A middle-aged man from Hong Kong has developed the world’s first case of rat hepatitis E.

Researchers say it is unclear how the 56-year-old male contracted the virus — but trash receptacles outside his home were reportedly infested with rats.

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“There had been no previous evidence that this strain of the virus could be transmitted to humans,” reported the BBC.

“The human version of hepatitis E is usually spread through contaminated drinking water.”

Doctors discovered the man had the strain of hepatitis when tests on him revealed abnormal liver function following a liver transplant.

Further tests showed that the individual was carrying a strain of hepatitis “highly divergent” from the strain that affects humans, researchers from the University of Hong Kong said.

“We postulate that contamination of food by infected rat droppings in the food supply is possible,” they said in a report, as the BBC noted.

The man is said to be recovering from the serious acute disease.

Symptoms of the human strain of hepatitis E include jaundice, tiredness, fever, nausea, vomiting, and abdominal pain.

The human version of hepatitis E affects 20 million people globally each year, according to the World Health Organization.

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Most people do recover from the virus, although for some — such as those with an immune deficiency disorder or pregnant women — it can prove fatal.

It is common for diseases to spread from animals to humans — more than 60 percent of emerging infectious diseases begin in animals, said the BBC.

A sustained period of hot and humid weather has caused rodent problems in Hong Kong to escalate, multiple sources reported.

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Hong Kong has been hit hard by outbreaks of disease — in 2003, almost 300 people died from severe acute respiratory syndrome — or SARS.

The bubonic plague, also known as the Black Death, which is carried by rats, swept through mainland China and Hong Kong in the late 19th century, killing thousands of citizens.

In 2014, the Chinese city of Yumen was sealed off and 151 people were placed in quarantine after a man died of bubonic plague.

The 38-year-old man died after he had been in contact with a dead marmot, a small animal in the squirrel family.

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