MODE OF INSECT TRANSMISSION. 83 



Hence it seems possible that errors may result 

 in assuming that a primary non-infective period 

 in the insect points, in an obligate manner, to the 

 protozoan nature of a parasite. 



So far we have considered three types of trans- Types of 

 mission by insects ; the first, in which the insect is n 

 infective immediately or soon after ingesting dis- 

 eased blood but not for a prolonged period (the 

 flea in plague) ; the second, in which the carrier 

 is infective almost immediately and for a prolonged 

 period later (ticks in Eocky Mountain spotted 

 fever) ; third, in which a harmless incubation 

 period is followed by a prolonged virulent period 

 (malaria and yellow fever). 



A fourth possibility, which is perhaps not yet 

 thoroughly established, is that of an immediate 

 infective period, followed by a non-infective, fol- 

 lowed again by an infective. The conditions as as- 

 certained so far indicate that this may be the case 

 in the transmission of trypanosomes by tsetse flies. 

 Until lately it was the experience that the flies 

 could convey nagana only for one or two days 

 after their feed on infected blood. Lately, how- 

 ever, Kleine 12 has found that an incubation period 

 of about twenty days, or a little less, occurs in the 

 fly, and from then on, even up to eighty-three 

 days (Taute), it is able to transmit infection. 

 This work has been done with the flies, Glossina 

 morsitans and G. palpalis, in connection with the 

 trypanosomes of nagana (T. brucei), and sleeping 

 sickness (T. gamltiense). The results have been 

 corroborated by Bruce. The first stage of infec- 

 tivity of the flies then remains for explanation. 



12. Deutsche med. Wchnschr., March 18, May 27 and July 

 22, 1909. 



