188 YELLOW FEVER [CH 



French investigators were so careful to exclude all chance of 

 error, that there is little doubt that under certain conditions, 

 a Stegomyia infected with yellow fever is able to transmit the 

 infection to its offspring. The reappearance of yellow fever in 

 places where it had apparently become extinct may possibly 

 t>e explained by this factor. 



Once a mosquito has become infected, it probably continues 

 so for the remainder of its life, but there are not sufficient 

 observations on this question. In nature the insects are 

 certainly still infective after being nourished on sugar for two or 

 three weeks, as shewn in the case of the St Nazaire epidemic. 



It is possible for the virus of yellow fever to be preserved 

 indefinitely in the mosquito and thus the cycle of development 

 must be quite different from that of Plasmodium. This was 

 clearly shewn by an interesting experiment performed by 

 Marchoux and Simond. Mosquitoes, that had fed on an 

 infected patient more than 12 days previously, were ground 

 up in thick syrup and some freshly hatched Stegomyia fed on 

 this mixture. Fifteen days later these insects were found to 

 be infected, although they had never fed on a yellow fever 

 patient. 



Vaccination. Although no successful means of preventing 

 infection by this means have yet been discovered, the results 

 obtained by the use of various sera are of some interest. 



We have previously referred to the extreme susceptibility 

 to temperature of the virus of yellow fever. Warming for 

 five minutes at 55 C. destroys all its activity. If preserved 

 at a temperature of 29 to 30 C. in an ordinary tube 

 plugged with cotton, the serum loses its virulence within 

 48 hours. The same temperature acts much more slowly if 

 the infected serum is covered with a layer of oil in order to 

 protect it from the air. At the end of five days some serum 

 kept in this manner was still infective, and produced an attack 

 of yellow fever when injected into a non-immune person. It 

 should be added that it only produced a very mild attack. 

 After being thus preserved for eight days the serum lost its 

 virulence entirely and could be injected into non-immunes 

 without producing any infection. 



