G. F. Petrie and C. R. Avari 
315 
as judged by the weight of the rats and the degree of infection in the 
blood. It appears, however, that the difference in weight of the various 
lots of rats taken is not sufficiently marked to justify any conclusion on 
this point. 
TABLE Y. 
Comparing the degree of infection with the average weight of M. rattus 
and M. decumanus infected with trypanosomes. 
Mus rattus. 
Mus decumanus. 
Degree of 
Total rats 
Average weight of 
Degree of 
Total rats 
Average weight of 
infection 
examined 
rats examined 
infection 
examined 
rats examined 
+ 
107 
47*6 grammes 
+ 
80 
62 - 9 grammes 
+ + 
86 
59-1 „ 
+ + 
68 
63-8 
+ + + 
134 
58-0 
+ + + 
81 
58-4 
Note 
— +=few, + + 
= fairly numerous, + + + = 
numerous or very numerous. 
No systematic observations have yet been carried out on the pre¬ 
valence of rat lice in Bombay. In view of Nuttall’s (1908) successful 
transmission experiments with rat lice such observations would be 
of interest. From general observations, however, it is fairly certain 
that the season of prevalence of various insect parasites (human-, rat-, cat- 
fleas and bugs) is the same for all, so that it is probable that the rat flea 
prevalence curves provide a fair index of the prevalence of rat lice. 
Reference may be made here to the relative importance from an 
epidemiological standpoint of the rat flea and the rat louse in the 
transmission of T. lewisi, assuming that both are capable of transferring 
the infection from rat to rat in nature. There would seem to be little 
doubt that the habits of these insects in relation to their host are import¬ 
ant in this connection. Thus lice are well known to live constantly 
upon their host, and this habit must necessarily limit the opportunities 
for conveyance of infection from rat to rat, although it is readily con¬ 
ceivable that young rats while in the nest, and especially while suckling, 
might pick up infected lice. Rat fleas on the other hand, from the fact 
that they spend much of their existence apart fi'om their host, are more 
likely to convey the infection from rat to rat over considerable distances. 
Interpretation of the trypanosome prevalence curves on the view that the 
conditions for successful transmission vary at different seasons of 
the year. 
It is conceivable that the climatic conditions at a particular season 
of the year are especially favourable in respect of the persistence or 
