Meteorological factors affecting scrub typhus occurrence: a retrospective study of Yamagata Prefecture, Japan, 1984-2014

Abstract

Climate change, by its influence on the ecology of vectors might affect the occurrence of vector-borne diseases. This study examines the effects of meteorological factors in Japan on the occurrence of scrub typhus, a mite-borne zoonosis caused by Orientia tsutsugamushi. Using negative binomial regression, we analysed the relationships between meteorological factors (including temperature, rainfall, snowfall) and spring-early summer cases of scrub typhus in Yamagata Prefecture, Japan, during 1984-2014. The average temperature in July and August of the previous year, cumulative rainfall in September of the previous year, snowfall throughout the winter, and maximum depth of snow cover in January and February were positively correlated with the number of scrub typhus cases. By contrast, cumulative rainfall in July of the previous year showed a negative relationship to the number of cases. These associations can be explained by the life-cycle of Leptotrombidium pallidum, a predominant vector of spring-early summer cases of scrub typhus in northern Japan. Our findings show that several meteorological factors are useful to estimate the number of scrub typhus cases before the endemic period. They are applicable to establish an early warning system for scrub typhus in northern Japan.

Keywords: Leptotrombidium pallidum; life-cycle; negative binomial regression; snowfall; temperature.

Fig. 1.

Geographical distribution of four meteorological stations (red plot) where meteorological data were acquired for this study. The cumulative number of scrub typhus cases in each city of Yamagata Prefecture during 1984–2014 is shown on the map. The colour altitude map was obtained from the Geospatial Information Authority of Japan.

Fig. 2.

Scatter plot between the number of scrub typhus cases and meteorological factors, which were significant (P < 0·01) for each negative binomial regression adjusted by population, in Yamagata Prefecture, Japan, 1984–2014. A regression curve was drawn with the population fixed to 1·13 million, representing the population in 2014. The average temperature in August of the previous year showed the greatest association to the number of scrub typhus cases. Six of seven meteorological factors showed a positive association to the number of the cases.

Fig. 3.

Scatter plot showing actual and predicted numbers of scrub typhus cases in spring–early summer in Yamagata Prefecture, Japan, 1984–2014. Values of the two groups showed good fitness, with adjusted R² = 0·74.

Fig. 4.

Results of actual and predicted numbers of scrub typhus cases in spring–early summer in Yamagata Prefecture, Japan, during 1984–2014. All actual numbers of cases were included within the prediction interval.