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(CNN) - In 2018, infectious disease experts from the University of Hong Kong encountered an unusual patient.
The 56-year-old man, who had undergone a liver transplant, showed abnormal liver functions without apparent cause.
The tests found that his immune system was responding to hepatitis E, but they were actually unable to find the human strain of the hepatitis E virus (HEV) in his blood.
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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 these four was known to infect humans.
Using the tests for that human strain of negative HEV, the researchers redesigned the diagnostic test, ran it again, and discovered, for the first time in history, rat hepatitis E in a human.
"Suddenly, we have a virus that can jump from street rats to humans," said Dr. 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 wondered if it was a "single incident, a patient who was in the wrong place at the wrong time."
But then it happened again. And again.
Since that first study, 10 more Hong Kong residents have tested positive for rat hepatitis E, also known as rat HEV. The most recent case came a week ago; A 61-year-old man with abnormal liver function tested positive on April 30. And there could be hundreds more undiagnosed people infected, Sridhar said.
The human strain of hepatitis E is generally transmitted through faecal contamination of drinking water, according to the World Health Organization.
But the rat strain poses a new mystery: No one knows exactly how these people are becoming infected. In the two years since the discovery, researchers have not yet identified the exact route of transmission from rats to humans. They have theories, perhaps the patients drank contaminated water like the usual human strain, or manipulated contaminated objects, but nothing has been definitively proven.
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The recent 61-year-old patient has authorities particularly puzzled; There were no rats or rat droppings in her home, no one else in her home has shown symptoms, and she has no recent travel history.
"According to available epidemiological information, the source and route of the infection could not be determined," the Hong Kong Center for Health Protection (CHP) said in a statement on April 30. The man is still in the hospital and the CHP investigation is ongoing.
What we know and what we don't know
The investigative team and city authorities have been trying to better understand this new health threat since 2018.
They have made some progress. His diagnostic tests have been refined and improved. They have spread awareness among the health care sector so that doctors know they should be tested for the hepatitis C virus and launched public awareness campaigns.
Scientists are testing rat populations across the city to try to identify groups before they can jump into humans, which has provided data on how many rodents in the city carry rat HEV and which areas have the most rats.
But there are still many things that are still unknown. They do not know how long the incubation period for this virus lasts, that is, how long it takes for patients to become ill after exposure. They are still trying to find a treatment, as the drug used to treat the human variant of hepatitis E has had mixed results in patients with rat HEV.
And, of course, the biggest question that still bothers scientists is how.
Not knowing how the virus jumps from rats to humans makes it very difficult to prevent new infections, or even make sense of all the data the researchers have collected. For example, people living in rat-infested areas should theoretically be at higher risk, however, some infected patients come from neighborhoods with low rat numbers.
"What we know is that rats in Hong Kong carry the virus, and we tested humans and found the virus. But how exactly it jumps between them, whether the rats contaminate our food or another animal is involved, we don't know, "Sridhar said. "That is the missing link."
One solution might be to get rid of all the rats in Hong Kong, but rat eradication is a long and complicated feat that is not very feasible. It would be necessary to reduce the places where they can take refuge, as well as their access to food, with measures such as preventing food from being discarded in back alleys.
For now, all the authorities can do is urge people to take preventative measures, such as washing their hands before eating, storing food properly or in the refrigerator, and keeping the house clean and sanitized with minimal nesting places for rodents.