Reassortant H5N1 Viruses Are Emerging as a Zoonotic Threat in Asia
Highly pathogenic avian influenza H5N1 is again moving in ways that routine surveillance can miss. A CDC Emerging Infectious Diseases perspective argues that the current focus on clade 2.3.4.4b, though justified by its global spread and spillover into mammals, may be obscuring a parallel evolution in long-endemic clade 2.3.2.1 viruses across Asia. In poultry systems where these viruses have circulated for years, reassortment appears to be generating new gene combinations with direct implications for human infection risk.
The concern is not theoretical. Since October 2023, all reported human infections with H5N1 in this regional context have involved reassortants. In Cambodia, recent human cases have carried hemagglutinin, neuraminidase, and nucleoprotein segments from clade 2.3.2.1e viruses, with internal genes likely drawn from clade 2.3.4.4b and low pathogenicity avian influenza viruses. In India and Bangladesh, recent human infections have involved clade 2.3.2.1a reassortants with similarly mixed internal gene origins. A person traveling from India to Australia in 2024 was also infected with a related reassortant virus.
The pattern is abrupt rather than gradual.
Long-Endemic Poultry Lineages Are Changing Their Genetic Makeup
Clade 2.3.2.1 emerged in 2007 and spread across Asia and into Europe. Continued circulation in Asia led to the recognition of subclades 2.3.2.1a, 2.3.2.1b, and 2.3.2.1c in 2014, with further subdivision of 2.3.2.1c into 2.3.2.1d through 2.3.2.1g. The current geography of these viruses reflects poultry production systems rather than a single uniform epidemic. Clade 2.3.2.1a is now detected mainly in poultry in Bangladesh and India, while clade 2.3.2.1e circulates in Cambodia, Laos, and Vietnam. The remaining 2.3.2.1g subclade is restricted to poultry in Indonesia.
The article traces a sharp change in human case patterns. Before 2015, clade 2.3.2.1 caused only one human infection across Cambodia, India, and Bangladesh. The viruses then became enzootic and ecologically stable in poultry, with relatively few recognized human infections for years. Since 2023, human infections have risen again. Cambodia reported 34 H5N1 cases between February 2023 and November 19, 2025 after nearly a decade without confirmed infections. India and Bangladesh reported six human cases in 2025 alone.
Those numbers matter because they align with a genetic shift, not just a surveillance artifact. The recent human infections are tied to viruses that no longer preserve the older clade structure alone. Instead, they combine segments from viral backgrounds with different ecological histories, including enzootic H5N1 lineages, the globally spreading 2.3.4.4b clade, and low pathogenicity avian influenza.
Reassortment Has Become the Central Evolutionary Concern
Reassortment has long been a defining mechanism in influenza biology. Every human influenza pandemic virus of the 20th century arose through reassortment. In the current H5N1 setting, the concern is that this process is no longer incidental. It may be actively reshaping the zoonotic behavior of clade 2.3.2.1 viruses in Asia.
Earlier 2.3.2.1a and 2.3.2.1e viruses were already known to reassort in the past, but their gene constellations appeared relatively stable in the decade before the recent cases. By contrast, clade 2.3.4.4b viruses have undergone frequent reassortment. When those two backgrounds mix, the resulting virus may acquire internal genes that alter replication, transmission, or host adaptation while retaining surface proteins that remain rooted in the endemic lineage. In practical terms, that can create a virus that looks familiar by clade label but behaves differently at the bedside and in the field.
The current hypothesis is straightforward: reassortant viruses may have emerged and circulated in poultry before being recognized in humans, gaining either a transmission advantage in birds or a fitness advantage that increased the frequency of human exposure. The article does not claim that direct causality has been proven. It does, however, argue that the association is strong enough to demand much tighter genomic and phenotypic scrutiny.
Surveillance Still Misses the Genome-Level Changes That Matter
Existing surveillance systems in endemic poultry settings are not built to capture this kind of shift reliably. Many programs still depend on clade-level reporting and targeted checks for known mammalian-adaptive mutations or antigenic markers rather than routine whole-genome sequencing with integrated analysis. When surveillance stops at the clade label, it can miss a virus that has acquired a new internal gene constellation with meaningful phenotypic consequences.
In the article’s framing, that is the central blind spot. A reassortant can be underdetected, or detected only after human cases appear, if surveillance does not capture the full segment architecture across poultry, environmental, and human samples. The authors call for an integrated real-time approach that links genomic detection with phenotypic assessment across animal and human health sectors. That type of system would allow public health teams to ask not only whether a virus exists, but whether its genetic makeup has changed in ways that should alter risk assessment.
Retrospective analysis is the immediate scientific priority. Archived poultry samples collected before recognized human cases could reveal whether the reassortants were already circulating in birds. If they are found in those samples, previous poultry circulation becomes more likely. If they are absent, a different possibility emerges: the viruses may have gained a stronger mammalian-adaptive phenotype after spillover. The article notes that no systematic retrospective screening has yet been reported.
Why the Internal Gene Segments Matter Clinically and Epidemiologically
Influenza biology gives these internal gene segments real importance. Hemagglutinin and neuraminidase define key antigenic and receptor-binding features, but the internal segments govern replication, packaging, host range, and overall viral fitness. A reassortant that preserves a familiar H5 surface while importing internal genes from another lineage can alter the balance between poultry adaptation and mammalian infection in ways that standard clade tracking may not reveal.
That matters for human health because the recent rise in Asian cases appears tied to exactly this kind of mixed-genome virus. Cambodia’s recent cases, the 2025 cases in India and Bangladesh, and the travel-associated infection in Australia all point to a regional problem with broader implications. The article’s argument is not that every reassortant will become efficiently transmissible between humans. It is that reassortment is now a major driver of zoonotic risk in endemic H5N1 lineages and deserves the same level of attention that point mutations have received for years.
The remaining gap is operational. Better sequencing, better retrospective sampling, and better linkage between animal and human surveillance are needed before the true prevalence and directionality of these reassortants can be established. Until then, the dominant limitation is simple: current systems are likely seeing only part of the virus evolution that is already under way.