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The remarkable reduction in murine typhus fever in southern United States is generally attributed to large scale control measures, particularly DDT dusting to kill rat fleas. Notwithstanding that such control operations were not in effect in Puerto Rico, the reported rate dropped steadily and there was a 100 per cent decrease in the City of San Juan (137 reported cases in 1944 and none in 1954). In an attempt to explain the decline of this disease a rat ectoparasite study together with human and rat serological surveys were undertaken. Ectoparasite data from 600 rats collected in the City of San Juan from April 15, 1954, through May 4, 1955, when compared with data from 775 rats trapped during the same period in 1946–47 demonstrated that the flea vector, Xenopsylla cheopis, had decreased in population sufficiently to account for the lack of human cases; for only 16 per cent of the rats in 1954–55 were infested with an average of 0.42 specimens per rat as compared with 54 per cent infested and 1.75 specimens per rat in 1946–47. Decrease in flea population was greater in the residential section known as Santurce where only 5 per cent of the rats were infested than in the largely business district, San Juan Islet, where 26 per cent of the rats were still infested. As regards Santurce, data from earlier surveys indicated that there has been a consistent downward trend in rat flea population during the last eight years. Complement-fixation tests on 279 rats resulted in only 3.2 per cent positive at dilutions of 1:8, indicating that infection among rats is negligible. Of 368 human subjects, 3.8 per cent were positive at dilutions of 1:4 or above using the complement-fixation test; since a positive reaction may be given years after recovery and because of very low titers as well as other reasons it is believed that the reported rate is essentially correct and there are few if any unreported cases.
On the basis of this evidence from Puerto Rico it is concluded that a reduced typhus rate is not proof of the efficacy of control operations. Further, it is possible that control was instituted in southern United States after the epidemic had already receded, and that without the large scale control projects there the disease might have followed the same course it has in Puerto Rico. No explanation has yet been found for the decrease in the reproductive potential of rat fleas during the last decade in San Juan, P.R., since rats are abundant and the climate has been favorable for flea development, at least as regards temperature and humidity. Improvements in construction, food storing, general sanitation and mosquito control may have had an incidental effect on rat fleas, but do not account for the great reduction in population which has occurred.