Scientists have identified two new coronaviruses in humans, although neither was proven to cause illness or spread to other people. One study identified pigs as the animal host of one virus, and another study found that a coronavirus had likely stemmed from dogs, the first time a canine coronavirus has been shown to infect humans. 

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“This research clearly shows that more studies are desperately needed to evaluate critical questions regarding the frequency of cross-species [coronavirus] transmission and potential for human-to-human spread,” Ralph Baric, a virologist at the University of North Carolina, Chapel Hill, who was not involved in either study, tells Science.

The dog study, published May 20 in Clinical Infectious Diseases, stemmed from a question Gregory Gray, an infectious disease epidemiologist at Duke University, had early in the pandemic, reports NPR. He wondered whether there were other coronaviruses already infecting people that might one day spark outbreaks, and he asked his graduate student, coauthor Leshan Xiu, to design a test that would detect not just SARS-CoV-2, but previously unknown coronaviruses as well.

The team used its diagnostic test to screen more than 300 nasal swabs taken from pneumonia patients in Malaysian Borneo in 2018. Eight patients, or 2.7%, showed evidence of prior exposure to a novel coronavirus, and seven were children, Science reports. For a previously undetected virus, “that’s a pretty high prevalence,” Gray tells NPR. “That’s remarkable.” 

Gray sent samples of the virus to Anastasia Vlasova, an expert in animal coronaviruses at the Ohio State University, and she found that the virus was actually a chimera—portions of its genome matched a feline coronavirus, while another part was similar to a porcine coronavirus. But the majority of its genome was most similar to two coronaviruses previously isolated from dogs, and she was able to grow the virus in canine cell cultures. “Canine coronaviruses were not thought to be transmitted to people,” Vlasova tells NPR. “It’s never been reported before.”

It’s not clear if the virus, dubbed CCoV-HuPn-2018, caused the patients’ pneumonia, and it’s not yet known if the virus is capable of jumping from person to person or how an adult immune system might react if it did. “We don’t really have evidence right now that this virus can cause severe illness in adults,” Vlasova says in a press release. 

She did find that the virus had a key mutation—a deletion—that isn’t found in other canine coronaviruses but is found in those that infect humans. While it warrants further study to determine if this mutation is necessary for initiating a cross-species jump, Vlasova adds that she “cannot rule out the possibility that at some point this new coronavirus will become a prevalent human pathogen. Once a coronavirus is able to infect a human, all bets are off.”

Speaking to NPR, Xumin Zhang, a virologist at the University of Arkansas for Medical Sciences, says, “As the authors are careful to say in their paper, they have not proven what’s called Koch’s postulates,” meaning they would need to infect a human with the virus in order to show that it causes pneumonia. Such an experiment would be unethical, he adds, but they could instead test more samples to see how common the virus is in pneumonia patients and then use animal models to test their hypotheses.

The pig study, released as a preprint on medRxiv in March and since submitted to a peer-reviewed journal, identified a new coronavirus in serum samples from three Haitian children who came to the hospital with fevers between 2014 and 2015. Researchers at the University of Florida were able to grow the virus in monkey cells, and a genomic analysis showed that it closely matched known delta-coronaviruses in pigs. 

Coronaviruses parse into four groups—alpha, beta, gamma, and delta—and delta-coronaviruses were once thought to only infect birds, Science reports. But in 2012, one appeared in pigs in Hong Kong, thought to have jumped from songbirds. The same virus caused a 2014 outbreak in swine in the US, and delta-coronaviruses have since been shown to infect human cells as well. 

The coronaviruses most dangerous to humans—SARS-CoV, SARS-CoV-2, and MERS-CoV—have all been betas. While delta-coronaviruses cause significant outbreaks in animals, the same has not been true in humans. Alphas, including the dog-derived coronavirus isolated in Malaysia, have also never triggered epidemics in humans, Texas A&M University virologist Benjamin Neuman tells Science, “but that doesn’t feel like much comfort in the wild world of viruses.”

Taken together, the studies suggest that coronaviruses are likely circulating in animals at higher rates than previously thought. “I think the more we look, the more we will find that these coronaviruses are crossing species everywhere,” University of Iowa virologist Stanley Perlman, who was not involved in the work, tells Science. 

The goal moving forward, Gray says in the press release, will be to seek out these viruses before they cause illness in people. “We are likely missing important animal viruses that are beginning to adapt to humans,” he says, adding that places where animals and people intermingle, such as open markets or farms, might be good places to screen for “early warning[s] of a new virus which may become a future pandemic virus.”