#18,903
One of the more intriguing influenza findings of the past dozen years has been the discovery of a previously unknown type of flu - Influenza D - infecting swine, cattle, and possibly even humans as well.We first learned of this new flu early in 2013 when researchers reported finding a novel influenza virus in swine from Oklahoma - initially classified as a novel Influenza C virus - but which would later be designated as Influenza D.
Their research – published PLoS Pathogens – was called Isolation of a Novel Swine Influenza Virus from Oklahoma in 2011 Which Is Distantly Related to Human Influenza C Viruses, and it immediately caused a stir in the flu research community.
And while we hadn't seen any evidence that Influenza D could cause symptomatic illness in humans, by 2016 (see Serological Evidence Of Influenza D Among Persons With & Without Cattle Exposure), researchers reported finding a high prevalence of antibodies against Influenza D among people with cattle exposure. They wrote:Additionally, the authors found that this new virus could infect, and transmit, in both ferrets and pigs. The following year, in mBio: Characterizing A Novel Influenza C Virus In Bovines & Swine, cattle were added to the list, and appeared to be the virus's primary reservoir.
IDV poses a zoonotic risk to cattle-exposed workers, based on detection of high seroprevalence (94–97%). Whereas it is still unknown whether IDV causes disease in humans, our studies indicate that the virus may be an emerging pathogen among cattle-workers.
Despite these high antibody positivity rates in occupationally exposed individuals, a small retrospective seroprevalence study conducted in 2011 on U.S. and Canadian blood samples found low (1.3%), but measurable HI antibody titers to this novel virus (cite).
However, a more recent (2023) study cited:
A related and more comprehensive study in Italy found a relatively sharp increase in seroprevalence for IDV in its general human population from 5.1% in 2005 to 46% in 2014 [18].
Also from Italy, in 2024 the EID Journal published a research letter (Detection of Influenza D Antibodies in Dogs, Apulia Region, Italy, 2016 and 2023) which found:
Dogs are known to be susceptible to influenza A viruses, although information on influenza D virus (IDV) is limited. We investigated the seroprevalence of IDV in 426 dogs in the Apulia region of Italy during 2016 and 2023. A total of 14 samples were positive for IDV antibodies, suggesting exposure to IDV in dogs.All of which brings us to a new report, published Friday in Emerging Microbes and Infections by Chinese researchers, who report:
- an extremely high seroprevalence (73.4%) in humans tested in Northeast China between 2020-2024
- efficient airborne transmission in Ferrets
- the discovery of a mutation in the P3 gene of a local strain (DHY11) which appears to boost its polymerase activity by about 50% compared to the classical strain
- which, in turn, appears to endow DHY11 with enhanced viral replication and transmissibility in mammals
Efficient airborne transmission of influenza D virus in ferret models and serological evidence of human exposure in Northeast China
Hongbo Gao, Weiyang Sun, Pengyang Lu, Zhipeng Dong, Jiajing Wu, Yuanguo Li, show all
Article: 2564308 Published online: 10 Oct 2025
ABSTRACT
Newly emerging influenza D virus (IDV), first identified in swine in 2011, has demonstrated broad mammalian tropism with notable prevalence in bovine populations and occupational exposure-associated seroprevalence among cattle workers. This zoonotic expansion raises concerns that IDV could acquire capability for human-to-human transmission via sustained evolving in mammal hosts.
Here, we evaluated the infectivity and transmissibility of a currently circulating IDV strain, D/bovine/Jilin/HY11/2023 (abbreviated as D/HY11), isolated from cattle in Northeast China in 2023.
D/HY11 was able to replicate efficiently in human primary respiratory epithelial cells and exhibits respiratory tract tropism in mammals. More importantly, we found that D/HY11 could efficiently transmit through the air between ferret models (5/6). Serological surveillance (2020–2024) revealed alarming exposure rates, with no significant difference in positivity between rural and urban populations: 73.37% (449/612) in the general population and an even higher rate of 96.67% (58/60) among individuals with respiratory symptoms.
The extraordinary high IDV seropositivity among people in Northeast China highlights the possibility of silent spread in mammals with mild symptoms.
Among generic anti-influenza drugs tested in vitro, only polymerase inhibitors demonstrated effective suppression of IDV replication. And the D/HY11 strain exhibited enhanced polymerase activity compared to the classical IDV strain, with preliminary evidence implicating the P3 gene as a potential contributing factor to this functional enhancement.
Our pathogenetic and serological findings indicate that IDV may have acquired the capacity for human-to-human transmission during its ongoing evolution, and currently circulating IDV strains already pose a potential panzootic threat.
(SNIP)
In summary, it is likely that IDV outbreak has metastasized into an ongoing problem for cattle and humans. Unobserved subclinical infections could be important in transmission, silently sustaining epidemics at the population level. The possibility of unseen chains of transmission may silently spread through cattle, other farm animals, and humans. Close biosecurity and active surveillance in affected areas are critical to monitor any changes in outbreak dynamics.
The good news is there is currently no clear evidence that Influenza D (IDV) produces clinical symptoms in humans, which for now, limits its public health threat.
But around the world we are seeing signs of this virus continuing to reassort, and evolve, as it expands its host range. Last year, the EID Journal carried this report:
Characterization of Influenza D Virus Reassortant Strain in Swine from Mixed Pig and Beef Farm, France
Stéphane Gorin1, Gautier Richard1, Séverine Hervé, Eric Eveno, Yannick Blanchard, Agnès Jardin, Nicolas Rose, and Gaëlle Simon
Abstract
Influenza D virus was isolated from pigs on a mixed pig and beef farm in France. Investigation suggested bull-to-pig transmission and spread among pigs. The swine influenza D virus recovered was a reassortant of D/660 and D/OK lineages. Reported mutations in the receptor binding site might be related to swine host adaptation.
While all of this may not be enough to raise alarm bells, Influenza D is certainly building up its zoonotic resume. And given the limits of global surveillance, testing, and the sharing of data, we probably don't know the half of it.