The influence of climatic factors on the development of hemorrhagic fever with renal syndrome and leptospirosis during the peak season in Korea: an ecologic study

Abstract

Background: Hemorrhagic fever with renal syndrome (HFRS) and leptospirosis are seasonal rodent-borne infections in the Republic of Korea (Korea). The occurrences of HFRS and leptospirosis are influenced by climatic variability. However, few studies have examined the effects of local climatic variables on the development of these infections. The purpose of this study was to estimate the effect of climatic factors on the occurrence of HFRS and leptospirosis in Korea.

Methods: Daily records on human cases of HFRS and leptospirosis between January 2001 to December 2009 were analyzed. The associations of climatic factors with these cases in high incidence provinces were estimated using the time-series method and multivariate generalized linear Poisson models with a maximal lag of 12 weeks.

Results: From 2001 to 2009, a total of 2912 HFRS and 889 leptospirosis cases were reported, with overall incidences of 0.67 and 0.21 cases per 100,000, respectively, in the study areas. The increase in minimum temperature (1 °C) at a lag of 11 weeks was associated with 17.8% [95% confidence interval (CI): 15.1, 20.6%] and 22.7% (95% CI: 16.5, 29.3%) increases in HFRS and leptospirosis cases, respectively. A 1-h increase in the daily sunshine was related to a 27.5% (95% CI: 18.2, 37.6%) increase in HFRS at a lag of 0 week. A 1% increase in daily minimum relative humidity and a 1 mm increase in daily rainfall were associated with 4.0% (95% CI:1.8, 6.1) and 2.0% (95% CI: 1.2, 2.8%) increases in weekly leptospirosis cases at 11 and 6 weeks later, respectively. A 1 mJ/m² increase in daily solar radiation was associated with a 13.7% (95% CI: 4.9, 23.2%) increase in leptospirosis cases, maximized at a 2-week lag.

Conclusions: During the peak season in Korea, climatic factors play a significant role in the development of HFRS and leptospirosis. The findings of this study may be applicable to the forecasting and prediction of disease outbreaks.

Keywords: Climatic factors; Generalized linear poisson model; HFRS; Leptospirosis; Rodent; Seasonality; Vector-borne disease; Zoonosis.

Fig. 1

Map of geographical areas of Korea showing the eight and five provinces selected for HFRS and leptospirosis, respectively

Fig. 2

Monthly and yearly distribution of HFRS (upper figure) and leptospirosis (lower figure) cases with climatic factors in selected Korean provinces

Fig. 3

Weekly HFRS (upper figure) and leptospirosis (lower figure) cases with climatic factors from 2001 to 2009 in selected Korean provinces

Fig. 4

Poisson regression model of weather variables and weekly number of HFRS (left) and leptospirosis (right) cases from 2001 to 2009 in selected Korean provinces. The figure indicates the lag in weeks between HFRS and leptospirosis cases with climatic factors. The Poisson regression model shows the relation of climatic factors in 12 weekly lag durations in the development of HFRS and leptospirosis starting from week 27 to the last week of the year (53).†weekly average daily minimum temperature (°C), ^#weekly average daily minimum humidity (%), ^‡weekly average daily rainfall (mm), ^§weekly average daily sunshine (hours), *weekly average solar radiation (mJ/m²). Percent change of risk and 95% CI were estimated using a regression coefficient (β) and the following equation: percent change of risk = (exp[β] - 1) × 100 and 95% CI = (exp[β] - 1 ± 1.96 × standard error)