OSAKA (Kyodo) -- Drugs used to treat human patients for hepatitis C infections cleared the virus from chimpanzees deliberately infected for research, including some infected more than 40 years ago, a recent study by a team of Japanese researchers showed.

The study, published in the July issue of the Journal of Medical Virology by researchers from Kyoto University and Kumamoto University, found that direct-acting antivirals for humans could also effectively eradicate long-standing hepatitis C virus infection in chimpanzees.

"We should focus on the remaining lives and welfare of chimpanzees used in experiments. I hope the findings of this study will be put to use," said Satoshi Hirata, a professor at Kyoto University.

Beginning in the 1970s, researchers in Japan infected chimpanzees with a then-unidentified virus -- later determined to be hepatitis C -- to study its symptoms and treatments. Hirata said many chimpanzees outside Japan were also infected for experiments, and a study published in 2022 found infected chimpanzees lived about 10 years less than uninfected ones.

Between 2023 and 2024, the team administered powdered human drugs mixed into food or drinks to eight infected chimpanzees for various periods depending on the amount of virus in their bodies.

Six had no detectable virus six months after the treatment ended, while two still tested positive. Of the two, one did not respond even after another drug was added, possibly due to the virus developing a drug resistance.

While the liver function improved in the six chimpanzees whose virus was cleared, two later died of liver cancer.

They are believed to have already developed the cancer by the time the treatment was administered as they had been infected for 40 years.

"It is necessary to regularly monitor whether infected chimpanzees have developed tumors," Hirata said.