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Global warming could increase the risk of schistosomiasis infection in China and expand infection into the country's northern territories, say researchers.

A study published in the March issue of Advances in Climate Change Research suggests that increasing temperatures could increase numbers of the Schistosome worm, which causes the disease and the Oncomelania snail in which it lives.

Scientists found the worm in China's southern regions and distributed along China's Yangtze River, but the worm could spread to northern regions if the temperature of 15.2 degrees centigrade for the worm to grow is reached in those areas.

"A warming trend is likely to be one of the reasons contributing to the epidemic increase of schistosomiasis during recent years," said Yu Shanxian of Zhejiang Meteorological Institute and lead researcher of the study.

Yu's team analysed average daily temperatures between 1950 and 2003 using data from China's National Meteorological Information Center.

The researchers found that an increase in the length of time environmental conditions were ideal for the growth of both the Oncomelania snail and the Schistosome worm.

In the most affected regions, the time period in which daily average temperature were no lower than 15.2 degrees centigrade increased by up to 15 days. The period where the temperature was no lower than 5.9 degrees centigrade ― the temperature required for Oncomelania snail growth ― increased by 15–30 days.

They also found that the temperature increases had extended to the north by one to two latitudes.

The temperature increase had also shortened the length of the reproductive cycle for snails and worms, leading to an increase in the numbers of both.

Yu pointed out that though an increase in schistosomiasis infection is possible theoretically, they found no evidence that any region previously free of the snails and worms was now affected due to warmer temperatures.

However, previous studies have shown that both were able to survive in northern China if the temperature reached a satisfactory level for growth.

Yang Kun, a research fellow with Jiangsu Institute of Parasitic Diseases said that climate is only a single factor affecting the spread of schistosomiasis. "Other factors are still not clear," he said.

But both Yu and Yang agreed that China's current projects to divert water from the south to the north could increase the spread of the disease.

Schistosomiasis is endemic in 74 developing countries, according to the World Health Organization.

Link to full article/paper in Advances in Climate Change Research
[in Chinese]

Reference: Advances in Climate Change Research 3, 106 (2007)

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