Shanghai's food authority has advised people to avoid uncooked shellfish during the winter, when outbreaks of gastroenteritis generally peak.

People are advised to avoid eating uncooked shellfish such as oysters and clams as they may contain noroviruses, the Shanghai Food and Drug Administration said in a notice released on Friday, Shanghai Oriental Morning Post reported.

Noroviruses are a common cause of food poisoning and are often found in polluted waters. They appear to be most active in cold weather. Symptoms include nausea, diarrhea, vomiting and abdominal cramps and the effects usually appear within 24 to 48 hours of exposure and typically last one to three days, the report said.

The virus is especially dangerous to children and seniors as it can cause lung infections and other life-threatening complications, the report said.

Norovirus cases were reported last winter in Beijing and Guangdong Province, along with Japan and Italy, according to Xinhua News Agency in previous reports.

(Shanghai Daily December 17, 2007)