for roughly 24 to 48 hours, and many patients feel they are recovering.
- Toxic phase: a minority of patients develop jaundice, bleeding from the gums and nose, albuminuria, renal failure and shock.
- Outcome: survivors usually recover without lasting organ damage, while the toxic phase carries a high mortality.
The phases are best remembered as a sequence rather than a list of symptoms. Clinicians monitor fluid balance, renal function and bleeding closely in the toxic phase, because early recognition of organ failure changes the outcome.
Where the exam tries to catch you
Questions on this topic usually test the clinical sign, the pathology and the rules of vaccination rather than the virus itself.
- Trap: a single vaccine dose is valid for only ten years. Correct: current WHO recommendations consider one dose to give lifelong protection, though the certificate becomes valid 10 days after vaccination.
- Trap: Faget sign is specific to yellow fever. Correct: it is classic for yellow fever but is also seen in typhoid and some other infections, so it is supportive rather than diagnostic.
- Trap: jaundice in yellow fever is caused by haemolysis. Correct: it is hepatocellular, from midzonal necrosis of the liver with Councilman bodies.
- Trap: a specific antiviral cures yellow fever. Correct: no specific antiviral is licensed, so supportive care and prevention by vaccination are the mainstays.
Frequently asked questions
What is Faget sign in yellow fever?
Faget sign is relative bradycardia, in which the pulse rate stays low despite a high fever. It is classically associated with yellow fever. Clinicians also recognise it in typhoid and some other infections, so it is supportive rather than diagnostic.
What are the two phases of yellow fever?
Yellow fever has an acute infection phase with fever, headache and myalgia, followed by a short remission. Some patients then enter the toxic phase, with jaundice, haemorrhage, renal failure and shock. The toxic phase carries a high mortality, so early supportive care matters.
Which mosquitoes transmit yellow fever?
Aedes aegypti transmits urban yellow fever, breeding in water stored around homes. In forested areas, Haemagogus and Sabethes species maintain the sylvatic cycle, mainly in South America. Aedes aegypti also transmits dengue, Zika and chikungunya.
How long after vaccination does a yellow fever certificate become valid?
A yellow fever vaccination certificate becomes valid 10 days after the single dose is given, as required by the International Health Regulations. Current WHO recommendations consider one dose to provide lifelong protection. Travellers arriving from endemic countries may be asked to present the certificate at the border.
What is the characteristic liver finding in yellow fever?
Councilman bodies, which are apoptotic acidophilic hepatocytes, are the characteristic liver finding in yellow fever. The liver shows midzonal necrosis with relatively little inflammation and microvesicular steatosis. Councilman bodies are also seen in other viral hepatitis.