Hundreds of Vietnam veterans are fighting a battle they never imagined nearly a half-century after the war. A rare bile duct cancer caused by parasites in raw or poorly cooked river fish can appear decades after infection and leave patients with just a few months to live.

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The worms infect an estimated 25 million people, mostly in Asia, but are less known in America, the Associated Press reported. These worms can easily be wiped out with a few pills early on, but when they’re less well known a cancer known as cholangiocarcinoma can develop.

The parasites typically go undetected, sometimes living for more than 25 years without revealing the infection.

The Veterans Administration acknowledges that liver flukes are likely killing some former soldiers. About 700 cholangiocarcinoma patients have passed through the agency’s medical system in the past 15 years, but less than half of those infected submitted claims for benefits — they were unaware of the potential link to their time in service.

Liver flukes are found mainly in parts of Southeast Asia, China and South Korea — anyone who consumes specific types of freshwater fish such as tilapia are at risk. Many American soldiers consumed the fish as a matter of survival during their service in the war. Health officials there are unsure of how many people may actually die from cholangiocarcinoma caused by the parasites, as there are few cancer registries.

The parasites typically go undetected, sometimes living for more than 25 years without making people aware they’re infected. But these organisms can cause inflammation and scarring and, over time, can lead to cancer. Symptoms are often jaundice, itchy skin and rapid weight loss. But by then, the disease is usually advanced.

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In the United States, cholangiocarcinoma is extremely rare, with roughly 5,000 people diagnosed each year, including some Asian immigrants who ate infected fish in their native countries, the AP reported. Liver flukes aren’t the only risk factor for the disease; others include hepatitis B and C, cirrhosis and bile duct stones.

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Patients hope that doctors in the U.S. might better understood bile duct cancer and the potential risks to those who served in Vietnam, so that they start automatically screening them for the parasite or the disease. Ultrasounds can be used to check veterans for inflammation. As with any cancer, early detection is key.