Uncooked fish can cause human liver cancer, but most sushi lovers need not worry since the condition comes only from eating certain freshwater fish.

The human liver fluke, a freshwater parasite endemic to areas of Thailand, Japan, and Siberia, triggers human liver cancer by creating harmful cell mutations, encouraging tumor growth, and stopping normal cell death, according to a study released yesterday in the journal PLoS Pathogens.

The research could help prevent millions of people from developing liver cancer.

The International Agency for Research on Cancer "has classified Opisthorchis viverrini as a Class One carcinogen," said Sutas Suttiprapa, a scientist at George Washington University and a co-author of the PLoS Pathogens papers.

"I think that over the next year or next few years there will be a big campaign to treat and give these people some knowledge to stop eating raw fish."

There are actually three species of microscopic human liver fluke. O. viverrini is endemic to streams and lakes in Thailand, Laos, and Cambodia and was the actual species of fluke tested by the scientists. Another fluke lives in Siberia, and yet another is found in Japan and Korea.The parasite is only found in freshwater however, so eating sushi from the ocean is safe to eat.

Like many parasites, the human liver fluke has a complicated life span. An egg is first released from the human host. A snail eats the egg, which hatches and begins to develop.

Eventually a free swimming larvae emerges from the snail and attaches to the skin of a freshwater fish. When a human eats the fish raw, or as most people in Thailand are infected, by eating a delicacy of fermented fish, the parasite emerges from the small intestine and takes up residence inside the liver.

Once in the liver three things happen that eventually lead to cancer. First, the body tries to kill the fluke by producing free oxygen radicals. The fluke is largely immune from these attacks, however, so the radicals rebound, enter the body's own liver cells, and mutate human DNA.