YELLOW FEVER, a specific infective tropical fever, the virus of which is transmitted by a mosquito. The disease occurs endemically in certain limited areas. The area of distribution in cludes the West Indies, Mexico, part of Central America, the W. coast of Africa and Brazil.
The incubation period of yellow fever is generally four or five days, but may be as short as twenty-four hours. The illness usually starts like any other febrile attack. But there soon occurs a peculiar look of the eyes and face, which is characteristic : the face is flushed, and the eyes suffused at first and then congested or ferrety, the nostrils and lips red, and the tongue scarlet—these being the most obvious signs of universal congestion of the skin, MUCOUS membranes and organs. There is high fever and albumen will nearly always be found in the urine. After two or three days the temperature falls below the normal; the pulse becomes slow and feeble, the skin cold and of a lemon-yellow tint, the act of vomiting effortless, the first vomit being clear fluid, but afterwards black from admixture of blood. This prostration may end in recovery, with copious flow of urine, which even then is very dark-coloured from the presence of blood or may increase and end in death. Much blood in the vomit and in the stools, to gether with all other haemorrhagic signs, is of evil omen.
The case mortality averages from 12 to 8o%. In Rio in 1898 it reached the appalling height of 94.5%. In cities where it is endemic the case mortality is usually lower.
Modern Researches.—The dreaded "vomito negro" which for four centuries claimed more than 5o% mortality among its victims, has been relegated to a place of secondary importance since the institution, in 1901, of the anti-mosquito campaign. Since 1910 no epidemic invasion of yellow fever into temperate regions has occurred, and some of the most noted endemic centres in Ecuador, Mexico and Brazil have been freed of the disease, probably forever; no case has been reported in the entire Western Hemisphere for many months. The use of oil for destroying mosquito larvae has now been practically abandoned in favour of placing in the tanks which serve as water supply for houses in the tropics one or two small fish, which eagerly devour the "wrigglers." Connor, in Guayaquil, first used this method successfully in 1919, and it has since been used in other countries with excellent results. The fish must be of small size and able to withstand
handling and transportation. Those chiefly used are the minnows, Gambusia affinis, Dormitator latifrons and Fundulus heteroclitus and the common "lefa" of South America (Pygidium piurae C.) some being top-feeding and others bottom-feeding fish.
Bacteriological Investigations.—While the transmitter of the yellow fever germ had been experimentally proved, by the American Army Board under Reed, to be the mosquito, Aedes aegypti (formerly known as Stegomyia fasciata or S. calopus), the microbe which produced the disease had remained unknown until 1918 when a minute spiral organism, subsequently named Leptospira icteroides, was isolated from the blood and organs of yellow fever patients,.in Guayaquil, Ecuador. Inoculation of cultures of this organism reproduced all the characteristic symp toms and lesions of yellow fever in guinea pigs, marmosets and young puppies. Yellow fever was also transmitted directly to guinea pigs with blood taken from yellow fever patients and the same micro-organism from experimentally infected animals.
Investigations previously conducted by the American Army Board in Havana had demonstrated that the germ of yellow fever could pass through the pores of certain bacteria-proof filters (see FILTER-PASSING VIRUSES), and this fact alone had been sufficient to disprove the relation to yellow fever of a dozen or more dif ferent bacteria. Leptospira icteroides, on the other hand, proved to be a filter-passer and to conform with other well-known char acteristics of the yellow fever virus; thermal death-point; trans missibility by Aedes aegypti; ability to produce typical fever, ex tensive haemorrhages into the gastro-intestinal tract, resulting in the "black vomitus" and melaena, severe nephritis, general jaundice and the characteristic changes of liver and kidney (fatty degeneration and necrosis). Another important proof that con nects Leptospira icteroides with yellow fever is that this micro organism is killed by the blood serum of persons who have just recovered from yellow fever but not by that of healthy persons or persons recovering from other diseases.