ETIOLOGY 



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vomiting sickness of Jamaica, in which black vomit is absent (p. 1695). At 

 the same time it is interesting to note the general similarity between pappataci 

 fever, dengue, and yellow fever, which appear to form a group of closely allied 

 diseases. 



Yellow fever has been reciorded by Hudellet at Dinguira, Mahina, 

 Ouida, between Kayes and the Niger in the Sudan, while cases 

 have been recorded in Java. 



iEtiology. — ^The causal agent exists in the blood of an infected 

 person, as Can be proved by the fact that the subcutaneous inocula- 

 tion of o-i c.c. of the infected blood into non-immune persons 

 produces attacks of the disease, but it cannot be spread to man by 

 post-mortem wounds. This causal agent appears to be, at least in 

 one of its stages, of exceedingly small size, for diluted blood filtered 

 through a Pasteur-Chamberland B bougie can still cause the disease 

 if inoculated intravenously into a non-immune. 



According to Seidelin, it is a small protozoon {Paraplasma flavigenum 

 Seidelin, 1909) found in the red blood cells, but the parasitic forms described 

 by this author are now considered to be artefacts. 



It is obviously a living organism, and not a chemical substance, 

 because of the time it takes to develop in man and the mosquito, 

 for the incubation period in man is usually three days. The mos- 

 quito, in order to become infected, must bite a patient during the 

 first three days of his illness, and then fourteen days must elapse 

 before the infected mosquito can transmit the disease. 



The proof of the transmission of the disease by Stegomyia calopus 

 was worked out by Reed, Carroll, Agramonte, and Lazear, by con- 

 structing a gnat-proof building divided into two gnat-proof com- 

 partments, into the first of which infected mosquitoes were liberated 

 and allowed to bite a non-immune, while other non-immunes slept 

 in the second compartment. The man in the first compartment 

 developed an attack of yellow fever, while the others did not. 



Non-immunes living in gnat-proof houses, with articles of clothing 

 and bedding soiled with urine, faeces, black vomit, etc., from cases 

 of yellow fever, did not contract the disease, though after this 

 experiment was finished some were infected by means of the bites 

 of infected mosquitoes, thus proving that they were not immune. 

 , Marchoux and Simond extended this knowledge by showing that 

 a Stegomyia can live for some thirty days, and lay seven batches of 

 eggs. Those laid after the twelve days' incubation by an infected 

 mosquito are also infected, and can transmit the contagium vivum 

 to a second generation of mosquitoes, but these cannot infect non- 

 immunes until fourteen days after hatching as imagines. 



The new generation of Stegomyia were fed upon glucose, and were 

 made to bite a recent non-immune arrival in Brazil, but no infection 

 followed. A week later the same mosquito was allowed to bite 

 the same person, who then developed a typical attack of yellow fever, 

 from which he subsequently recovered and was found to be immune. 



This is believed to indicate that a mosquito requires a feed of blood 

 before it can produce infection. 



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