YELLOW FEVER

acute infectious tropical and subtropical disease, sometimes occurring in the temperate zones. The disease, caused by a virus, infects humans, all species of monkeys, and certain other small mammals. The virus is transmitted among susceptible hosts by several species of mosquitoes. There is considerable uncertainty over the origin of yellow fever. Western Africa has long been regarded as the home of the virus, but the first recorded outbreaks of the disease were in central and coastal South America after the Spanish conquest in the 16th century. For the next 300 years, yellow fever was one of the great plagues of the world. The tropical and subtropical regions of the Americas were subjected to devastating epidemics, and serious outbreaks occurred as far north as Boston and as far away from the endemic centres as Spain, France, England, and Italy. By the late 19th century there were several theories about the cause and transmission of yellow fever, and in 1881 Carlos Juan Finlay of Havana, Cuba, suggested that the infectious agent was transmitted by the mosquito now known as Aedes aegypti. In his investigation of Finlay's theory, Major Walter Reed of the U.S. Army demonstrated in 1900 the transmission of yellow fever from one human to another through the bite of A. aegypti. Reed was further able to show that mosquitoes (rather than bodily contagion, as had previously been thought) were the only vector of the disease. Reed's discoveries were quickly taken up by the American surgeon William Crawford Gorgas, who was able to practically eliminate yellow fever from Havana through the control of the Aedes mosquito. Gorgas' success was repeated in Rio de Janeiro and then in Panama during the building of the Panama Canal. The last outbreak of yellow fever in the United States occurred in 1905, when New Orleans and other ports of the South were invaded. There are two substantially different patterns of transmission of the virus: (1) urban, or classical, yellow fever, in which the transmission is from person to person by the Aedes aegypti mosquito, and (2) jungle yellow fever, in which the transmission is from a mammalian host (usually a monkey) to any one of a number of forest-living species of mosquito to humans. In South America, jungle yellow fever is transmitted by mosquitoes belonging to the genus Haemagogus, while in Africa it is transmitted by the mosquito A. africanus. The distinct transmission cycle involved in jungle yellow fever was first recognized in 1933, after which it became clear that the virus was endemic in huge areas of the Amazon and Orinoco river basins in South America and in the forests of tropical central and western Africa. The course of yellow fever is rapid. After the bite of the infecting mosquito, there is an incubation period of several days while the virus multiplies within the body. The onset of symptoms is then abrupt, with headache, backache, rapidly rising fever, nausea, and vomiting. This stage lasts two or three days, after which the patient either begins to recover or proceeds to a deeper febrile state marked by high fever, slow pulse rate, and the vomiting of dark, altered blood. Death may occur six or seven days after the onset of symptoms. Because the virus destroys liver cells, jaundice (yellowing of the skin and eyes by deposition of bile pigment) is a common symptom in persons with yellow fever and is in fact responsible for the name of the disease. The yellow-fever patient's convalescence is prolonged, but, when recovery does occur, it is complete and is accompanied by a lifelong immunity. The mortality rate of yellow fever varies greatly, depending upon the strain of virus and, to a certain extent, upon the race of the patient. Many persons may experience only a mild infection that lasts a few days. There is no specific treatment for those with yellow fever. Good nursing and supportive care, particularly reduction of fever, are important both in maintaining comfort and in reducing mortality. Yellow fever is an outstanding example of a completely preventable disease. Originally, the control of Aedes aegypti mosquitoes was the only preventive procedure available, as, for example, the campaign against them that made the construction of the Panama Canal possible. Live-virus vaccines, which produce active immunity without clinical illness, are the second great preventive measure. Eradication of the Aedes aegypti mosquito gives effective protection to the populace of cities. Where people must travel or live in regions where the forest cycle of the virus is maintained through host animals, individual immunization is necessary. In these jungle-yellow-fever regions, human cases will continue as long as there remain unimmunized persons, for there is no known practical way of eliminating the virus of yellow fever from the animal and mosquito populations of the vast tropical forests in South America and Africa.