5. In one instance it was thought that the progeny of infected mosquitoes transmitted the disease. Rosenau and Goldberger, in 38 experiments, failed to obtain such result.
Paraplasma flavigenum.—In 1909, Seidelin reported certain minute protozoa as existing in the red cells of yellow fever patients. He considered them as related to the piroplasms and gave the name Paraplasma flavigenum. It is stated that the parasite has been found as late as the fourteenth day from the onset of the attack. The idea is advanced that there may be carriers of yellow fever. These claims are generally denied.
Epidemiology.—There are numerous records which attest the almost universal susceptibility to yellow fever. In the Orwood epidemic, Carter has reported that of 46 persons entering an infected house, “Gray Mansion,” 45 contracted yellow fever.
On the “Lombardia,” with a complement of 249, there were 242 cases and 134 deaths. The 7, who escaped, were immunes.
The idea that the colored race possesses immunity is now thought to be connected with the contraction of the disease in infancy or childhood, attacks at this period of life being very mild and difficult of diagnosis.
Immunity.—As proof that such immunity is not racial we may note that in Ecuador the natives of the endemic area about Guayaquil possess an immunity due to mild attacks in childhood, but the natives of Quito, 300 miles distant, where there is no yellow fever, do not possess it, and many residents of Quito have contracted the disease when passing through Guayaquil to take steamer to Europe.
There has been some inclination to question the immunity conferred by an attack of yellow fever but Carter has shown that in quarantine practice we can admit such immunes with perfect safety. Thirty thousand such immunes were allowed to enter Key West and Tampa from Havana between 1888 and 1898 and no case of yellow fever developed from them. During the same period 450 non-immunes from Havana gave 13 cases in the quarantine stations.
The Yellow Fever Mosquito.—A knowledge of the life history of Stegomyia calopus explains the epidemiology of yellow fever. This culicine species is widely distributed in the tropical and subtropical world, extending from 38° north to 38° south latitude. It is rarely found at a greater altitude than 3000 feet. Petropolis, a railway-connected suburb of Rio, has an altitude of 2300 feet, with cool nights, at times about 9°C, and a freedom from Stegomyia. Persons having occupation in Rio during the day but returning to Petropolis in the afternoon escape yellow fever. In this connection it is generally accepted that the female Stegomyia only bites between 5 o’clock P.M. and midnight. While the first feeding may occur earlier in the day, all subsequent feedings, which alone could be infectious, occur late in the afternoon or at night. (Recent observations show Stegomyia to bite in daytime,—not at night.)
These views, which were advanced by Marchoux, would explain the apparent freedom from infection of those leaving infected areas by the early afternoon. Seidelin, however, claims that these mosquitoes will continue to bite in the day after numerous feedings of blood.
It is recognized that railways are unimportant factors in transporting these mosquitoes, differing in this respect from ships which offer better conditions.