In 2018, experts at the University of Hong Kong discovered an unusual patient. This patient had an infectious disease that was incredibly uncommon—actually, virtually unheard of at the time.

The 56-year-old man, who had undergone a liver transplant, was showing abnormal liver functions with no obvious cause.

When the researchers ran the redesigned diagnostic test, they found rat hepatitis E in a human for the first time.

This was such an unprecedented infection that the team asked themselves if this patient was a “one-off incident, one patient who was in the wrong place at the wrong time.”

Since 2018, 10 more Hong Kong residents have tested positive for rat hepatitis E (also known as rat HEV). The most recent case happened in April 2024 when a 61-year-old man with abnormal liver function tested positive.

The human strain of hepatitis E can be transmitted through fecal contamination of drinking water, according to the World Health Organization, but researchers are unsure how the rat strain is infecting people.

There are some theories as to why more and more humans are getting infected with rat hepatitis E. One way of transmission could be through drinking contaminated water like the usual human strain.

The research team and authorities in the city have been working to figure out why this threat has been on the rise since 2018. So far, the team has spread awareness among the healthcare sector for doctors to be on the lookout for signs of hepatitis E.

Scientists are testing rat populations throughout the city to help identify clusters of infected rats before the virus jumps to humans. This hunt for more clusters has also provided data on how many rodents in the city carry the diseases and which areas have the most rats.

Researchers do not know how long this virus’ incubation period is, which means how long it takes for patients to show noticeable symptoms of illness after exposure.

Because we don’t know how the virus jumps from human to human, it is tough to prevent further infections.

“What we know is the rats in Hong Kong carry the virus, and we test the humans and find the virus. But how exactly it jumps between them – whether the rats contaminate our food, or there’s another animal involved, we don’t know,” said Sridhar. “That’s the missing link.”