More than 20 Chinese, sickened after eating crayfish, suffered the effects from Haff, a rare disease often associated with the consumption of fish, Chinese disease control experts said Tuesday.

Twenty-three residents in Nanjing, capital of east China's Jiangsu Province, fell ill beginning in late July, suffering pains in the back and waist.

Symptoms reported by patients, such as muscle and joint pain without a fever, or neural paralysis, matched those of Haff disease, experts from the Chinese Center for Disease Control And Prevention (China CDC) and the Nanjing Food Safety Committee said at a press conference Tuesday in Nanjing.

"Haff disease is usually caught by patients who have eaten aquatic products during the previous 24 hours," said Wu Yongning, researcher at the Institute for Nutrition and Food Safety (INFS) under the China CDC.

"Medical experts all over the world have not discovered the exact cause of the disease," added Wu.

It was previously suspected that the cause of the poisoning was excessive residue of a chemical called oxalic acid, which is believed to be illegally used to clean crayfish.

The crayfish-related poisoning had again aroused wide concern over food safety among Chinese consumers.

However, experts from the China CDC and its Beijing and Jiangsu branches did not find any suspected chemical from both crayfish sold in the market or blood sample of the patients, Wu said.

Among those sickened, 22 patients have recovered and been discharged from hospital, with another person remaining in hospital for continuing treatment.

Haff disease is the development of rhabdomyolysis, or swelling and breakdown of muscle mass with a possibility of kidney failure and even death, Wu said.

The disease was first detected in 1924 at Haff Beach on the Baltic coast, and more than 1,000 patients have been identified with having contracted it since then.

In 2000, six residents from Beijing were also found to have come down with Haff disease after they ate crayfish.

"It might be the toxin borne by individual crayfish that caused the disease, because others who had eaten crayfish together with the patients did not fell ill," Wu said.

He speculated that a small number of crayfish might carry some unknown toxin, which was possibly taken from a certain area of the waters, but this could not be proven with certainty, yet.

The team offered advice and monitoring methods to prevent further cases. Local food safety inspection departments are also required to closely monitor the outbreaks to investigate the exact cause.

(Xinhua News Agency September 8, 2010)