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Rat Ectoparasite Surveys in Relation to Murine Typhus Fever in Puerto Rico^*

A rat ectoparasite survey of the City of San Juan, Puerto Rico, was made from 7 December 1956 to 31 December 1960. Data obtained from 964 rats collected from 15 April 1957 to 4 May 1958, 747 rats collected in 1958–59, and 573 in 1959–60 were compared with similar data obtained in 1946–47 and 1954–55. In 1946–47 54% of the rats were infested with Xenopsylla cheopis; in 1954–55, 16%; in 1957–58, 6.0%; in 1958–59, 3.3%; and 1959–60, 2.6%. Other common rat ectoparasites, Laelaps nuttalli, Ornithonyssus bacoti, Listrophoroides expansus, and Polyplax spinulosa increased in abundance. In particular, L. nuttalli showed marked increases. Ornithodoros puertoricensis which affected 26.8% of the rats in 1946–47 infested only 5.4% in 1959–60. Designed rat flea insecticidal control measures have never been applied in Puerto Rico. The rat population has always been high. Fleas were uncommon in poorly sanitated areas and more often found in well sanitated places subject to other pest control activities. Weather Bureau records indicated that precipitation, humidity or temperature could not account for the rat flea reduction. Of 998 rat sera subjected to the complement fixation test with U. S. Army group specific typhus antigen at 1:10 dilution, 804 were negative and 194 were anticomplementary. In 1954 and succeeding years including 1960 there were no cases of murine typhus fever reported from the City of San Juan and in 1958, 1959, and 1960 none in Puerto Rico as a whole. Since a great reduction in rat flea population occurred and no infection in rats was found, there is no reason to doubt the reported morbidity or to postulate that the use of antibiotics had anything to do with the rate. It was concluded that investigations on the ecological circumstances of the association of arthropods, particularly mites, with rat fleas are necessary, if the question of why the rat flea population was decimated is to be answered.

^* This investigation was supported by research grants E-854 and E-2887, National Institute of Allergy and Infectious Diseases, National Institutes of Health, U. S. Public Health Service. Medical students with summer fellowships under graduate research training grant 2E-15 to the senior author from the same source also participated in some of the work.