Scientists in China have found a new virus that has likely made dozens of people sick since 2018 and may be natively carried by shrews. The discovery is the latest reminder that microbes spread from animals to people represent an ever-present potential public health threat. So far, at least, the evidence suggests that the virus is only causing sporadic infections and that it isn’t being transmitted between people.

The researchers laid out their findings in a paper published last week in the New England of Journal of Medicine. According to their report, the virus was first identified via a throat swab collected from a patient as part of an active disease surveillance program that tests people with fever who have had recent exposure to animals. After confirming the presence of a never-before-documented virus in the patient, the researchers subsequently found a total of 35 cases in which people with reported illness were acutely infected with the same unknown virus between late 2018 and early 2021. All of these cases were located around the Shandon and Henan provinces in eastern China. The team decided to call their discovery the Langya virus (LayV).

Simply finding a new microorganism in people doesn’t necessarily mean that it’s causing illness. And the authors caution that their evidence to date hasn’t met Koch’s postulates, a common criteria used by scientists to establish a causative link between a microbe and disease. But they do have some compelling circumstantial data. In 26 of the 35 patients, for instance, the authors say they found no pathogen that could possibly otherwise explain their symptoms. These symptoms included fever (reported in all 26 cases), fatigue, and cough, along with signs of kidney damage and low platelet counts in some. No deaths were reported, however.

“The case for causation of human illness with this virus—I think it’s pretty strong,” Emily Gurley, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health who has studied similar viruses, told Gizmodo.

Based on its genetics, the study authors argue that the virus belongs to the genus henipavirus. The genus is nestled within the larger family Paramyxoviridae, which has many members that cause human disease, such as measles and mumps. Other known dangerous henipaviruses include Nipah virus and Hendra virus. But Langya virus appears to be most closely related to the Mòjiāng virus, a mysterious microbe linked to three cases of fatal pneumonia documented in China in 2012 (one reason for its mystery is that it’s never been isolated from potential animal hosts, though other related viruses have since been discovered).

Benhur Lee, a professor of microbiology at the Icahn School of Medicine at Mount Sinai, notes that there has been an ongoing debate as to whether Mòjiāng virus and its relatives should truly be considered henipaviruses. And he says it’s possible that the International Committee on Taxonomy of Viruses (ICTV), the organization that formally names viruses, will soon decide to split Mòjiāng-like viruses into a related but distinct branch separate from henipaviruses, which could include Langya virus.

In any case, along with coronaviruses and influenza viruses, many scientists are concerned about the danger posed by emerging henipaviruses and Mòjiāng-like viruses. Henipaviruses are most commonly found in bats, which can and have spread new infectious diseases to humans, and many of these henipaviruses seem to possess the potential ability to spill into other species, humans included, Lee said. Meanwhile, Mòjiāng-like viruses have been found in other animals all over the world, though their potential to cross over and cause human illness is less clear.

During their investigation, the researchers in China also looked for the Langya virus in nearby animal populations. And while it was found in a very small minority of dogs and goats, it was found in 27% of wild shrews, suggesting that they may be its predominant hosts. The original Mòjiāng virus is believed to have come from rodents, which shrews are not, but according to Lee, there have been other Mòjiāng-like viruses found in samples collected from shrew roadkill. At the same time, Gurley notes that the authors didn’t present or may not have any available information on the types of animal contact that could have led to these human cases.

Out of all these related viruses, Nipah has been the most concerning to date, since it’s routinely caused outbreaks in parts of Asia (including last year), has a fatality rate as high as 100%, and has occasionally spread from human to human. By contrast, the cases identified with Langya virus so far have had no clear connection to one another, and there have been no reports of close contacts later catching the infection.

Given these data points, Lee said, “my feeling is that the danger of transmission is low.”

Similarly, Gurley says that unless you’re living and spending plenty of time with shrews in these areas, you probably shouldn’t be concerned about Langya virus. And she notes that while spillover events from animals to humans are fairly common, most do not lead to pandemics or serious outbreaks.

Of course, every once in a blue moon, humanity does come across a zoonotic pathogen that can cause mass illness, as our current calamity with covid-19 has harshly reminded us. So even if Langya virus doesn’t turn out to be a serious concern, it’s an example of why scientists and public health organizations must remain vigilant in hopes of stopping the spread of the next germ that can truly become a nightmare.

“I think the takeaway should be that it’s good people are doing this type of research to understand what’s happening around us. Because unless we go looking, we’ll never really know,” Gurley said. “This is how you can pick up something early so that you can do something about it if you start seeing person to person transmission.”