A rare bile duct cancer that may be linked to time served in the Vietnam War is quietly killing some former soldiers. The disease can be caused by liver flukes, a parasite found in raw or undercooked fish that is common in parts of Asia. Some veterans are fighting for the Department of Veterans Affairs to recognize their disease as service-related so they can receive benefits, but most claims are denied.

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WHAT ARE LIVER FLUKES?

Liver flukes are parasites ingested in raw or undercooked freshwater fish. They are endemic in parts of Southeast Asia, including Vietnam, along with other areas, mainly in China and South Korea. Some 25 million people are infected with the worms. Liver flukes die when frozen, but they can survive fermentation or pickling. Visitors traveling in endemic areas can also be infected.

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WHAT IS CHOLANGIOCARCINOMA, AND HOW DO LIVER FLUKES CAUSE IT?

Cholangiocarcinoma is a rare cancer that affects the bile duct. Liver flukes are a risk factor; others include hepatitis B and C, cirrhosis and bile duct stones. After the worms are ingested, they can live for more than 25 years in the bile duct, causing inflammation and scarring that can eventually lead to cancer. The disease is difficult to treat, with many victims dying within months of diagnosis. A patient typically does not experience any symptoms, such as jaundice, until the end stages.

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CAN THIS CANCER BE TREATED?

Bile duct cancer is unusual because it can be prevented in some cases. Pills can wipe out liver flukes early on, but the medicine is not effective in later stages after the worms have died and scarring has occurred. Surgery is possible in some cases, but the survival rate is only about 30 percent for five years, said Dr. Gregory Gores, a gastroenterologist and executive dean of research at Mayo Clinic. Affected countries, such as Vietnam and Laos, have not conducted extensive research to determine the extent of the problem. The world's highest rate of cholangiocarcinoma — about 84 new cases per 100,000 people — is found in northeastern Thailand where many people eat a popular raw fish dish. In the U.S., cholangiocarcinoma is extremely rare, with around 1.7 in 100,000 people diagnosed each year.

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WHAT IS THE CONNECTION TO VIETNAM VETERANS?

Men who served in the Vietnam War and ate raw or poorly cooked fish, sometimes while on patrol in the jungle after their rations ran out, could have been infected by liver flukes. Left untreated, they can experience symptoms related to bile duct cancer decades later. Because the disease is so rare and awareness about liver flukes is poor in the U.S., many veterans may not be aware of the possible connection to their service time.

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ARE VETERANS WITH THIS CANCER ELIGIBLE FOR FINANCIAL HELP?

Each case is examined individually, and it's up to veterans to prove to the Department of Veterans Affairs that their cancer is "as likely as not" related to their service time. The VA says fewer than 700 cholangiocarcinoma patients have passed through its medical system in the past 15 years. In part because they are unaware of the potential link to their wardays, only 307 of the veterans submitted claims for benefits over that period. Even though the VA sometimes approves the link between wartime service and cholangiocarcinoma, the vast majority of claims — 3 out of 4 — are rejected, according to data obtained by The Associated Press through the Freedom of Information Act.