In this paper we have described the first part of an experimental study of the epidemiology of mouse typhoid. One set of data has been presented on the basis of which little or no analysis has been attempted. The immediate object has been rather to collect materials than to undertake to account for the phenomena encountered. It is obvious that the factors involved in the inquiry are intricate, but it is believed that they are not necessarily or all beyond disentanglement. About 500 mice in all have been studied in an experimental village, brought together in increments among a population in which mouse typhoid experimentally induced was prevailing.

The results have been presented according to two phenomena; namely, mortality or death rate, and bacillus carriage rate. The material does not lend itself to consideration according to morbidity rates. If it were established that every instance of attack, when not fatal, was attended by carrier production for the bacillus of mouse typhoid, reliable morbidity tables could be constructed. In the absence of this certain criterion, the materials here presented can be dealt with only as mortality data. This fact is attended with obvious disadvantages in respect to the epidemiological material assembled regarding infectious disease in man. In spite, however, of the drawbacks, it is already evident that the results obtained by the sort of inquiry here described may come to throw no inconsiderable light on moot problems on the origin, mode of spread, and manner of decline of epidemic diseases in general.

The analysis of the strains by selecting single cells and thus establishing substrains has yielded results which may eventually have value in explaining fluctuations in virulence. Among the positive data arising from the experiments with such cultures are, first, that there have been obtained by mechanical means from the ordinary bacteriologically pure culture, single cell strains exhibiting slightly different pathogenic activity, whether administered by mouth or parenterally, and second, that more regular results are obtained with intraperitoneal injections of these strains than with the parent strain. Among the negative results to be recorded are the failures of two single cell strains to incite an epidemic among mice under conditions known to be suitable when the parent strain is used.

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