Arthropod-borne tularemia in Japan: clinical analysis of 1,374 cases observed between 1924 and 1996.

Sixteen cases of tularemia transmitted by arthropods are used to characterize arthropod-borne tularemia in Japan. Arthropod-borne tularemia accounted for 1.2% of a total of 1,374 cases of tularemia observed between 1924 and 1996. The number of cases reported in Japan was low when compared with the number of cases reported in the United States. Arthropod-borne tularemia, however, is increasing and has reached 10.1% incidences during the last 16 yr. No arthropod-borne cases were reported prior to 1951. The occurrence of tularemia infection caused by contact with diseased hares was diphasic with the higher peak occurring during the winter, whereas the occurrence of arthropod-borne tularemia was common from spring to autumn. Among the 16 cases that we studied, 5 were initiated by tick-bites, 5 by the crushing of ticks found on domestic dogs, and 1 by an unidentified insect. In the remaining 5 cases, the vectors were not identified although arthropod bites were confirmed. These arthropod-borne cases were observed exclusively in the northeastern area of Honshu, the main island of Japan, and the age of patients ranged from 23 to 74 yr. The arthropod vectors associated with tularemia and factors influencing incidence of arthropod-borne tularemia in Japan are discussed.