Rats have long been a vector of disease for humans, but it's especially alarming when an exotic virus that used to only infect rats suddenly jumps to humans. That's now what appears to be happening with a rat-specific strain of hepatitis, after two people were confirmed to have contracted the rat disease, reports The Guardian.

“Because the strain is very different from the human strain, people think it wouldn’t be able to jump to humans,” said Siddharth Sridhar, one of the principal researchers at Hong Kong University. “This was a clinical discovery.”

Both cases occurred in Hong Kong, a city plagued by a rapidly growing rat population.

The first case cropped up in September, when a 56-year-old man had a hepatitis strain previously known only in rats in Vietnam. With only one case, however, no major flags were raised. When a second case was confirmed, a 70-year-old woman with a compromised immune system, scientists began to worry that this rat strain was legitimately adapting for transmission to humans.

The woman's case was discovered after blood samples of 70 patients in a hospital who had been diagnosed with hepatitis E were analyzed, which revealed that the rat-specific version was actually at play with her. Scientists have noted that the strains in both of the confirmed cases so far are "uncannily similar," however there is not yet any known link between the patients. They do, however, live within two miles of one another.

Human hepatitis E is typically transmitted when traces of human feces containing the virus contaminate water, objects, or hands that come into contact with the mouth. Contaminated drinking water is by far the most common vector, but it can also be contracted from eating undercooked meat. It's unclear how the rat strain jumps to humans, but researchers suspect a similar vector, via rat feces that might accidentally contaminate food or water.

Officials in Hong Kong are calling the incidents a wake-up call to improve public hygiene, and to institute new measures to control the city's booming rodent infestation. For now, there's no reason to fear a widespread outbreak, but given that rats have long inhabited human environments, there's always the risk that these viruses could mutate into new forms that might jump between the species.

A rat form of hepatitis might be making the jump to humans
There are now two confirmed cases of humans in Hong Kong contracting hepatitis from rats.