(CNN) For the first time, a case of rat hepatitis E has been discovered in a human in Hong Kong.

A 56-year-old man has been diagnosed with the disease, researchers from the University of Hong Kong said. It was not previously known the disease could be passed from rats to humans.

"Previous laboratory experiments have found that rat hepatitis E virus cannot be transmitted to monkeys, and human hepatitis A virus cannot be transmitted to rats," said Dr. Siddharth Sridhar, clinical assistant professor at the University of Hong Kong, explaining that monkeys are very close to humans when it comes to disease susceptibility.

The risk of rat hepatitis E affecting humans has been underestimated, Sridhar said during a news conference.

The man developed the disease after undergoing a liver transplant following chronic infection with hepatitis B. He continued to show signs of abnormal liver function, said Sridhar, with no obvious cause.

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