Leading scientists have issued an urgent warning that two little-known animal viruses could 'easily' spark the next pandemic if they jump to humans.
The alert comes as the US battles a severe winter illness season, with influenza sickening around 20 million people and killing more than 11,000 since October, according to federal data.
But researchers say the greater long-term danger may lie elsewhere.
In a new review, scientists warned that influenza D - a virus that mainly infects cattle - and canine coronavirus, a highly contagious virus found in dogs, have the potential to mutate and spread among humans.
Neither virus is known to circulate widely in people. However, both spread easily in animals and are poorly monitored, meaning mutations that allow human-to-human transmission could go undetected.
The warning was published in January in a CDC-backed paper and echoes growing concerns about animal-borne viruses with pandemic potential, including Nipah virus, an incurable pathogen currently spreading in parts of India.
Dr John Lednicky, a research professor in the Department of Environmental and Global Health at the University of Florida and a co-author of the paper, said: '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.
'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.'
Scientists are warning that two viruses have the potential to spread to humans and trigger pandemics. Pictured above are USDA workers disinfecting a farm in Minnesota in 2015 for bird flu
Influenza D virus (IDV) was first identified in US pigs in 2011 and has since been detected in cattle, chickens, deer, giraffes and even kangaroos. Like other flu viruses, it mutates readily.
Researchers wrote that IDV strains can 'reassort and recombine', suggesting the virus is rapidly evolving. It is most closely related to influenza C, which infects humans - particularly children - but is not routinely tracked by US health authorities.
IDV is also thought to play a role in bovine respiratory disease, known as shipping fever - the most costly infectious illness in North American cattle.
The disease can cause pneumonia, heart inflammation and immune suppression, and can kill up to two per cent of a herd.
Previous studies by the same team found antibodies to influenza D in up to 97 per cent of cattle workers in Colorado and Florida, indicating widespread exposure.
The second virus, canine coronavirus (CCoV), is unrelated to SARS-CoV-2 but is highly infectious among dogs, particularly in kennels.
It typically causes gastrointestinal illness and is rarely tested for in humans.
'So far, influenza D virus has not been associated with serious infections in humans,' Dr Lednicky said.
The researchers wrote that influenza D and canine coronavirus have the ability to mutate and spread quickly without proper monitoring.
'However, canine coronavirus has - but diagnostic tests are not routinely performed, so we don't know how widely it affects people.'
In 2021, his team isolated a canine coronavirus strain from a US medical worker who fell ill after travelling to Haiti. That same year, researchers at the University of Texas Medical Branch identified a near-identical strain in a child hospitalized with pneumonia in Malaysia.
The virus has since been detected in people with respiratory illness in Thailand, Vietnam and Arkansas - showing it can circulate across continents.
The researchers warned that without stronger surveillance, improved testing and vaccine development, both viruses could quietly gain the ability to spread between humans.
'Our knowledge of these viruses is limited,' they wrote. 'Even so, the available evidence suggests they pose a major threat to public health.'