C. Strickland and N. H. Swellengrebel 
437 
I. The prevalence of T. lewisi in rats under 
NATURAL CONDITIONS. 
(a) Endemic foci. Many observers have reported upon the natural 
prevalence of T. lewisi in rats. The statistics relating to this prevalence 
of the parasite cannot always be compared, because of their being based on 
observations made in different ways. Thus Lingard (1895) reported on 
all the rats he examined irrespective of their age, while Petrie and 
Avari (1909) only noted the presence of the parasite in young rats. 
Working in Bombay Vandyke Carter (1887) found 12 “/o of the rats 
infected, Lingard (1895) about 35 7o. Petrie and Avari (1909) about 
45 “/oi in the Philippine Islands Musgrave and Clegg (1903) found that 
about 65 7o of the rats at Manila harboured the parasite at certain 
times of the year; in the United States Swingle (1907) at Nebraska 
found four out of seven infected, while Yakimoff (1907) in St Peters¬ 
burg found a higher proportion infected in the summer months than in 
the winter but he gives no figures. 
Prof. Nuttall kindly allows us to report upon the results of the 
similar observations which he conducted with one of us (C. S.) during 
1909 in Cambridge, where 25 "/o of rats {M. decumanus) of all ages were 
found to be infected. 
Having regard to the manner in which all of these percentages of 
infection were obtained it would appear that the prevalence of T. lewisi 
varies considerably in different parts of the world. The figures given 
by Vandyke Carter, Lingard, and Petrie and Avari for Bombay indicate 
moreover that the prevalence of the parasite may vary in a given 
locality from time to time. 
(b) Variation of incidence in endemic foci {localised endemicity.) 
Carter, Musgrave and Clegg, and Swingle have observed a marked 
localised endemicity of the infection, and our observations conducted in 
Cambridge also reveal the local character of rat trypanosomiasis. 
Thus, 20 young rats from Comberton, Cambs, shewed no trypano¬ 
somes, while a large percentage from Cherryhinton Brook, Cambs, 
were found to be infected. We at an early date learnt to know the 
particular localities from which we could rely for a supply of T. lewisi 
infected rats in and about Cambridge. 
Of the Cambridge rats which were infected 12 “/o were young, 
33 7o adults, and 29 "/o old animals. 
