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Yellow Fever Virus

• 1. Yellow fever

• 1.1. Summary

• Yellow fever was one of the most lethal diseases before the development of the vaccine. It is a major health concern for unvaccinated travellers to tropical regions in South America and Africa. It is transmitted by mosquitoes (Aedes aegypti) bites in a cycle which involve these mosquitoes biting also monkeys and human beings, which act as hosts for the virus. The yellow fever virus is a member of the Flaviviridae family, which comprises about 70 viruses, most of which are arthropod-borne.

• 1.2. Epidemiology

• Up to 5000 cases are reported annually in Africa and 300 annually in South America, although it is believed that numbers are underestimated. In Africa the human population is seasonally exposed in and around villages and small cities so the highest risk of disease are children without naturally acquired immunity. In South America the virus is transmitted in poorly populated forested areas and it occurs mainly with workers and farmers in the borders of the forested areas.

• 1.3. Clinical Manifestations

• Yellow fever can present itself in three forms: subclinical infection, nonspecific abortive febrile disease and fatal hemorrhagic fever. The incubation time for the disease is 3-6 days. After this period, the onset of fever, myalgia, lower back pain, irritability, nausea, malaise, headache, fotofobia and dizziness is oftenly abrupt. These findings are not specific to Yellow Fever and can be found in any acute infection. During this period the patient can be a source of virus for mosquitoes.
• On physical examination the liver can be enlarged with tenderness, Faget sign (slow pulse rate despite high fever) can be found. Skin might appear flushed with reddening of conjunctivae and gums. Between 48-72h after onset and before the jaundice, hepatic enzymes starts to rise. Laboratory studies may show leukopenia with relative neutropenia. This is called period of infection and may last for several days and may be followed by a remission period which last about 48h, with the disappearance of the fever and the symptoms. Patients with the abortive form of the disease recover at this stage.
• After the third to sixth day of the onset of the symptoms the patient may present return of the fever, vomiting, renal failure (oliguria), jaundice, epigastric pain and hemorrhagic diathesis. The viremia terminates during this stage and the antibodies appear in the blood. The patient may evolve with multiorgan failure during this phase. Also in this stage, AST concentrations might exceed ALT, probably due to myocardial and skeletal muscle damage. Serum creatinine and bilirubin levels also rise at this stage. Hemorrhagic manifestations may include petechiae, ecchymoses, epistaxis, melena, metrorrhagia, haematemesis. Laboratory studies may show thrombocytopenia, reduced fibrinogen levels, presence of fibrin split products, reduced factors II, V, VII, VIII, IX and X, which suggest a multifactorial cause for the bleeding with a consumption coagulopathy. Myocardial disfunction may be demonstrated by abnormalities in the ST-T segment in the electrocardiogram. Encephalitis is very rare.
• 20-50% of the patients with the hepatorenal disease die after 7-10 days of the onset.

• 1.4. Diagnosis

• Diagnosis can be made by serology, detection of viral genome by polymerase chain reaction, immunohistochemistry on postmortem tissues, viral isolation or histopathology. No commercial test is available and diagnostic capabilities are restricted to selected laboratories only. Serologic diagnosis is made by dosing IgM antibodies with ELISA. The virus might be isolated by inoculating it in mice, cell cultures or mosquitoes. PCR is generally used to detect viral genome in clinical samples that were negative by virus isolation or other method.

• 1.5. Treatment

• Preferred regimen: No specific treatment is available for yellow fever. In the toxic phase, supportive treatment includes therapies for treating dehydration and fever. Ribavirin has failed in several studies in the monkey model.

• Note: An international seminar held by WHO in 1984 recommended maintenance of nutrition, prevention of hypoglycemia, maintenance of the blood pressure with fluids and vasoactive drugs, prevention of bleeding with fresh-frozen plasma, dialysis if renal failure, correction of metabolic acidosis, administration of cimetidine IV to avoid gastric bleeding and oxygen if needed.

• 1.6. Prevention

• The Yellow fever 17D is highly effective, safe, attenuated vaccine that has been used for over 60 years. It should be taken my travellers who are going to endemic areas of the disease. Revaccination is needed after 10 years from the first dose. The side effects of the vaccines are rare but they include yellow fever associated viscerotropic disease and neurotropic disease. Immunization is contraindicated during pregnancy and in patients with immunodeficiency due to cancers, HIV/AIDS, or treatment with immunosuppressive agents.