A small pilot study hints that a startling number of Vietnam veterans may be infected with a liver parasite that can induce a rare type of cancer, the Associated Press reports.

The study, conducted by the Northport VA Medical Center in New York, involved blood samples from 50 Vietnam veterans. Testing performed at Seoul National University in South Korea found that more than 20 percent of those samples were positive or borderline positive for antibodies against the parasite, a liver fluke.

The results are preliminary and require follow-up research. It’s also unclear how the 50 blood samples were chosen. That said, the results hint that many veterans may have the cancer-inducing infection and not yet know. The study follows a report last year by the AP, which raised questions about the rate of that otherwise rare type of cancer in veterans.

The results of the pilot study were “surprising,” Sung-Tae Hong, the tropical medicine specialist behind the testing, said.

Liver flukes are prevalent in East Asia and are contracted by eating raw or undercooked fish. After being swallowed, the flat, leaf-like parasites (Opisthorchis viverrini, O. felineus, and Clonorchis sinensis) can migrate into the liver, latching on to the lining to suck blood. Though damage from the infection develops quietly—sometimes taking several decades before it’s noticed—the final results can be deadly.

The flukes cause tissue damage that snowballs into ulcerated lesions and chronic inflammation. Over time, their eggs become embedded in tissue, encased by legions of immune cells. These damaging clusters can block bile ducts, which drain bile from the liver into the small intestines. The parasites also excrete toxic metabolites, which can cause cells to proliferate, prodding the development of cancer.

Infection with liver flukes has long been linked to bile duct cancer, called cholangiocarcinoma. It’s a relatively rare type of cancer in the US, with an incidence of around two cases out of 100,000. However, those rates have been on the rise.

Last year, the Associated Press raised questions about the prevalence of cholangiocarcinoma among Vietnam veterans and whether their cancer treatments should be covered by the Department of Veterans Affairs as service-related. About 700 veterans with cholangiocarcinoma were seen at VA facilities in the past 15 years. But less than half filed for service-related benefits. It’s likely that many did not know that their cancer may be linked to parasite exposure during their time in Vietnam. Moreover, the VA rejected 80 percent of the requests.

The AP reported that those rejections appeared to be haphazard decisions. And though the VA has posted information on its website warning that veterans may have been exposed to liver flukes, the VA continues to reject most requests.

The VA notes that there are no studies on whether cholangiocarcinoma is more common in Vietnam veterans than other groups. The new study is the first to try to address that unanswered question.

In a statement to the AP, VA spokesperson Curt Cashour said that the VA is “taking this seriously. But until further research, a recommendation cannot be made either way.”