Yellow Fever

Organism The yellow fever virus is a Flavivirus.

Clinical features One of the haemorrhagic group of arbovirus infections, yellow fever presents with a sudden onset of fever, headache, backache, prostration and vomiting. Jaundice commences mildly at first and intensifies as the disease progresses. Albuminuria and leucopenia are found on examination, while haemorrhagic symptoms of epistaxis, haematemesis, melena and bleeding from the gums can all occur. In endemic areas, the fatality rate is low except in the non-indigenous areas. The death rate may reach 50% during epidemics.

Diagnosis is made on clinical grounds after initial identification of an outbreak. Virus can be isolated from the blood in specialist laboratories. Specific IgM in early sera or a rise in titre in paired serum can be of value, but cross-reactions can occur with other fla-viviruses.

Transmission Yellow fever is a disease of the forest, maintained in the monkey population by Haemagogus, Sabethes and Aedes mosquitoes in America, and Aedes in Africa. The monkeys are generally not affected by the disease, but occasionally start dying, indicating that spread to the human population may soon begin. In South America, it may be a reduction in the monkey population that will make the canopy mosquito look for another blood meal and perhaps feed on humans. More commonly, it is the person who goes into the forest to cut wood or hunt and is bitten incidentally. When they return to their village or town, they are fed on by Ae. aegypti and an urban yellow fever transmission cycle is set up (see Fig. 15.3). In Africa, three different kinds of mosquitoes are involved. Ae. africanus remains in the jungle canopy rarely feeding on humans, but should the monkey descend to the forest floor or even enter areas of human habitation, it is fed on by Ae. simpsoni, Ae. furcifer-taylori or Ae. luteocephalus. The mosquito then bites a person on the edge of the forest, who returns to the village soon to suffer from yellow fever. Fed upon by the peri-domestic mosquito Ae. aegypti, an urban cycle is started (Fig. 15.3). The extrinsic cycle of infection takes 5-30 days in the mosquito depending on temperature and type of mosquito. Trans-ovarian infection can also occur.

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