People are catching hepatitis from rats in Hong Kong. But scientists don’t know how.

A new strain of the hepatitis E virus that usually only infects rats is now sickening people in Hong Kong, with nearly a dozen human cases reported so far, according to news reports. But exactly how the virus jumps from rats to people is a mystery.

The first case of a human contracting rat hepatitis E virus, or rat HEV, was reported in Hong Kong in 2018. Since then, 10 more people in the area have tested positive for rat HEV, according to CNN. The most recent case was reported on April 30, in a 61-year-old man who was hospitalized with abnormal liver function, CNN reported.

Scientists still don’t know how people are getting infected. The latest case is particularly baffling because no rats or rat feces were seen in the 61-year-old man’s home, and he had not recently traveled anywhere.

It’s possible people are getting infected through ingestion of food or water contaminated with rodent feces, or through the handling of contaminated objects; but these theories haven’t been proven, CNN reported. There could also be an intermediate animal involved in passing the virus from rats to people.

“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,” Dr. Siddharth Sridhar, a microbiologist at the University of Hong Kong and one of the researchers who first discovered rat HEV in people, told CNN.

Hepatitis is an inflammation of the liver, and hepatitis E is a liver infection caused by the hepatitis E virus, according to the Centers for Disease Control and Prevention (CDC). The human strain of hepatitis E is usually transmitted through water contaminated from infected human feces, and infections are more common in developing countries, according to the CDC. (In rare cases, people can catch the disease from eating certain animals infected with the virus, including pigs, boar and deer.) 

Symptoms can include fever, nausea, vomiting, jaundice (yellowing of the skin or eyes) and joint pain. Many people with the infection have mild symptoms or no symptoms, but the disease can be more serious for people with weakened immune systems. Complications are rare, but the disease can cause liver failure.

Outside of Hong Kong, the only other reported case of a human contracting rat HEV occurred in Canada in 2019, in a man who had recently traveled to Africa, CNN reported.

It’s likely that the disease is occurring in other countries, but it hasn’t been recognized because doctors aren’t testing for it, CNN reported. Doctors need to use a specific test to look for rat HEV in people, but it hasn’t been widely adopted.

“We need ongoing vigilance in the public to control this unusual infection. I really hope that public health authorities take the first step and look at how much their populations are actually being exposed to rat hepatitis E,” Sridhar said.

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Originally published on Live Science.  

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