| Budget Amount *help |
¥2,000,000 (Direct Cost: ¥2,000,000)
Fiscal Year 1989: ¥500,000 (Direct Cost: ¥500,000)
Fiscal Year 1988: ¥500,000 (Direct Cost: ¥500,000)
Fiscal Year 1987: ¥1,000,000 (Direct Cost: ¥1,000,000)
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| Research Abstract |
In order to gain perspectives of the endemicity of tsutsugamushi disease, the present study was aimed at clarifying the mechanism of recurrent outbreaks of the disease by investigating ecological aspects of the vector chiggers, which have been suggested to have characteristic environmental requirements and restricted distributions. We have intended to specify the habitat type for Leptotrombidium pallidum to incorporate results to be obtained into discussion whether the recent trend in environmental changes is advantageous to the vector chigger to propagate abundantly or not. L. pallidum was found in many types of habitats, but the most common habitat was large paddy banks, which have been made during the recent paddy zone reconstruction program. Since the areas in which disease cases have been reported have gradually widen to cover most of the range of L. pallidum, environmental changes such as those mentioned above and, as pointed out previously, the reversion of agricultural lands to
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desolated fields in the recently depopulated districts are thought to have contributed to an increase in vector population density and are probably responsible for the recurrent outbreaks of tsutsugamushi disease transmitted by this vector, although the incidence of pathogenic rickettsiae in the chiggers has only been partially studied. So it is postulated that tsutsugamushi disease caused by L. pallidum will continue to be endemic, at least at the current scale, especially in reconstructed paddy zones as well as in further depopulated districts. The other vector chigger, Leptotrombidium scutellare, was shown to be distributed at well drained sites at the foot of mountains in the Gotenba-Oyama District, shizuoka Prefecture. The specific habitat for this species was the sparse miscanthus grassland on granular soil in sunny places, which is sporadically encountered in the district. It is difficult to incorporate these findings into the recurrent outbreak of tsutsugamushi disease transmitted by L. scutellare, but is not difficult to postulate that outbreaks of many cases at a time might readily occur within restricted areas, because of the vector's specific distributional pattern. Less
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