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Japanese encephalitis incidence during Three Gorges Dam construction investigated
Changes in temperature and rainfall were key variables in the incidence of Japanese encephalitis in the areas near China’s Three Gorges Dam between 1997 and 2008, according to recent study findings.
In the study, researchers gleaned data from three geographical regions (East, Middle and Western regions) in Chongqing, China, along the Yangtze River, consisting of 12 districts.
They analyzed dimensional patterns of Japanese encephalitis virus (JEV) spread by calculating the annual occurrence for each of the 12 districts, each of the three regions, and the entire area from 1997 to 2008. From this, they determined monthly incidence of JEV for each region and district, and charted epidemic curves for the three regions. This information was used to establish the JEV patterns between 1997 and 2008. Using the population of each area at a given time period, the researchers calculated the incidences for each district, each geographic region, and the total area. The investigators then used Zero-Inflated Poisson Regression models to establish the climate-related variables affecting JEV transmission in the entire study area and each region, and used linear regression to determine climatic variations during the period spanning the Three Gorges Dam construction. The dam was completed in 2012.
The study found that throughout the entire city of Chongqing, the occurrence of JEV was reduced between 1997 and 2008, with notable fluctuations seen in 2000, 2001 and 2006. The highest incidence of JEV was observed in the eastern region, which is in closest proximity to the Three Gorges Dam. The western region had the lowest incidence. Seasonal climatic variations during this period were discovered via linear regression models. According to the models, there was a significant positive correlation between temperature (with a 1- to 3-month lag) and incidence of JEV, and a significant inverse relationship between rainfall (with a 0- to 4-month lag) and JEV occurrence.
The researchers said the overall decrease in JEV occurrence may have masked any harmful effects of dam construction on climate fluctuations and JEV.
“The effects of dam construction on the changes of climatic variables and in turn on the JE incidence must be further studied,” the researchers wrote. “At least by now, the negative impact of [Three Gorges Dam] on [Japanese encephalitis] incidence might be too slight to neutralize the positive effects of improved sanitation and economic development in Chongqing.”
Disclosure: This work was supported in part by The Ohio State University Office of International Affairs to Qinghua Sun, and in part by the National Science Foundation.
Perspective
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Investigators of this study attempted to identify climatic factors associated with the transmission of Japanese encephalitis virus (JEV) in three areas near the Three Gorges Dam (TGD). The study analyzed data from 1997-2008, during the construction of the TGD.
The paper does highlight the existing JE risk in China, which is important for travel/tropical medicine providers. It illustrates some interesting ecological issues:
- Higher temperatures usually lead to shorter extrinsic incubation periods, and mosquitoes also breed faster at higher temperatures to a certain point. Some entomological literature describe these regarding dengue and there may be literature describing this regarding JE/Culex mosquitoes.
- Heavy rainfall may wash away the eggs/larvae, hence reducing the survival/maturation of mosquitoes. Some data from dengue/Aedes mosquito studies may correspond to this finding too.
Some other points worth mentioning include: First, TGD construction was associated with schistosomiasis in new areas of China, where the fresh water flooded areas previously not known to have schistosomiasis. Second, the ecological factors have complex interplay.
However, this raises questions about whether anyone has studied flight pattern alterations with heavy rainfall. This might be important because birds can spread JE. And if there has been correlation of heavy rainfall with mosquito counts, which might support or contradict the theory that heavy rainfall washes away eggs/larvae. Also, whether rice fields have been eliminated (or reduced overall) to expand towns/cities in the region, which can lead to decline in mosquitoes and hence JE virus.