For the first time, a case of rat hepatitis E has been discovered in a human in Hong Kong.
The man was diagnosed after doctors found his liver was functioning abnormally following a transplant, which was prompted by a chronic infection of Hepatitis B.
"This study conclusively proves for the first time in the world that rat HEV can infect humans to cause clinical infection", the university added.
But one man's infection isn't necessarily a sign of an onslaught of rat-mediated hepatitis infections to come.
The university said rat HEV is only distantly related to strains of hepatitis E which more commonly affect humans. "We don't know if in the future there will be a serious outbreak of the rat hepatitis E virus in Hong Kong".
It is usually spread through contaminated drinking water. "Doctors later found that he had a strain of hepatitis that was "highly divergent" from other strains found in humans".
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Hepatitis can be caused by various factors, including viruses or alcohol.
Symptoms of the human strain of hepatitis E include jaundice and sometimes tiredness, fever, nausea, vomiting and abdominal pain. To the transplanted organ, "stuck", the man used immunosuppressants. The animal form of the disease is thought to infect wild boars, domestic pigs and deer, as well as rats and other rodents.
Many people clearly have hepatitis, based on their symptoms, but they test negative on all the human strains known to exist, Adalja said.
There's no evidence of an imminent epidemic, the researchers said, but more work is needed to understand how and why the man got infected.
A sustained period of hot and humid weather has caused rodent problems in Hong Kong to escalate, multiple sources reported.
The human version of hepatitis E is normally spread by ingesting contaminated water. It was not previously known the disease could be passed from rats to humans.
Cases of HEV typically present in one of two ways, either as large outbreaks and sporadic cases in areas where HEV is endemic (genotype 1) in Asia and Africa, (genotype 2) in Mexico and West Africa and (genotype 4) in Taiwan and China or as isolated cases in developed countries (genotype 3), reports the WHO.