HONG KONG — Researchers said Wednesday they have found a second patient in Hong Kong who contracted a strain of hepatitis carried by rats, in what appears to be the first known human cases in the world. The finding surprised the researchers, though it wasn't immediately clear whether there were significant implications for human health.

"Because the rat ... strain is very different from the human strain, people think it wouldn't be able to jump to humans," said Siddharth Sridhar, one of the principal researchers at Hong Kong University. "This was a clinical discovery."

The first case came out in September. Researchers confirmed a 56-year-old man had a hepatitis E strain previously known only in rats in Vietnam. Doctors later found he had a strain of hepatitis that was "highly divergent" from other strains found in humans, BBC News reported.

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The second case was found after blood samples from more than 70 hepatitis E patients were tested.

A 70-year-old woman with a compromised immune system was found to have been infected with the hepatitis strain, the Hong Kong Health Department said Wednesday in an emailed response to questions.

She had been admitted to a hospital in May 2017 with abdominal pain, headache, anorexia and other symptoms. The blood samples collected from her at the time recently tested positive.

Both patients were hospitalized only weeks apart and live within three kilometers (two miles) of each other.

"They form a cluster - they're linked in time and space," Sridhar said. The strains found in the patients "are uncannily similar."

However, the Health Department said "the sources and routes of infection of these two cases could not be determined."

While rats are known to transmit a number of other diseases to humans, including plague, Lassa fever and leptospirosis, these are the first reported cases in humans of the rat variation of hepatitis E.

Other strains of hepatitis E are often transmitted among humans through contaminated water, particularly in East and South Asia. In China, many are stricken after eating undercooked pork and innards. China has developed and licensed a hepatitis E vaccine but it isn't yet available outside the country.