Autopsies of the guinea pigs showed a fatty and yellow liver with nephritis. Some of the injected guinea pigs showed only fever but seemed to have acquired an immunity to subsequent injections of a virulent virus. In the blood, liver and kidneys of the animals showing the jaundice and albuminuria Noguchi was able to demonstrate spirochaetes by dark-field illumination. He also obtained cultures from such animals. Leptospiras were demonstrated in the blood of 3 out of 27 human cases but only after prolonged search. Cultures have been obtained from human yellow fever blood. For culturing, a medium is used consisting of 1 part of serum and 3 parts of Ringer’s solution made semisolid with 0.3% agar and contained in tall tubes. One cc. citrated yellow fever blood is introduced into the lower part of the medium. A thin layer of liquid petrolatum is poured on the top of the medium. We need partial oxygen tension but not anaerobiasis. Optimum growth temperature is 33°C. L. icteroides is from 4 to 9 microns long by 0.2 wide and tapers gradually to extremely thin sharp points. These organisms will pass the pores of V and N Berkefeld filters thus placing them in the group of filterable viruses. The virulence of different strains varies, with some strains as little as 0.00001 cc. of culture proving fatal for guinea pigs. Monkeys, rabbits and birds were not susceptible to infection but the marmoset and puppy seemed to respond as did the guinea pig.
By having Stegomyia (Aedes calopus) feed on infected guinea pigs as well as the human case of yellow fever, and subsequently allowing these mosquitoes to bite normal guinea pigs, the disease was transmitted in a few cases. In certain infected mosquitoes Noguchi found leptospiral organisms with dark-field illumination. Mosquitoes fed on infected guinea pigs became infectious in 8 days, this shorter period over human feedings being due, probably, to the greater abundance of organisms in the blood of the infected guinea pigs over human yellow fever blood.
Guiteras doubts the etiological relation of Noguchi’s leptospira to yellow fever on the ground that in view of the susceptibility of animals to this organism the disease should exist as an epizootic, which is improbable in view of the ease with which yellow fever is eradicated when measures applied solely to man and the mosquito are practiced. Among other points of objection he notes the recovery of the spirochaete from the blood later than the third day of the disease.
Fig. 42.—Leptospira icteroides in blood of a guinea pig inoculated with a culture × 1000. (After Noguchi.)
Infection by Injection of Blood.—The subcutaneous injection of as little as 0.1 cc. of the blood of a yellow fever patient in the first three or four days of the disease, or the serum of this blood, which has passed through the pores of a Chamberland filter (F but not B according to the French Commission), will bring about the infection of a susceptible person after an incubation period of from one day and fifteen hours to twelve days and eighteen hours.
In the natural method of transmission the mosquito, Stegomyia calopus, is the intermediary. In order that the female of this culicine species may transmit the disease it is necessary that she bite a yellow fever patient in the first three days of his illness, after which a period of approximately twelve days must elapse before the mosquito can transmit the disease. In such case the period of incubation varies from two days one hour to six days two hours. Carroll was bitten by a mosquito which had fed on a yellow fever patient twelve days previously, and four days later experienced a very severe attack, this fact being against the views of the French Commission that the disease shows a less severe form in those who may be bitten in the first period of the infectivity of the mosquito. A second case bitten five days later by the same mosquito that infected Doctor Carroll had a mild attack. Persons bitten by experimentally contaminated mosquitoes before an interval of twelve days had elapsed escaped infection.
Prior to the investigations of the American Commission our views as to the incidence and spread of yellow fever were chaotic.
Rush, in 1793, thought that the Philadelphia epidemic originated from “damaged coffee which putrefied on a wharf near Arch Street.”
In 1883, Doctor Friere reported that yellow fever was caused by a coccus, Cryptococcus xanthogenicus and claimed that he could confer immunity by vaccination with attenuated cultures. Carmona y Valle of Mexico and Carlos Finlay of Havana considered that the Micrococcus tetragenus was the cause but Sternberg, investigating these claims, showed that these cocci had nothing to do with yellow fever. In his work Sternberg isolated an organism which he designated “X.”