A man in Hong Kong has been found to have a strain of hepatitis E that had only been seen previously in rats, researchers said on Friday.

While rats have passed on several other illnesses to people, this was the first discovery in humans of the rat variation of hepatitis E, a liver disease. Researchers at the University of Hong Kong identified the infection last September in a 56-year-old man who had received a liver transplant in May 2017.

He was cured of the disease in March, after which they verified their findings before announcing the case.

Dr. Yuen Kwok-yung, chairman of the infectious diseases section of the microbiology department at the University of Hong Kong, called the case “a wake-up call.”

And Dr. Siddharth Sridhar, a clinical assistant professor in the university’s department of microbiology, said it suggested that Hong Kong needed to work harder on rodent control, as the city did during the SARS epidemic of 2003 and 2004.

In densely populated areas like Hong Kong, “infections that jump from animals to humans must be taken very seriously,” Dr. Sridhar said. “For these kinds of rare infections, unusual infections, even one case is enough to make public health authorities and researchers very alert about the implications of the disease. One is all it takes.”

Although the patient had received a liver transplant, researchers ruled out infection from his blood and organ donors, who all tested negative for the disease. The researchers instead highlighted rat droppings, piles of uncovered garbage and open passageways found near the patient’s home as major risk factors in rodent-borne diseases.

The researchers said that routine hepatitis E testing would have failed to identify the strain, which is significantly different from the one that typically infects humans. They detected the source of the patient’s infection in this case after finding similarities with infected rats in Vietnam.

When they tested rodent samples that health officials had collected in his neighborhood in recent years, they found Hepatitis E in at least one rat.