The gigantic Three Gorges Dam project and global warming could be to blame for the recent rodent outbreak in central China's Hunan Province.

The rodent scourge surrounding Dongting Lake, China's second-largest freshwater lake, had been for the most part controlled. In a recent two-week mouse offensive, residents of the 22 counties encircling the lake eradicated some 2.3 million of the estimated 2 billion rodents.

The rodents were driven out into populated areas when floodwaters burst out in late June.

Shi Dazhao, director of the Chinese Agricultural University's laboratory on the prevention and control of rodents, and a consultant to the Chinese Association for the Control of Rodents and Sanitary Insects (CPCA), attributed the magnitude of this year's rat pest problem to the gigantic dam project and global warming.

His assessment was partially echoed by Wu Chenghe, chief of the plantation protection station for Datong Lake, a sub-lake of Dongting Lake. Wu argued that interception of the upper watershed had lowered water levels and created ideal conditions for a rodent outbreak.

The fact that water levels did not rise during last year's flood season also helped boost rat numbers, he said.

But while the media have called it the region's most severe rat infestation in the past decade, Deng Zhi, a senior researcher with the Academy of Military Medical Sciences, told China Daily that rodent outbreaks were a natural annual event during flood seasons.

"Running for their lives, the rodents always come and go during flood seasons and there's hardly a way to stop them," he said. The root cause of this year's particularly massive rat attack was ongoing human polderization, he added.

"Polderization violates the laws of ecology," he said.

"It not only facilitates floods, but also creates an ideal condition for rodent reproduction."

Another reason is nearby counties' reliance on reeds for paper production, and a source of food of rodents.

"The more reeds people grow, the more rodents and paper factories there are, the more pollution there is, and the more serious the rat problem becomes," Yang Hualin, director of the CPCA, said.

Experts believe Hunan's rat control campaign was tricky to solve.

Shi said: "It's not that we can't kill all the rodents, but that it's not worth doing. Just how much money, time and effort ought the government to spend in trying to protect each hectare of farmland?

"But if we were to let the rats run wild, the crops would be ruined. And if we were to truly resolve the issue through lake restoration, where should all the surrounding residents be relocated and how will they live?" Shi proposed several immediate and long-term solutions to the problem. For instance, the government should try to control rodent reproduction before each flood summer, and farmers should avoid growing crops during the period.

Monitoring stations with an organized and trained workforce should also be established immediately, because "prevention is key to everything", he said.

In the long run, coordinated development and supervision at all levels for a national preventive scheme was a must, he said, adding that a disease prevention fund may be useful.

The Ministry of Health launched its first strategic national scheme on the prevention and control of emerging infectious diseases on June 20. But a comprehensive project to facilitate inter-departmental prevention and control efforts was still unavailable.

(China Daily July 16, 2007)