yellow_fever

Yellow Fever

Primary Distribution: Most of tropical and subtropical South America and Africa. Although yellow fever (YF) is not found in Asia, the vector mosquito is widespread in that region, hence there is potential for new areas of distribution. YF is a "reemerging" disease with potential for "explosive outbreaks" including in urban areas (Robertson et al; Tomori). Readers are referred to and encouraged to read Tomori (1999), whose article provided much of the material for this work.

Agent and Vector: The virus of yellow fever is an arbovirus (Flavivirus) transmitted by the Aedes aegypti mosquito from one human to another (the urban form of YF) or from monkeys to humans (the jungle or sylvan form of YF).

Incubation: 3-6 days

Clinical Findings and Treatment

Signs and Symptoms: Most patients with YF experience sudden onset flu-like (non-specific) symptoms and the disease is self-limited. In both mild and severe YF, the disease is characterized by two stages with a short intervening remission. The first stage of severe YF has a sudden onset of fever, chills, intense headache, lumbosacral back pain, myalgia, nausea and vomiting, conjunctival injection, dark urine, and bradycardia in relation to fever. A 12-24 hour remission usually ensues, and is characterized by decreased temperature, no headache, and general improvement. Remission is followed by an "intoxication" or hepatorenal stage that features reemergence of generalized symptoms including increased temperature, increased nausea and vomiting, abdominal pain, jaundice, and prostration (see complications below).

Complications: In severe YF jaundice, dehydration, decreased renal function, delirium, and hemorrhagic symptoms occur, with the latter including (depending on severity of illness) hematemesis, melena, metrorrhagia, petechiae, ecchymosis, and bleeding from mucous membranes. Terminal signs include progressive tachycardia, intractable hiccups, and shock.

Laboratory Findings: Albuminuria, leukopenia, abnormal liver function, increased prothrombin times.

Diagnosis: Serologic diagnosis of YF may be accomplished with hemagglutination inhibition (fourfold increase), complement fixation, neutralization antibodies, single radial hemolysis, ELISA or immunofluorescence for IgM antibodies.

Differential Diagnosis: Mild YF resembles many other such febrile illnesses. Early (anicturic) stages of more severe YF resemble malaria (and the infections may co-exist), typhoid fever, rickettsial infections (various louse and flea borne typhuses, Rocky Mountain spotted tick fever, Boutonneuse fever, various tick typhuses, trench fever, Q fever, erlichiosis), other arboviral fevers (various forms of encephalitis, dengue fever, other viral hemorrhagic fevers), and influenza. If hemorrhagic features or hepatorenal dysfunction are present, the differential diagnoses include viral hepatitis, viral hemorrhagic fevers (dengue fever, Lassa fever, Marburg fever, Ebola fever, Crimean-Congo fever, Rift Valley fever, and hemorrhagic fever with renal syndrome), and leptospirosis.

Treatment: Treatment is supportive, i.e, control of fever, vomiting, dehydration, and pain.

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References

Monath, T.P. (1991). Yellow fever. In G.T. Strickland (Ed.), Hunter's Tropical Medicine (7th ed.) (pp. 233-238). Philadelphia: W.B. Saunders Company.

Robertson, S.E., Hull, B.P., Tomori, O., Bele, O., LeDuc, J.W., & Esteves, K. (1996). Yellow fever: A decade of reemegence. Journal of the American Medical Association. 276(14), 1157-1162.

Tomori, O. (1999). Impact of yellow fever on the developing world. Advances in Virus Research. 53, 5-34.

Tsai, T.F. (1998). Yellow fever. Bulletin of the World Health Organization. 76(2), S158.