A shift in the distribution of cases of murine typhus in man from central Los Angeles to areas in eastern Los Angeles and Orange counties, California, has been recognized over the past 20 years. The absence of the classic flea vector, Xenopsylla cheopis, and of Rattus norvegicus seropositive for murine typhus in these foothill areas suggested that other transmission cycles were involved. During 1967, a field study was conducted in an endemic area of eastern Orange County. Serum was obtained from 443 of 477 animals (14 species), and ectoparasites were obtained from 401 animals. Serum-antibody tests were positive only for eight of 75 opossums (Didelphis marsupialis) and one of seven skunks (Mephitis mephitis). All ectoparasites tested were negative for rickettsiae, but Rickettsia typhi was isolated from the spleen of one seropositive opossum trapped at the residence of a person with typhus. The association of seropositive opossums with human cases, and the heavy infestations of the animals with Ctenocephalides felis fleas, which readily bite man, suggested that opossums and their ectoparasites are responsible for some of the sporadic cases of typhus in man.
Present address: Department of Medicine, Boston City Hospital, Boston, Massachusetts.
Present address: Rodent Control Evaluation Section, New York State Department of Health, Albany, New York.