In 2018, infectious disease experts at Hong Kong University encountered an unusual patient.

A 56-year-old man, who underwent a liver transplant, was exhibiting abnormal liver function for no apparent reason. Tests revealed that his immune system was responding to hepatitis E - but they could not actually find the human type of hepatitis E virus (HEV) in his blood, CNN reports.

Hepatitis E is a liver disease that can also cause fever, jaundice and an enlarged liver. The virus comes in four species, which circulate in different animals; at the time, only one of them was known to infect humans.

With tests for that human strain of negative HEV, the researchers redesigned the diagnostic test, executed it again - and found, for the first time in history, the hepatitis E mouse in a human.

" Suddenly, we have a virus that can be transmitted from rats to humans, " he said. Siddharth Sridhar, a microbiologist and one of the HKU researchers who made the discovery. It was such an unusual and unprecedented infection that the team asked if it was a " one-time incident, a patient who was in the wrong place at the wrong time ."

But since then it has happened again and again

Since the first study, 10 other Hong Kong residents have been positively tested for mouse hepatitis E, also known as HEV. The most recent case came a week ago; a 61-year-old man with abnormal liver function tested positive on April 30th. And there may be hundreds of other unidentified infected people there, Sridhar said.

The human origin of hepatitis E is usually transmitted through fecal contamination of drinking water, according to the World Health Organization.

But the origin of the mouse is a new mystery: no one knows exactly how these people become infected. In the two years since its discovery, researchers have not yet identified the exact route of transmission from rats to humans. They have theories - perhaps patients drink contaminated water or touch contaminated objects - but nothing has been conclusively proven.

The 61-year-old patient recently had no mice or rat feces in his home, no one else in his family has shown symptoms and he has no recent history of travel.

"Based on the available epidemiological information, the source and route of infection cannot be determined," the Hong Kong Center for Health Protection (CHP) said in a statement on April 30. The man is still in hospital, and the investigation is ongoing.

What we know, and we do not know

The research team and city authorities have been trying to better understand this new health threat since 2018.

They have made progress. Their diagnostic tests have been refined and improved. They have spread awareness among the healthcare sector so that doctors know how to test for HEV mouse, and public awareness campaigns began.

Scientists are testing the population of mice across the country to try to identify groupings before they can be thrown at humans, which has provided data on how many rodents in the city carry HEV and which areas have more mice.

But there is still much that remains unknown. They don't know what the incubation period of the virus is - it means how long it takes for patients to get sick after exposure. They are still trying to find a cure, as the drugs used to treat the human variant of hepatitis E have had mixed results in patients with HEV.

And, of course, the biggest unknown that continues to worry scientists is how.

Not knowing how the virus is transmitted from rats to humans makes it very difficult to prevent further infections - or even to understand all the data that researchers have collected. For example, people living in mouse-infected areas should theoretically be at higher risk, however some infected patients come from low-mouse neighborhoods.

" What we do know is that rats in Hong Kong carry the virus, and we test people and find the virus. But how exactly does he jump between them - if the rats contaminate our food, or if there are any other animals involved, we don't know, "Sridhar said. "It simply came to our notice then. "