Beijing - Twenty-three people in Beijing have fallen ill since May after eating snails, according to the Beijing Health Bureau.

Eighteen are still in hospital - five of them are seriously ill. The other five have made a complete recovery.

The youngest patient is a 13-year-old, who had a fever and a stiff neck a day after he had eaten an Amazonian snail salad and a spicy snail dish in a branch of the restaurant chain called Sichuan Legend.

All the patients had eaten the snails in two restaurants, both branches of Sichuan Legend. Doctors suspect that a batch of the Amazonian snails was contaminated.

According to Xu Rongman, researcher with the Institute of Microbiology and Epidemiology under the Academy of Military Medical Sciences, each Amazonian snail is an intermediate host of 3,000 to 6,000 parasites, which can harm the human nervous system, leading to headaches, facial paralysis, meningitis and fever.

The Beijing Office of Food Safety issued an urgent notice on Saturday, calling for tighter supervision over aquatic products and safety inspection in supermarkets, shopping malls and restaurants. The office also warned people against eating raw fish, shrimp, snail, crab, frog and snake.

Amazonian snails originate from South America and first came to China in the 1980s. The first patient of Angiostrongylus Cantonensis, the disease caused by the snails, was reported in Guangzhou, capital city of south China's Guangdong Province.