1912.] 



Trypanosoma garnbiense in Glossina palpalis. 



•243 



Toad and starvation experiments.. 

 Flies dissected between days 1 — 5 



Total 



2733 flies ; infected flies, 81. 

 •2U „ ; „ 17. 



8-1 „ ; „ 9. 



Deducting the last two batches from the total there remain 2415 flies, 

 of which 55 were infected ; that is 2-27 per cent, of the flies harboured 

 trypanosomes. 



This percentage, i.e. 2*27, is naturally not the measure of the infectivity 

 to the fly of any strain (or strains) of trypanosome. It is the percentage of 

 infected individuals produced by allowing 2415 flies to feed at random, in 

 groups, through a period of two and a half months, on a population of nine 

 infected monkeys. Each group receives of course only one or two feeds on 

 the infecting monkey, and is then fed on clean animals. 



Certain obscuring features, habitually neglected in dealing with trypano- 

 some infections, must be pointed out in figures handled in this way. The 

 nature of the individual strain must be considered, and the occurrence of 

 negative periods (i.e. periods when the vertebrate is not infective to fly) 

 must be duly taken into account. As they stand, the figures above cited 

 have no real meaning. The number of infected individuals obtained by 

 feeding flies at random upon an infected monkey or other vertebrate is 

 neither an index of the infectivity of the strain nor of the potential danger 

 of such an animal at large in a fly area. The percentage, however, of 

 infected individuals produced among flies fed during periods when the blood 

 is infective, gives the index of the virulence of the strain as regards fly. If, 

 on the other hand, batches of, say, 50 or 100 flies were fed on an infected 

 monkey for every day of its life during the course of the disease, the infected 

 glossinse thus produced would give an index of the infective power of the 

 monkey as a whole. 



It is obvious that there are two quite different aspects of the question, 

 and calculations in which they are treated as one must naturally be mis- 

 leading. In practice it seems usual to neglect this distinction, with the 

 result that there has been a tendency to underestimate the potential 

 transmitting capacity of the fly, and to over-rate its individual idiosyncrasy. 

 Given reasonably favourable conditions of temperature and moisture, it is 

 the strain of trypanosomes and not the fly that within a relatively wide 

 range plays the deciding role in limiting the number of infected glossina. 

 There is, of course, as has already been mentioned, a serious difficulty in the 

 way of the trypanosome in its attempt to establish itself at all in the glossina, 

 but that must be very nearly constant in all cases. 



To consider some of the experiments in greater detail. Monkey 113, 



