VI] MODE OF INFECTION, ETC. 47 



Following the bite of an infected fly, there is an incubation 

 period of from 3i to 7 days, during which the blood of the 

 patient is not infective. Then follows the first day of the attack 

 during which the blood becomes virulent and is capable of 

 infecting any Phlebotomus that may feed on it. 



The blood loses its infectivity within a very short period, 

 probably not more than 24 hours, and after recovery the 

 patient is immune against any further attacks of the disease. 



The flies become infected only if they feed on a patient 

 during the short time that his blood is infective. After ingest- 

 ing the virus, there is an incubation period of seven to ten 

 days before the insects become infective and beyond this 

 period they may again become non-infective, but experiments 

 on this point are far from complete. 



Causal agent. Up to the present the causal agent of Pap- 

 pataci Fever has not been discovered, but it belongs to the 

 group of ultra-microscopic organisms which cause dengue, 

 yellow fever, etc. Its presence in the blood is proved by the 

 fact that the disease may be produced by the inoculation 

 into a non-immune person of blood taken from an infected 

 patient on the first day of the attack. Moreover, the injection 

 of a small quantity of diluted serum filtered through a Cham- 

 berland F filter is also followed by a typical attack of fever. 

 The virus is sufficiently resistant to maintain its activity for 

 a week in vitro. 



It is evident that the fly is a true host for the organism 

 that causes the disease. If the Phlebotomus were only infec- 

 tive up to the seventh day after an infective feed, there woixld 

 be a strong suggestion that the virus was merely retained in 

 the alimentary canal and regurgitated at subsequent meals. 

 This supposition is disproved by the occurrence of a negative 

 phase of about seven days, during which the mfective agent of 

 the disease undergoes some development, finally resulting in the 

 fly becoming infective. 



Prophylactic measures. In view of the limited number of 

 observations on the breeding habits of Phlebotomus it is some- 

 what premature to advance any definite prophylactic measures. 

 If crevices in stone walls and amongst rocks constitute the 



