The Spatial Heterogeneity between Japanese Encephalitis Incidence Distribution and Environmental Variables in Nepal

BackgroundTo identify potential environmental drivers of Japanese Encephalitis virus (JE) transmission in Nepal, we conducted an ecological study to determine the spatial association between 2005 Nepal JE incidence, and climate, agricultural, and land-cover variables at district level. MethodsDistrict-level data on JE cases were examined using Local Indicators of Spatial Association (LISA) analysis to identify spatial clusters from 2004 to 2008 and 2005 data was used to fit a spatial lag regression model with climate, agriculture and land-cover variables. ResultsPrior to 2006, there was a single large cluster of JE cases located in the Far-West and Mid-West terai regions of Nepal. After 2005, the distribution of JE cases in Nepal shifted with clusters found in the central hill areas. JE incidence during the 2005 epidemic had a stronger association with May mean monthly temperature and April mean monthly total precipitation compared to mean annual temperature and precipitation. A parsimonious spatial lag regression model revealed, 1) a significant negative relationship between JE incidence and April precipitation, 2) a significant positive relationship between JE incidence and percentage of irrigated land 3) a non-significant negative relationship between JE incidence and percentage of grassland cover, and 4) a unimodal non-significant relationship between JE Incidence and pig-to-human ratio. ConclusionJE cases clustered in the terai prior to 2006 where it seemed to shift to the Kathmandu region in subsequent years. The spatial pattern of JE cases during the 2005 epidemic in Nepal was significantly associated with low precipitation and the percentage of irrigated land. Despite the availability of an effective vaccine, it is still important to understand environmental drivers of JEV transmission since the enzootic cycle of JEV transmission is not likely to be totally interrupted. Understanding the spatial dynamics of JE risk factors may be useful in providing important information to the Nepal immunization program.研究背景 为明确尼泊尔日本脑炎（Japanese Encephalitis, JE）病毒传播的潜在环境驱动因子，本研究开展生态学研究，旨在探究2005年尼泊尔日本脑炎发病率与县级尺度气候、农业及土地覆盖变量之间的空间关联。 研究方法 本研究采用空间关联局域指标（Local Indicators of Spatial Association, LISA）分析县级日本脑炎病例数据，识别2004至2008年的空间聚集区域；并以2005年数据为基础，拟合包含气候、农业及土地覆盖变量的空间滞后回归模型（spatial lag regression model）。 研究结果 2006年以前，尼泊尔远西部与中西部特莱平原区域存在一处大型日本脑炎病例聚集区。2005年之后，尼泊尔日本脑炎病例分布发生转移，中部丘陵区域出现聚集性病例。相较于年均气温与年均降水量，2005年疫情期间的日本脑炎发病率与5月月均气温、4月月总降水量的相关性更强。经简约的空间滞后回归模型分析显示：1）日本脑炎发病率与4月降水量呈显著负相关关系；2）日本脑炎发病率与灌溉土地占比呈显著正相关关系；3）日本脑炎发病率与草地覆盖占比呈无统计学意义的负相关关系；4）日本脑炎发病率与猪-人比例呈单峰型无统计学意义的相关关系。 研究结论 2006年以前，日本脑炎病例聚集于特莱平原地区，后续年份则逐渐向加德满都地区转移。2005年尼泊尔日本脑炎疫情的空间分布特征与低降水量及灌溉土地占比显著相关。尽管已有有效疫苗，但明确日本脑炎病毒传播的环境驱动因子仍具有重要意义，因为日本脑炎病毒的自然传播循环难以被完全阻断。厘清日本脑炎风险因子的空间动态特征，可为尼泊尔免疫规划工作提供关键参考依据。