#18,837
Less than two years ago - but before we first learned about the spillover of HPAI H5 into American Cattle - the big H5Nx livestock concern was its potential to infect and adapt to swine.
While detections in swine have been limited, over the years we've scattered evidence that H5N1 can infect pigs, albeit usually asymptomatically. A few past reports include:- An Unusual Report Of H5N1 in Pigs (Indonesia 2016)
In May of 2023, in Netherlands: Zoonoses Experts Council (DB-Z) Risk Assessment & Warning of Swine As `Mixing Vessels' For Avian Flu, we looked at growing concerns in Europe that avian H5N1 could increase its pandemic threat by spreading (and evolving) in farmed swine.
A week later, a report out of Italy confirmed an H5N1 spillover event at a `mixed species' farm (poultry & swine) in Italy, and the subsequent seroconversion of the majority of the pigs tested on that farm (see Study: Seroconversion of a Swine Herd in a Free-Range Rural Multi-Species Farm against HPAI H5N1 2.3.4.4b Clade Virus).
Once again, swine showed mild or subclinical signs of infection - and little or no detectable virus replication in the nasal cavity. While good news for the pig, it can make it challenging to identify spillover events without serological testing.Last November, we saw two pigs infected with a new, recently emerged genotype (D1.2) in Oregon (see USDA Confirms 2nd Pig on Oregon Farm Tested Positive for H5N1).
While research studies have shown only limited susceptibility of swine to the H5N1 virus (see EID Journal: Low Susceptibility of Pigs against Experimental Infection with HPAI Virus H5N1 Clade 2.3.4.4b) only a handful of H5 variants (subclades and/or genotypes) have actually been tested.
Until the emergence of H5 genotype B3.13 in late 2023, cattle were considered unlikely hosts for H5N1 as well. Since then, genotype D1.1 has also shown the ability to infect cattle.
Given that there are literally scores of genotypes circulating around the globe - and new ones continue to emerge - there are legitimate concerns that a more `swine-adapted' strain could eventually emerge.
And since pigs are susceptible to a wide variety of (swine, human, avian, etc.) influenza A viruses - the worry is they could serve as a `mixing vessel' - and produce a more dangerous hybrid virus.
All of which brings us to a lengthy review article - published yesterday in Frontiers Vet. Sci. - which examines the emerging threat of HPAI H5N1 clade 2.3.4.4b in swine populations.
While you'll want to read this review in its entirety, I'll draw your attention to a couple of points. First, the author's conclusion (excerpt below) that swine herd infection with HPAI is a `very likely event'.
Considering the cumulative findings from both experimental and field studies, natural infection with HPAI H5N1 clade 2.3.4.4b genotypes B3.13 and D1.1 appears to be a very likely event. Once introduced into the swine population, these viruses may act as dead-end infections or exhibit limited transmission among pigs (48, 49, 52).However, the epidemiological risk escalates depending on the virological context of the host population. In influenza-free herds, the virus could establish more readily due to the absence of immunity and competition, potentially increasing its adaptation and facilitating onward transmission (48).Conversely, if introduced into herds already endemic for swine IAV (swIAV), co-infection could lead to genetic reassortment, raising concern over the emergence of novel viruses with altered host range, pathogenicity, or transmissibility (15, 60).These scenarios highlight the critical need to evaluate the outcomes of HPAI H5N1 introduction under both naïve and swIAV-positive scenarios.
This review also identifies a number of critical knowledge gaps, including:
- A limited understanding of how HPAI H5N1 spreads between pigs in commercial farming operations
- Whether current tests for swine influenza virus surveillance have adequate sensitivity or specificity for detecting HPAI H5N1 infections
- Uncertainty about whether existing swine influenza vaccines provide cross-protection against HPAI H5N1
There is obviously much more. I've posted the abstract, and some excerpts below. I'll have a postscript after the break.
