An acute, febrile, mosquito-borne viral disease characterized in severe cases by jaundice, albuminuria, and hemorrhage. Inapparent infections also occur. The agent is a flavivirus, an arbovirus of group B. The virus multiplies in mosquitoes, which remain infectious for life. After the mosquito ingests a virus-containing blood meal, an interval of 12–18 days (called the extrinsic incubation period) is required for it to become infectious. Animal virus Arboviral encephalitides The virus enters the body through a mosquito bite and multiplies in lymph nodes, circulates in the blood, and localizes in the liver, spleen, kidney, bone marrow, and lymph glands. The severity of the disease and the major signs and symptoms which appear depend upon where the virus localizes and how much cell destruction occurs. The incubation period is 3–6 days. At the onset, the individual has fever, chills, headache, and backache, followed by nausea and vomiting. A short period of remission often follows. On about the fourth day, the period of intoxication begins with a slow pulse relative to a high fever and moderate jaundice. In severe cases, there are high levels of protein in the urine, and manifestations of bleeding appear; the vomit may be black with altered blood; and there is an abnormally low number of lymphocytes in the blood. When the disease progresses to the severe stage (black vomit and jaundice), the mortality rate is high. However, the infection may be mild and go unrecognized. Diagnosis is made by isolation of the virus from the serum obtained from an individual as early as possible in the disease, or by the rise in serum antibody. There are two major epidemiological cycles of yellow fever: classical or urban epidemic yellow fever, and sylvan or jungle yellow fever. Urban yellow fever involves person-to-person transmission by Aedes aegypti mosquitoes in the Western Hemisphere and West Africa. This mosquito breeds in the accumulations of water that accompany human settlement. Jungle yellow fever is primarily a disease of monkeys. In South America and Africa, it is transmitted from monkey to monkey by arboreal mosquitoes (Haemagogus and Aedes species) that inhabit the moist forest canopy. The infection in animals ranges from severe to inapparent. Persons who come in contact with these mosquitoes in the forest can become infected. Jungle yellow fever may also occur when an infected monkey visits a human habitation and is bitten by A. aegypti, which then transmits the virus to a human. Vigorous mosquito abatement programs have virtually eliminated urban yellow fever. However, with the speed of modern air travel, the threat of a yellow fever outbreak exists where A. aegypti is present. An excellent attenuated live-virus vaccine is available. |
