Our findings suggested that Cwa, Cwb, Dwa, and Dwb were high-risk JE infectious regions in Gansu, China. The BWk and Dwc may be the emerging epidemic areas. The weather conditions, especially the maximum temperature at the lag of 1 month for JE, may accelerate JE transmission. This town-level JE epidemic analysis is based on climate categorization, and the findings may shed new light on JE management and prevention, particularly in areas where the climate is similar.
The current study found that JE mostly occurred between weeks 31 and 37 of the year. This result was consistent with the studies conducted in provinces neighboring Gansu like Shaanxi and Ningxia, which indicated JE is prevalent from July to September(Li et al., 2016; W. Liu et al., 2020; Wang et al., 2020). A study conducted based on the geographical distribution of JE found that JE occurs in temperate areas (including Cwa, Cwb, Dwa, and Dwb) in the summer or early autumn(Chan et al., 2016; van den Hurk et al., 2009). A possible explanation is that JEV is transmitted through a basic cycle among mosquitoes and vertebrate hosts, primarily domestic pigs and ardeid birds, and the transmission cycle of JE was sensitive to the weather variables. This is because climate can influence the development of mosquitoes and viruses, and the dynamic distribution of available hosts and habitats.
The analysis of epidemic temporal characteristics indicated that BWk and Dwc might be emerging epidemic areas. The BWk were likelier to have a longer epidemic duration with fewer cases due to the highest β with lower α and γ with eight JE cases. Because those cases were not travelers from a JE epidemic area, climate change may contribute to these results. A study conducted in BWk climate subtype found positive temperature trends in 1995–2005(Mamtimin et al., 2011), and climate change may influence the hydrological cycle(Snyder et al., 2019). It is understudied whether this climate change accelerates JEV transmission. The Dwc had the highest γ with low α and β. Therefore, it was more likely to experience the temporally concentrated JE epidemic. This result may indicate that JEV’s distribution has expanded. The Dwc is a subarctic climate(Peel et al., 2007), which borders the Tibetan plateau. In general, the Tibetan plateau is free of JE. However, JEV was detected in Tibet in recent years, and it is not clear whether the increased detection of JE in the highlands is due to changes in environmental factors or is a consequence of extended surveillance(Baylis et al., 2016). More laboratory research is needed to investigate whether JEV distribution has expanded to Dwc.
The areas in a temperate climate (including Cwa, Cwb, Dwa, and Dwb) were at high risk of JE, with more JE cases with the moderate-high epidemic temporal index and showed the most high-high cluster (α, β, and γ). Notably, the Cwb was the dominant JE epidemic area in Gansu; compared to other climate zones in Gansu, it has more hot-spot (High-High cluster area) counties of α, β, and γ. This result may indicate that the Cwb has more favorable climatic conditions or breeding sites that may facilitate JE transmission—in that field study and laboratory research are needed in Cwb areas.
The current study highlights the average temperature at 24°C for JE at the lag of 1 month in Cwa, the maximum temperature at 28°C, 29°C, and 28°C for JE at the lag of 1 month is the most important predictor of JE incidence rate in Cwb, Dwa, and Dwb, respectively. As such, temperature is the dominant factor which influence the JE. Previous studys have also demonstrated that temperature is the primary factor influencing JE's transmission(Wang et al., 2014). Because the optimal temperature can decrease the larval development time and reduce the extrinsic incubation period for the virus(Wang et al., 2014), which further increases the mosquitoes' density and the virus transmission. For example, the 22.8–34.5°C(Murty et al., 2010) and 21.025.2°C(Bi et al., 2007) temperature intervals were optimal for JE transmission. Besides, the threshold values and type of temperature for JE responses varied in different climates. Precipitation was only found to be positively correlated with JE is Cwa. More detailed studies are needed to investigate whether the Cwa has more favorable climatic conditions and widely available breeding sites compared with other climate subtypes.
To the best of our knowledge, this is the first study to use the town as the unit of analysis in climate subtypes of Gansu to examine the relationship between weather conditions and JE. The CART analysis helps us find out the interactive effects of weather on JE with a further determination of the threshold values of the potential predictors of JE. Specific results of threshold limits from CARTs could be used for decision making and the lag time associated with the thresholds allows the government to prepare for the outbreaks. However, there are also some limitations to our study. Although this study could interpret the spatial heterogeneity among climate subtypes, it cannot explain why spatial heterogeneity exists within climate subtypes. Furthermore, this study could help to identify emerging JE epidemic areas, but more studies on etiology are essential for the determination of an emerging epidemic area of JE; these include disease surveillance, vector mosquito surveillance, investigation of the virus load in vector, host (pigs), and antibody levels in healthy people.