A mass swine epidemic is not what killed nearly 6,000 pigs that have floated downstream to Shanghai from Jiaxing, Zhejiang province, since last week, officials said on Tuesday.

"No abnormal animal epidemic was reported in Jiaxing," Jiang Hao, from the city's veterinary department, said at a news conference.

Local authorities are assisting Shanghai's veterinary departments in the investigation into what killed the pigs found floating on the Huangpu River, he said.

Dead pigs have been found in the upper Songjiang section of the Huangpu River, about 100 km from Jiaxing, since last week.

Shanghai authorities said on Monday that they have determined from the animals' ear tags that they came from Jiaxing. Tests have revealed that some pigs may have died from porcine circovirus, a common pig disease.

Many pigs had died in Jiaxing in the first two months of the year, local media reported.

According to the Jiaxing Daily, nearly all the 1,400 households in Jiaxing's Zhulin village keep pigs. In January, 10,078 pigs died, and another 8,325 died in February.

Bacteria can breed easily since pigs often live in overcrowded pens. Many pigs became sick and died. The report also said local villagers simply threw away the dead pigs because there is not enough land to bury the carcasses.

Jiang said the authenticity and accuracy of the media reports remain to be proven. He didn't provide the number of dead pigs in the past two months.

He said that about 7 million pigs are raised each year in Jiaxing, and that most are raised by local villagers. The mortality rate of the animals is at the normal level of less than 3 percent.

Earlier, an official from Zhejiang province's agriculture department said that many pigs froze to death earlier this year. But that wasn't confirmed by Jiaxing authorities.

"Many reasons may lead to the death of pigs. We are still investigating the cause," said a Jiaxing government spokesman named Hu.

"Pigs easily get sick and die in bad weather, especially when they don't have enough protection," said Zhang Jianguo, from Jilin's Jingqishen Organic Agriculture Co, a business that specializes in pig farming and processing.

In recent years, a centralized pig-raising model has been encouraged in the country. The unified management system can lower the risk of an epidemic, he said.

"Due to a lack of scientific method and environmental awareness, some villagers often failed to prevent diseases among pigs, which may lead to mass deaths," he said.

Jiang added that Jiaxing has 600 non-hazardous treatment stations for dead pigs, and that farmers who hand over dead pigs will receive a subsidy of 80 yuan ($12.75) per carcass.

He acknowledged that some individual farmers had disposed of the carcasses directly into the rivers without any treatment because they believed that the "dead pigs are very unlucky".

By Monday, a total of 5,916 carcasses had been retrieved from the river, according to Shanghai agriculture authorities.

Most of the dead pigs retrieved in Shanghai have been buried in 7-meter-deep holes and covered with at least 3 meters of thick soil. Some carcasses were incinerated.

Contact the writer at wanghongyi@chinadaily.com.cn