Another day, another germ to worry about. Scientists in Japan say they’ve shown that a previously undiscovered virus spread by ticks has been making people in the country sick since at least 2014. The Yezo virus, as it has been christened by its discoverers, is thought to cause low white blood cell and platelet counts as well as fever in its victims, similar to other tick-borne diseases.
In a paper published last month in Nature Communications, the researchers laid out their case for Yezo being a new human virus. According to the report, the first known appearance of Yezo was in a 41-year-old man who was admitted to their hospital in Sapporo, Japan, in mid-May 2019 with a high fever, appetite loss, and lower limb pain on both sides.
The symptoms had begun four days after he visited a nearby forest and three days after he found a tick attached to his right abdomen. Tests showed that the man had low levels of white blood cells and platelets, the component of blood that helps prevent or stop bleeding through clotting. Doctors assumed he had a tick-borne illness and gave him various antibiotics. Over the course of 19 days—15 of which were spent in the hospital—he gradually recovered and his symptoms resolved. But during those days, tests failed to identify evidence of infection from any known tick-borne illness, including Lyme disease, tularemia, and relapsing fever.
Just over a year later, in July 2020, the doctors came across another patient with similar symptoms. He had visited two hospitals before seeing them, but successfully recovered as well.
Eventually, the authors say, they were able to identify traces of the same unknown virus in the blood of both patients. From the blood of the second patient, they claimed to salvage enough of the virus to grow it fully intact in the lab and in mice. And finally, from these samples, they were able to genetically identify the mystery bug as belonging to a family of viruses all spread by ticks, called orthonairovirus. They decided to tentatively call it Yezo virus (YEZV), in reference to the historical name of Hokkaido, the larger island where it was first found.
Proving that a newly discovered virus can be transmitted to and cause illness in humans takes several lines of evidence, though, which the authors say they’ve collected.
For instance, they say they’ve found antibodies to Yezo in the wildlife of Hokkaido and in three of the major tick species known to inhabit the area. Most importantly, they also claim to have found evidence of Yezo infection in the preserved blood samples of at least five other people in the area who were suspected of being sick with a tick-borne disease, dating back to 2014. These patients, like the first two, appeared to develop fever and low platelet counts, while the blood of four patients overall showed levels of Yezo antibodies that became detectable as they recovered, which is what you would expect from an acute infection. Interestingly enough, four of these patients also had evidence of Lyme disease, suggesting they may have experienced a co-infection with both germs (Unfortunately, tick-borne co-infections aren’t rare).
“In this work, we demonstrate that YEZV is highly likely to be the causative pathogen of febrile illness, representing the first report of an endemic infection associated with an orthonairovirus potentially transmitted by ticks in Japan,” the authors wrote.
It will still take independent verification by other scientists before Yezo will be considered the real deal. But the authors suspect that it may be more widespread than currently documented, and they’re pushing for more research to be done into it as soon as possible.
“All of the cases of Yezo virus infection we know of so far did not turn into fatalities, but it’s very likely that the disease is found beyond Hokkaido, so we need to urgently investigate its spread,” said study author Keita Matsuno, a virologist at Hokkaido University’s International Institute for Zoonosis Control, in a statement from the university.