Veterans returned to the U.S. after fighting in the jungles of Vietnam a half century ago, but hundreds of them may be dying from a silent bullet — a slow-killing parasite living in the men decades after the war, a new study revealed.

Liver flukes, parasites that infect a human when raw or undercooked fish is eaten, are being investigated as the cause of a rare bile duct cancer among veterans who served in the Vietnam War. It could take years for symptoms to show up, but when they do, the host is left with tremendous pain and given just a few months to live.

The Department of Veterans Affairs this spring commissioned a small pilot study to look into the link between liver flukes and the cancer. More than 20 percent of the 50 blood samples submitted to the study came back positive or bordering positive for liver fluke antibodies, said Sung-Tae Hong, the tropical medicine specialist who carried out the tests at Seoul National University in South Korea.

Gerry Wiggins, who served in Vietnam from 1968 to 1969, agreed to take part in the study even though he didn’t have any symptoms. The 69-year-old said he has already lost friends to the disease. To his surprise, liver fluke antibodies were detected in him.

"I was in a state of shock," he said. "I didn't think it would be me."

SALMONELLA POISONING SYMPTOMS AND WARNING SIGNS

Further tests showed two cysts on his bile duct what could develop into cancer known as cholangiocarcinoma. They were removed and Wiggins is doing well.

Everyone who tested positive for the antibodies was notified, Northport VA Medical Center spokesman Christopher Goodman told the Associated Press. Goodman would not comment on the findings. The parasites infect about 25 million people worldwide, mostly in Asia. It’s very uncommon among Americans.

The parasites can be killed in infected humans through drugs during the early stages. But they can also live in humans without treatment for decades because no symptoms show up — much like veterans who fought in the jungles of Vietnam during the war. Over time, swelling and inflammation of the bile duct can lead to cancer. Jaundice, itchy skin, weight loss and other symptoms appear only when the disease is in its final stages.

Mike Baughman, 65, who has bile duct cancer that his doctor said was “more likely than not” caused by liver flukes, was granted a claim for service-related benefits early this year after being denied three times. Baughman was probably infected when his unit ate uncooked fish in the Vietnam jungle after they ran out of rations. He gets about $3,100 a month and said he's relieved to know his wife will continue to receive benefits after he dies. But he remains angry that other veterans' last days are consumed by fighting the same government they went to war for as young men.

"In the best of all worlds, if you came down with cholangiocarcinoma, just like Agent Orange, you automatically were in," he said, referring to benefits granted to veterans exposed to the toxic defoliant sprayed in Vietnam. "You didn't have to go fighting."

"Personally, I got what I needed, but if you look at the bigger picture with all these other veterans, they don't know what necessarily to do," he said. "None of them have even heard of it before. A lot of them give me that blank stare like, 'You've got what?'"

MOM JAILED OVER SON'S 'SENSELESS' STREP THROAT DEATH

The VA study, along with a call by Senate Minority Leader Charles Schumer of New York for broader research into liver flukes and cancer-stricken veterans, began after The Associated Press raised the issue in a story last year. The reporting found that about 700 veterans with cholangiocarcinoma have been seen by the VA in the past 15 years. Less than half of them submitted claims for service-related benefits, mostly because they were not aware of a possible connection to Vietnam. The VA rejected 80 percent of the requests, but decisions often appeared to be haphazard or contradictory, depending on what desks they landed on, the AP found.

The number of claims submitted reached 60 in 2017, up from 41 last year. Nearly three out of four of those cases were also denied, even though the government posted a warning on its website this year saying veterans who ate raw or undercooked freshwater fish while in Vietnam might be at risk. It stopped short of urging them to get ultrasounds or other tests, saying there was currently no evidence the vets had higher infection rates than the general population.

"We are taking this seriously," said Curt Cashour, a spokesman with the Department of Veterans Affairs. "But until further research, a recommendation cannot be made either way."

The Associated Press contributed to this report.