Climatic Impacts on Japanese Encephalitis, Three Gorges Dam, China

by Sarah King

Japanese encephalitis (JE) is a prevalent, mosquito-borne infectious disease found throughout the Asian Pacific Rim and Southeast Asia and most predominately in China (Bai et al. 2014). The Chinese province Chongqing has one of the highest incidence rates of JE in the country in combination with only four other provinces, make up 50% of the incidence of JE in all of China, with only 26% of the population. Consequently, Chongqing is an interesting place to study the effect of climatic change on Japanese encephalitis, which is exactly what Yuntao Bai and his colleagues did. Bai and his team set out to identify the most important climatic variables that induce the transmission and spread of the JE virus in Chongqing from 1997–2008, and what kind of geographical incidence patters arise in relation to climate change (Bai et al., 2014).

Japanese encephalitis (JE) continues to be prevalent at a relatively high rate in China compared to the rest of eastern Asia. China has done of good job administering vaccines for JE, which has helped reduce its incidence significantly since the 1980s, but it is still a health concern across the country. There are three reasons that JE is a continued health risk in China, even with the availability of a vaccine. First, there is minimal awareness of JE, so people do not know to get vaccinated. Second, the JE virus can mutate into vaccine resistant strains. Finally, changes in human behavior, such as leisure time and increased tourism, have contributed to the rate of human exposure to JE.

Bai and his team explored the climatic variables that advanced JE virus transmission and spread in the province of Chongqing, along the Yangtze River in 1997–2008. To do this, they obtained monthly reported cases of JE from the Chinese Center of Disease Control and Prevention (CDC), along with the monthly average temperatures, relative humidity, rainfall, and sunshine duration. Next, they used simple linear regression to model the trend of the climatic variables during 1997–2008 and then analyzed the incidence rate of JE in relation to the climatic variables. The researchers also included a GDP variable to control for the growing economy in the area. The final model contained incidence, GDP, temperature, and rainfall.

From the model, Bai et al. gained several significant findings. First, there was a positive association between the three-month lag temperatures and the incidence of JE. In other words, it takes three months for temperature to affect the incidence of JE. They explained that temperature can affect the development of mosquito larvae, which then can help or hinder the spread of the virus, depending on the specific conditions (warmer temperatures speed development, help the spread of the JE virus). Second, they found a negative association between precipitation and JE, which is contrary to most previous studies. Evidently, the excessive rain that Chongqing receives during the monsoon season most likely ruins mosquito breeding grounds or life cycles. Third, Bai and his colleagues found that there was a clear seasonal trend in the number of JE cases from July to September. They guessed that the combination of temperatures and precipitation levels during those months provides the best conditions for mosquitoes to breed and infect. Finally, there was a general decrease in the incidence of JE throughout the study period, but with markedly high rates in 2000, 2001, and 2006, corresponding to relatively higher temperatures and less precipitation.

Ultimately, Bai et al. found that temperature and rainfall are essential climatic variables for determining the incidence of JE. This study provides strong evidence to support the importance of these variables that future researches can use to predict what climate change will do to the incidence of JE in China. Although further research must be done to confirm this study, the current findings may be very useful for other research groups to determine useful variables for JE incidence projections.

Bai, Y., Xu, Z., Zhang, J., Mao, D., Luo, C., He, Y., Liang, G., Lu, B., Bisesi, M., Sun, Q., Xu, X., Yang, W., Liu, Q., 2014. Regional Impact of Climate on Japanese Encephalitis in Areas Located Near the Three Gorges Dam. PLoS ONE 9(1), e84326.

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