Gr. F. Petrie and C. R. Avari 
307 
and was continued without intermission until 19th October, 1906, a 
period of nearly 14 months 1 . The rats were brought from all parts of 
Bombay city and island for the purposes of the Plague Commission 
then working at the laboratory at Parel. The observations were made 
generally on five or six days of each week. Only “young” rats were 
examined for the presence of trypanosomes, i.e. in accordance with the 
arbitrary standard adopted by the Commission, those weighing 70 and 
100 grammes or less in the case of M. rattus and M. decumanus 
respectively; no rats below these weights were found pregnant. Young 
rats were selected with the idea of eliminating the factor of immunity 
acquired in adult rats either by their having harboured the infection at 
some previous time or by vaccination with doses of infected blood 
insufficient to bring about an infection. When brought to the labora¬ 
tory the young “ live ” rats were killed and at once dissected. Blood 
was pipetted from the heart and spread in a somewhat thick layer upon 
clean slides. The blood films after fixation with absolute alcohol were 
stained with carbol-thionin and examined under an oil immersion lens. 
It was found that carbol-thionin had the advantage as a diagnostic 
stain for trypanosomes of staining the red blood corpuscles very feebly 
so that a film of moderate thickness could be searched for the parasites 
without difficulty. 
In all, 1832 M. decumanus and 2651 M. rattus, a total of 4483 rats, 
were examined during the period of investigation. The results are 
shown in the tables (I, II and III) and in Chart I. It will be noted 
(Tables I, II and III) that, as might be expected, there is no marked 
difference in the incidence of the infection on males and females of 
both species. Table III also shows that the percentage of ili. rattus 
infected during the whole period does not differ markedly from the 
percentage of infected M. decumanus. 
It is interesting to compare these results with those obtained by 
previous observers in Bombay. Vandyke Carter in 1885 examined 210 
rats for trypanosomes and found 25 infected, i.e. nearly 12°/ 0 - From 
November 1890, to May 1895, Lingard in the course of his work on 
1 We may note that the primary object we had in view in undertaking the investigation 
was in order to determine if possible, a seasonal prevalence of trypanosomes in rats which 
might give a clue to the season of rat-flea prevalence, an important question in plague 
epidemiology. In this we were influenced by the experiments of Rabinowitsch and 
Kempner (1899) which seemed to warrant the belief that rat fleas could act as trans¬ 
mitters of the infection in nature. As it happened the Plague Commission were able to 
obtain direct evidence during the year subsequent to our investigations as to a season of 
rat flea prevalence in Bombay. 
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