135 



Carter (H. R.)- Spontaneous Disappearance of Yellow Fever from 

 Failure of the Human Host. — Trans. Soc. Trop. Med. Hyg., 

 London, x, no. 7, June 1917, pp. 119-139. 



The author assumes as a basis of his reasoning that one attack of 

 yellow fever gives permanent immunity from the disease. This 

 however, as he indicates, is a disputed ])oint, and has been discussed 

 by him in a previous paper [see this Review, Ser. B, iv, p. 109]. The 

 conditions necessary for the continued existence of yellow fever in a 

 community are the parasites of the fever, active Stegomyia fasciata 

 {calopus), and susceptible men. The parasites live in the mosquito 

 only during the life of the host ; therefore, for yellow fever to be trans- 

 mitted, a sick man must be bitten by a moscjuito which in its turn 

 must feed on another man susceptible to yellow fever, before the death 

 of either parasite or mosquito. If in a community there be no sus- 

 ceptible persons, the disease will disappear. Where a community 

 exists which is an endemic focus of yellow fever, that is, where the 

 three necessary conditions for the transmission of the disease exist, 

 if it be true that one attack of yellow fever produces in general a per- 

 manent immunity, there will be in time no individuals susceptible 

 to the disease left in that community, and yellow fever would disajjpear 

 as soon as the infected mosquitos died off and the parasites with them. 

 This community moreover would remain free unless the same three 

 factors for conveyance were again brought together. Natui'ally, a 

 younger generation would grow up susceptible to the fever and suscep- 

 tible immigrants might arrive, but unless the parasite be again intro- 

 duced there would be no yellow fever. The new generation, being 

 protected for some years at least by its immune environment, is of 

 much less importance in maintaining yellow fever than adult immi- 

 grants. 



Turning from theoretical deductions to facts, the author gives many 

 instances of yellow fever having disappeared from tropical towns in 

 which no sanitary work has been done and in which Stegomyia is still 

 abundant ; in fact this disappearance seems to be the rule in the case 

 of isolated communities of small or moderate size. The explanation 

 that the fever disappears owing to " the failure of the human host " 

 is believed to be true because it is logical and accords with the known 

 facts and because none other is apjjarent. 



It is obvious that this explanation depends absolutely on the doctrine 

 that an attack of yellow fever confers immunity against another 

 attack. A commission of the Pasteur Institute, working at Rio 

 de Janeiro, has definitely stated that " yellow fever is kept up in 

 endemic centres by recurrent attacks among the indigenes " and this 

 view is accepted by many modern writers. If this be so, the recur- 

 rences might well make their appearance indefinitely, independently 

 of immigration or new births. 



The possibility of disappearance by the process described above is 

 therefore not dependent upon the permanence of the immunity given 

 by one attack, but the chance of its occurring in any place at any 

 definite time is directly dependent on it, and reaches its maximum 

 if one attack gives permanent immunity. The frequent disappearance 

 of the fever, when immigration is absent, is evidence against the 

 recurrence of attacks. 



