YELLOW FEVER EXPEDITION 
503 
insects the time factor does not seem to have been considered. Thus Nuttall 1 
shews that bugs and fleas which had been fed on animals full of plague and 
other bacteria were incapable of giving the disease to other animals when directly 
transferred to them ; whether they could have done so after an incubation period, and 
whether the bacteria could have been discovered in their salivary or ejecting apparatus 
is not known. 
Another point which also reflects upon the mildness of the fever conferred is the 
small quantity of albumin in many in the urine in the experimental cases ; thus out 
of the twelve gnat infections in five the quantity is described as a ' trace,' and in one 
other there was no albumin until thirty-six hours after the fever had subsided. 
The control cases were formed by the seven susceptible persons accommodated 
at the experimental station camp 'Lazear' (II), where the incidence of the fever 'was 
strictly limited to those individuals who had been bitten by contaminated mosquitoes.' 
It may then be taken as proved that Stegomyia fasciata (Culex) may be capable of con- 
ferring the disease some ten, twelve, or more days after having fed upon yellow fever 
patients during the first, second, or third day of the fever. The cases which were 
bitten by gnats which had been fed upon yellow fever cases later in the disease, viz. :-— 
Case 6 (I) one mosquito fed ten days before on fifth day of fever, 
Case 8 (I) one mosquito fed thirteen days before on fifth day of fever, 
Case (III, p. 18) one mosquito fed forty days before on fourth day of fever, 
proved negative. The question of how long the infective agent remains in the circu- 
lating blood, whence it can be extracted and transferred by gnats is important for 
combatting the prevalence of the disease by means of isolation. It can hardly be 
doubted that the infective agent does not remain for prolonged periods in the circula- 
tion of those who have passed through the disease, otherwise it is difficult to under- 
stand the comparative readiness with which the disease disappears in localities, or has 
done so in the past, without any overt act directed against the gnats. Conversely 
there is a tenacity with which the fever remains endemic in the Latin-American countries 
of Central and South America ; here there are four possibilities : one already dealt 
with, the continuance of the parasite in the circulation of the ' immune ' ; second, the 
occurrence of second or further attacks amongst the ' immune,' with which may be 
included more or less primary attacks in the children of the ' immune ' ; thirdly, the 
prolonged survival of infected gnats ; fourthly, that the infecting agent is able to re- 
main alive independently of man and gnat. 
Apparently it is with the third method that Dr. Reed considers that the foci of 
the fever are maintained by proving that two gnats ( S. fasciata ) which had been kept 
for fifty-seven days after their infecting feed upon a yellow fever patient on the 
second day of illness, were capable of conveying the infection to a susceptible person 
— . « 
1. Nuttall, Zur Aufkldrung der RoIIe, ivelche Insekten bei der Verbreitung der Pest spielen u. s. iv. Centralblatt f. Bacteriologie, 
I Abth XXII, 1897, p. 87, and Zur Aufklarung der Rolle ivelche stechende Insekten bei der Verbreitung ion Infectionskrankheiten 
spielen, ibid XXIII, p. 625. 
