JO TROPICAL MEDICINE AND HYGIENE 



fusion or by the injection of the blood of a malarial 

 patient into another person, but this method of trans- 

 mission can play no part in the natural dissemination of 

 the disease. Ross records fifty-one successful inocula- 

 tions by many observers with blood, either intravenously 

 or by subcutaneous injection. The natural method, and, 

 as far as we know, the only method under ordinary con- 

 ditions, is by certain mosquitoes. 



The proofs of the mosquito malaria hypotheses are as 

 follows : 



(1) The development day by day of the parasites can 

 be followed in a batch of mosquitoes fed on a person 

 in whose blood gametocytes are found. This is so 

 definite that, knowing the time that has elapsed since 

 the mosquito was fed, the appearances of the malarial 

 parasite in it can be predicted with certainty. No such 

 bodies are found in other mosquitoes bred from larvae. 

 When these parasites have reached a certain stage of 

 development the sporozoites are found in the salivary 

 glands of these mosquitoes, and if such mosquitoes be 

 allowed to bite any susceptible person he will, after a 

 period of incubation, develop malaria of the same type 

 as that of the man on whom the mosquito fed. This 

 period of incubation is not constant, and differs with the 

 species of malaria, and also from that found by direct 

 inoculation of blood. Ross contrasts these from records 

 he has collected as follows : 



AVERAGE PERIOD IN DAYS BEFORE Shortest period 



_ SYMPTOMS affer mos uito 



Blood inoculation Mosquito inoculation 



P. malarise ... ... 17 ... ... ... ... 



P. vivax 10 18 15 



P. falciparum 8 n 6 



If quinine is being taken the incubation period is pro- 

 longed, and if taken regularly in small doses may be 

 prolonged by weeks or months, and symptoms not appear 

 till two or three weeks after the quinine is discontinued. 



(2) Mosquitoes were allowed to feed on a patient with 



