A SYDNEY man is critically ill after eating a slug as a dare, leading authorities to issue health warnings on devouring raw gastropods.

The 21-year-old contracted rat lungworm disease - a rare form of meningitis - after the stunt.

Rat lungworm disease is caused by Angiostrongylus cantonensis, a parasitic worm that is carried in the pulmonary arteries of rats. Larvae are excreted in the rat's droppings, which are often eaten by slugs and snails.

The disease can cause fatal swelling of the brain and spinal cord.

However, Jeremy McAnulty from New South Wales Health said most people do recover. He said the worm goes away by itself because the body's immune system eventually gets rid of it.

Dr McAnulty said people should not eat raw slugs or snails and should ensure they wash their hands after touching them.

"It is also important to thoroughly wash and cook any produce that could be contaminated by animals," he said.

The worm was found mainly in Asia and the Pacific Islands. It was rare for people to contract rat lungworm disease in Australia, but it occurred occasionally from eating an uncooked slug or snail, Dr McAnulty said.

Infected people may have no symptoms or only mild short-lived symptoms, he said.

Sometimes the infection causes meningitis, with symptoms such as headaches, stiff neck, tingling or pain in the skin, fever, nausea, and vomiting, Dr McAnulty said.

"Even if infected, most people recover fully without treatment," he said.

"However, it can sometimes cause severe meningitis.

"Because humans are not the natural host of the parasite, the parasite eventually dies without treatment."

Rat lungworm infection cannot be passed on to other people.