A newly emerging strain of influenza D virus detected in China has sparked fresh pandemic fears after researchers discovered “alarming exposure rates” suggesting the virus may have already developed the ability to spread between humans.
Scientists from the Changchun Veterinary Research Institute found that nearly three-quarters of people tested in Northeast China showed evidence of exposure to the virus, with rates soaring to 97 per cent amongst those with respiratory symptoms. The findings have prompted urgent calls for global surveillance of what experts describe as a “panzootic threat” that could be spreading silently across continents.
The research team, led by Hongbo Bao, studied the newly emerging strain called influenza D virus (IDV), specifically the D/HY11 variant that emerged in cattle in Northeastern China in 2023. Their findings suggest the virus has evolved significantly from its origins as an animal-only pathogen.
Evidence of Widespread Human Exposure
The most striking discovery came from analysis of archived blood samples collected between 2020 and 2024. Researchers found that 74 per cent of 612 volunteers tested in Northeast China had antibodies against the D/HY11 strain, indicating widespread exposure to the virus.
Even more concerning, the rate jumped to 97 per cent amongst individuals who had sought medical care for respiratory symptoms. These figures far exceed what would be expected from isolated animal-to-human transmissions, raising serious questions about whether the virus has already achieved human-to-human spread.
“This raises the possibility of cryptic transmission in humans with mild or asymptomatic infections via the emerging D/HY11-like viruses,” the researchers stated. They warned that IDV may have been circulating in northeastern China since at least 2020.
The research team expressed particular concern about the lack of routine testing. “Currently, no routine IDV testing is carried out anywhere in the world, facilitating concerns about the silent spread of this panzootic virus and the potential emergence of new varieties,” they wrote.
Airborne Transmission Confirmed
Laboratory experiments revealed troubling characteristics of the D/HY11 strain that distinguish it from typical animal influenza viruses. The virus demonstrated efficient replication in human airway cells and showed the ability to spread through airborne transmission between ferrets, the gold-standard model for predicting human flu transmission.
Researchers placed infected ferrets in specially designed cages to test whether the virus could spread to healthy animals without direct contact. The virus successfully transmitted through the air, a trait typically associated with viruses capable of spreading easily amongst humans.
The virus also showed an ability to infect multiple organ systems, including the brain in mice studies, suggesting it could cause more severe illness than typical seasonal flu strains. Testing revealed that whilst some newer antiviral drugs like baloxavir showed effectiveness against D/HY11, the virus proved resistant to common flu medications.
Silent Global Spread
IDV was first identified in 2011 in swine in the United States, but cattle have since been recognised as its primary reservoir. What makes this virus particularly concerning is its demonstrated ability to infect an unusually broad range of mammals.
Beyond cattle and pigs, the virus has been detected in goats, sheep, horses, camels, and dogs across Europe, North and South America, Asia, and Africa. Recent studies have even found evidence of IDV antibodies in domestic cats in northern China, with pet cats showing higher infection rates than strays.
“In recent years, IDV often arrives silently in a new country or continent and does not display symptoms,” the study authors warned. This silent spread makes detection and containment particularly challenging, as infected animals and potentially humans may show no obvious signs of illness.
The virus shares approximately 50 per cent similarity with influenza C virus, which causes mild respiratory illness in humans, particularly children. However, IDV has evolved distinct characteristics that set it apart, including enhanced stability that may facilitate cross-species transmission.
Pandemic Risk Assessment
The findings add to growing concerns about influenza viruses with pandemic potential. Whilst global health authorities remain focused on the H5N1 bird flu currently spreading through dairy cattle in the United States, IDV represents a different but equally concerning threat.
Dr Nicola Lewis, Director of the Worldwide Influenza Centre at the Francis Crick Institute in London, recently told the World Health Organization that “the chances that disease X will be an influenza virus are probably greater than for any other known pathogen group.” Disease X refers to a hypothetical unknown pathogen that could cause the next pandemic.
What makes IDV particularly worrying is its apparent ability to evolve rapidly. Research published in Nature Communications revealed that IDV has a significantly higher substitution rate than influenza C virus, consistent with its global distribution and multi-host tropism.
The virus’s HEF protein, responsible for receptor binding and membrane fusion, has demonstrated the ability to bind to human tracheal epithelium, indicating that IDV could pose a genuine threat to public health.
Implications for Global Health
The discovery comes as health systems worldwide remain on high alert following the COVID-19 pandemic and ongoing concerns about H5N1 bird flu. The WHO currently assesses the risk from H5N1 to the general public as low, despite 70 human infections reported in the United States since 2024, because no human-to-human transmission has been documented.
However, IDV presents a different challenge. Unlike H5N1, which requires close contact with infected birds or cattle, the evidence suggests IDV may already be spreading amongst humans, albeit potentially with mild or no symptoms.
The research highlights critical gaps in global influenza surveillance. Whilst extensive monitoring systems exist for influenza A and B viruses, IDV has largely escaped attention. The virus is not included in routine testing panels, meaning cases could be misdiagnosed as common respiratory infections.
Agricultural and Economic Concerns
For the agricultural sector, IDV poses immediate challenges beyond its pandemic potential. The virus has been implicated in bovine respiratory disease complex (BRDC), a major cause of illness and economic loss in cattle farming worldwide.
Studies have shown that whilst IDV typically causes mild symptoms on its own, it often occurs alongside other respiratory pathogens, potentially weakening animals’ immune systems and making them susceptible to more severe infections.
The high seroprevalence in cattle, reaching over 90 per cent in some regions, suggests the virus is already endemic in many farming areas. This widespread circulation provides ample opportunity for the virus to continue evolving and potentially acquire new characteristics that could enhance its ability to infect humans.
Call for Urgent Action
Researchers are calling for immediate implementation of global surveillance systems for IDV, similar to those established for other influenza viruses. They emphasise the need for routine testing in both animal and human populations, particularly amongst agricultural workers who have close contact with cattle.
The study authors stressed that “unobserved subclinical infections could be important in transmission, silently sustaining epidemics at the population level.” They warned that chains of transmission may be spreading undetected through cattle, other farm animals, and humans.
Development of diagnostic tests, vaccines, and treatment protocols should begin immediately, experts argue, rather than waiting for clear evidence of a pandemic threat. The COVID-19 pandemic demonstrated the devastating consequences of being unprepared for a novel respiratory virus.
Lessons from Past Pandemics
History provides sobering context for the current findings. The 1957 H2N2 “Asian Flu” pandemic killed an estimated 1.1 million people worldwide, including 116,000 in the United States. That pandemic emerged when a novel influenza strain to which humans had little immunity began spreading.
More recently, the 2009 H1N1 pandemic demonstrated how quickly a new influenza virus could spread globally in the modern era. That virus, which originated in pigs, infected millions worldwide within months of its emergence.
The current situation with IDV differs in that the virus appears to be spreading slowly and causing mild illness, providing a potential window for intervention before it evolves into a more dangerous form. However, influenza viruses are notorious for their ability to mutate rapidly and unpredictably.
As one expert noted, pigs represent an ideal “mixing vessel” for influenza viruses, as they can be infected by bird, human, and swine flu strains simultaneously. This creates opportunities for genetic reassortment that could produce novel pandemic strains.
The emergence of IDV as a potential human pathogen underscores the ongoing threat posed by zoonotic diseases and the urgent need for comprehensive “One health” approaches that consider the interconnections between human, animal, and environmental health.
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