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The above-mentioned considerations bearing upon the influence of insect prevalence and the influence of meteorological conditions on the prevalence of trypanosome infections in rats may be summarized as follows:
(1) There are definite seasonal variations in the prevalence of trypanosome infections in M. rattus and in M. decumanus.
(2) No definite correlation is forthcoming between the seasonal prevalence of T. lewisi and the seasonal prevalence of rat fleas. The seasonal prevalence of the transmitting insects is apparently a factor of subsidiary importance in the causation of the trypanosome prevalence.
(3) The evidence adduced above indicates that the dominant factor determining the seasonal prevalence of trypanosome infections is the atmospheric temperature, the optimum temperature approximating to 79° F. It is probable that temperature operates by influencing a developmental cycle of the trypanosomes in the transmitting insects.
(4) It is possible that in addition to temperature the atmospheric humidity may play a part by influencing the direct or mechanical transference of the infection.