A 70-year-old woman in Hong Kong has just become the second ever recorded case of rat-specific hepatitis E in humans, worrying Chinese researchers. One case could be an anomaly. Two cases could mean that the disease is becoming transmissible between species.

"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 woman, who has a compromised immune system, was admitted to hospital in May of 2017, complaining of a variety of symptoms consistent with the human version of hepatitis E, including abdominal pain, loss of appetite, weight loss, and headaches, the Hong Kong Health Department said on Wednesday.

She was treated and sent home, and has since recovered.

But in September of this year, when an immunocompromised 56-year-old Hong Kong man from the Choi Wan Estate in Kowloon became the first known case of rat hepatitis in humans, health officials started investigating.

They tested the saved blood samples of 70 patients in the hospital who had been diagnosed with hepatitis E, and the woman - from the Wong Tai Sin district, also in Kowloon, just 3 kilometres from the Choi Wan Estate, and hospitalised within weeks of the man - came up positive.

However, while the two strains are what Sridhar described as "uncannily similar," there is no direct link between the two patients.

The human version of hepatitis E is spread when traces of human faeces containing the virus contaminate water, objects, or hands that come into contact with the mouth. It can also be contracted from eating undercooked meat, but by far the most common vector is faeces-contaminated drinking water.

Exactly how it's transmitted by rats is unclear, but Hong Kong in recent years has been plagued by a growing rat population.

When the first case was discovered, researchers speculated that the man had eaten food contaminated with rat droppings; or, possibly, been bitten by a rat without noticing.

The source of the woman's infection could not be determined either. She reported that she hadn't noticed rats in her home, and that she didn't recall any contact with rats. But that doesn't mean the rats weren't there.

"Not seeing did not mean there was no contact," Sridhar said. "It was possible that rodents' excreta somehow got into food which the patient ate."

In September, experts at Hong Kong University said the first case was a wake-up call to improve public hygiene, and Hong Kong's Centre for Health Protection has urged the Food and Environmental Hygiene Department to implement rodent control measures.