YELLOW FEVER EXPEDITION 
507 
after infection, but then the number of parasites introduced is relatively far larger 
than would be the case probably in gnat-borne yellow fever. 
In Texas fever 1 the incubation between injection of blood of sick animals and 
onset of fever ' may be in from six to ten days, depending upon the number of 
microparasites originally introduced, the predisposition and age of the animals, and 
the season of the year.' With inoculations by means of voung ticks ' the high 
temperature appeared generally in fifteen days after the first young ticks had been 
put on the animal. 
Another point in comparing yellow fever with a zooparasitical disease like 
malaria is the question of the fertilization of the gnats ; it seems to be that the proper 
sexual conception of the female Anopheles is essential for the development of the 
malarial parasite ; the report of the U.S. Commission is silent upon whether the 
successfully-infected mosquitoes were fertilized or brought forth eggs. Whilst there 
may be no direct connexion of the fertility of the transferring insect, with its 
capability of fostering an animal parasite, possibly the mere increased nutritional 
circumstances after a feed of blood would be sufficient for aiding the development of 
bacterial parasites. 
Another point in which yellow fever differs from the known infections caused 
by protozoal parasites is the absence of splenic enlargement, which is found in 
malaria, Texas fever, and tsetse fly disease. Though certainly, it must be admitted, 
that our knowledge of diseases of this nature is too scanty for discussion. 
The points then which seem at variance with a zooparasite in yellow fever are 
(1) the shortness of the natural incubation period, (2) the readiness and rapidity with 
which the parasite disappears from the blood, (3) the considerable degree of 
immunity 1 which is rapidly attained, (4) the short course of the illness and the 
usual absence of remote relapses or recurrences, and (5) the absence of splenic 
enlargement. At the same time none of these are incompatible with diseases of 
bacterial origin. 
In regard to the uncertainty of diagnosis in some cases and their importance in 
the continuance of the disease, the following remarks may be made : — In Cuba a special 
form of fever, which is termed ' Borras ' (or Fiebre de Borras), is recognized amongst 
the inhabitants (generally juvenile) ; this has been considered distinct from yellow 
fever by Cuban physicians. In Para the fevers of children seem usually diagnosed as 
' Febre palustre ' ; throughout Brazil another fever called ' Febre remittente bilioso 
dos paizos quentes ' (remittent bilious fever of hot countries). 
1. Smith ami Kilborne, Investigations into Texas or Southern Cattle Fever, Washington, 1893. pp. 15 and 106. 
z. Theobald Smith and Kilborne, p. 134 ; 'These experiments demonstrate the important fact that one attack of Texas 
fever does not necessarily protect the animal from a second attack. Of the eighteen cases seven may be said to have remained 
practically unaffected during the second exposure. Of the remaining eleven three died during the second exposure 
Hence we must be careful in giving even in these cases too much credit to the first attack in warding off the second 
following one But it may be laid down that as a general proposition that a single attack is not sufficient to 
produce complete immunity.' 
