Abstract
The ecology of the unfed larval trombiculid mite of Leptotrombidium pallidum infected with Orientia tsutsugamushi was studied in an endemic area, in northern Kyoto, Japan. Surveys were conducted in seven detailed study areas of a rice field heavily infested with 0. tsutsugamushi, once every spring and autumn during the years 1997 through 2001. A total of 5,295 unfed larvae from 4 genera and 7 species were collected from soil samples. The dominant species was L. intermedium (61.1%), followed by L. pallidum (34.5%). 0. tsutsugamushi was detected in only 2 species, L. pallidum andL. intermedium, and the infection rates were 15.6% (285/1,826) and 0.1% (3/3,237), respectively. The main vector species of 0. tsutsugamushi to humans and field rodents in the study area is considered to be L. pallidum. Serotypes of 0. tsutsugamushi in L. pallidum and L. intermedium were determined as Gilliam- and Karp-type rickettsiae, respectively. The locations of infective spots composed of infected unfed larval L. pallidum were patchily distributed in each detailed study area. One to 18 infected unfed larvae were identified in each infective spot. The numbers of infected mites in these spots reflect the infection risk of tsutsugamushi disease for humans exposed to infected unfed larvae in an infective spot. The location of infective spots was changeable, and the numbers of spots gradually decreased year by year. In each of the seven detailed study areas, infection rates of L. pallidum from the spring of 1997 to the spring of 1999 varied from 11.0% to 26.4%. However, both the infection rate and the number of infected larvae gradually decreased from the autumn of 1999, and no infected larvae were collected in the spring of 2001. These data could lead to a possible explanation of the process whereby an endemic area gradually disappears, and also an uncontaminated area could be converted to infective spots.
