Two viruses that mostly live in animals are starting to look a little too comfortable around humans. So far, they have not triggered a global emergency.
Still, researchers say the ingredients for a future outbreak are present – from frequent animal exposure to patchy testing and poor early detection of new respiratory viruses.
In a recent review, infectious disease experts argue that influenza D virus and canine coronavirus deserve much more attention than they’re getting.
The worry isn’t just that these viruses can infect people now. It’s that if either one evolves to spread efficiently from person to person, it could move fast through a population with little existing immunity.
Viruses hiding in plain sight
One reason the alarms aren’t louder is that both viruses can slip by without being obvious. Influenza D, so far, has mainly been linked to silent infections in humans.
Researchers have linked canine coronavirus to more serious illness in a few cases, but because doctors rarely test for it, no one really knows how widespread it may be.
“Our review of the literature indicates these two viruses pose respiratory disease threats to humans, yet little has been done to respond to or prevent infection from these viruses,” said John Lednicky, a professor at the University of Florida.
“If these viruses evolve the capacity to easily transmit person to person, they may be able to cause epidemics or pandemics since most people won’t have immunity to them.”
The team’s bigger message is familiar from the past few years: you don’t want to start paying attention only after a virus has learned the trick of easy human-to-human spread.
Influenza D reaches humans
Scientists first identified influenza D virus in 2011 and tied it mainly to pigs and cattle. Since then, they have found it in many other animals, from poultry and deer to giraffes and kangaroos.
In cattle, the virus is believed to contribute to bovine respiratory disease, a costly condition that affects animal health and farm productivity.
What makes influenza D especially interesting – and potentially unsettling – is how often exposure appears to occur at the human-livestock interface.
Quiet human exposure raises concern
The authors point to earlier studies of cattle workers in Colorado and Florida showing that up to 97 percent of people working with herds carried antibodies to influenza D. Antibodies do not necessarily mean a current infection, but they do suggest that people have encountered the virus.
So far, this exposure seems mostly subclinical, meaning infected individuals do not notice symptoms. That may sound reassuring. But it also cuts both ways. A virus that spreads quietly can move widely without being detected.
Researchers also note that influenza D shows signs of evolving quickly. A strain recently isolated in China has developed the capacity for human-to-human transmission.
That kind of shift is what turns a virus from “interesting” to “urgent,” even if its real-world impact remains uncertain.
A virus from dogs
Canine coronavirus (CCoV) is a different virus from SARS-CoV-2. In dogs, it’s best known for causing gastrointestinal illness.
Scientists are paying attention now because they have linked rare human infections to pneumonia hospitalizations in Southeast Asia.
“So far, influenza D virus has not been associated with serious infections in humans,” Lednicky said. “However, canine coronavirus has, but diagnostic tests are not routinely performed for the virus so the extent at which the virus affects the population at large is not known.”
If clinicians aren’t testing for something, cases don’t show up in the data. And if cases don’t show up in the data, the virus can “fly under the radar” for years.
Crossing from dogs to humans
The review pulls together several key findings suggesting that canine coronavirus is not just an isolated curiosity.
In one study led by Lednicky, a University of Florida team isolated a canine coronavirus from a medical team member. The individual had traveled from Florida to Haiti in 2017 and later developed mild fever and malaise. The researchers named the strain HuCCoV_Z19Haiti.
A few years later, a separate group led by Gregory Gray reported another strain, CCoV-HuPn-2018, isolated from a child hospitalized in Malaysia. Notably, researchers described the Malaysia strain as nearly identical to the one the Florida team identified.
Signals of wider spread raise concerns
The story did not stop there. Since then, researchers have detected CCoV-HuPn-2018 in people with respiratory illness across Thailand, Vietnam, and even Arkansas, suggesting the virus has already crossed regional boundaries.
That pattern does not automatically mean the virus spreads easily from person to person. Travel, repeated animal-to-human spillover, and limited surveillance could all produce scattered cases.
Still, the fact that the virus keeps appearing in different places is exactly why researchers are calling for better tracking and monitoring.
Preparing before the next virus crisis
The review argues that the current knowledge base is thin, and that thin knowledge is part of the risk. When scientists lack data on infection rates, symptoms, and exposure risk, they struggle to design smart prevention.
“Our knowledge about the viruses’ epidemiology and clinical manifestations are limited to a modest number of research studies,” the authors wrote.
“Even so, the limited data regarding these novel, newly detected viruses indicate that they are a major threat to public health.”
Their proposed direction is straightforward. The plan calls for stronger monitoring in both animals and humans and pushes for better diagnostics that people actually use. It also urges early planning for treatments and even vaccines, rather than waiting for a crisis to force action.
The takeaway isn’t that panic is warranted. It’s that complacency is expensive. These viruses already send warning signals, from antibodies in workers to scattered pneumonia cases across countries.
The whole point of preparedness is treating those signals as a reason to look harder, not a reason to shrug.
The study is published in the journal Emerging Infectious Diseases.
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