Front. Vet. Sci., 12 August 2025Sec. Veterinary Infectious DiseasesVolume 12 - 2025 | https://doi.org/10.3389/fvets.2025.1648878Juan Mena-Vasquez , Ana Marco-Fuertes. Marie Culhane, Montserrat TorremorellAbstract
Highly pathogenic avian influenza (HPAI) H5N1 represents a significant threat to wildlife, livestock, and public health. The recent detection of HPAI H5N1 clade 2.3.4.4b genotypes B3.13 and D1.1 in dairy cows, poultry, wild birds, wild mammals, and humans, along with the recent detection of D1.2 genotype in outdoor pigs, reflects an accelerated shift in the ecological and transmission dynamics of the virus. Given the pigs’ role in influenza ecology, these shifts present a serious threat to the swine industry and public health, accentuating the urgency for a coordinated One Health response.
However, the current understanding of swine influenza, particularly in preventing and preparing for potential HPAI H5N1 incursions, has not been fully discussed. Furthermore, the consequences of such incursions on the swine industry and consequently on public health have not been explored extensively. This review addresses the knowledge gaps related to HPAI H5N1 clade 2.3.4.4b infections in pigs. Assessing the risks of HPAI H5N1 in pigs and the consequences for cross-species transmission is crucial. Preventing the introduction of HPAI into pigs and minimizing spillover risks through evidence-based strategies is vital to ensuring food security, maintaining a safe food supply, sustaining animal production systems, and preventing human infections, including potential pandemics.(SNIP)
Conclusion
The expanding host range and ongoing evolution of HPAI H5N1 clade 2.3.4.4b highlight the urgent need for comprehensive surveillance, preparedness strategies, and support for scientific investigations. While experimental and field data so far suggest limited replication and transmission of HPAI H5N1 in pigs, the emergence of mammalian-adaptive mutations, even at low frequencies, demonstrates the potential for further viral evolution in this host and highlights the need for close monitoring of this virus in swine populations. As pigs serve as key hosts in the influenza virus ecosystem, they remain a critical species for the generation of reassortant viruses with pandemic potential.Current swine influenza control strategies, including vaccination, biosecurity, and surveillance, offer a valuable foundation. However, they must be evaluated and potentially adapted to address the distinct risks posed by HPAI H5N1. In particular, cross-species transmission, mammalian adaptation, and the risk of zoonotic spillovers underscore the importance of integrating virological, epidemiological, and immunological monitoring.
In addition, vaccine development strategies should explore the feasibility and efficacy of homologous and broadly protective HPAI H5N1 vaccines for use in pigs. Understanding the HPAI H5N1 immune responses in swine following natural infection or vaccination will be critical for shaping effective control programs.Finally, mitigating the public health threat posed by HPAI H5N1 in pigs will require a One Health approach that includes coordinated efforts across veterinary, human health, and wildlife sectors to assess occupational risks, preexisting immunity in exposed populations, and to implement mitigation strategies that consider both animal and public health. The current panzootic reinforces the importance of proactive, science-based policies to prevent the emergence of the next pandemic.
As we've seen with other recent reviews (see here, here, and here), the authors of this report offer a number of crucial steps we should be taking to help mitigate the HPAI H5 threat.
But whether producers and regulatory agencies are listening - or are inclined to adopt them in a meaningful way - remains to be seen.
The USDA's surveillance program for Influenza A in swine (IAV-S) is strictly voluntary and relies on passive sample submission. The latest USDA summary report (below) illustrates some of the limitations of current surveillance.
As the USDA notes:
Due to the voluntary nature of this surveillance, the information in this report cannot be used to determine regional and/or national incidence, prevalence, or other epidemiological measures, but it may help identify IAV-S trends.
Somewhat unnervingly, we are probably testing more than a great many other nations around the globe.
The good news is - with this level of global surveillance - when the next pandemic finally does emerge, governments can honestly state:
They `never saw it coming'.
