17 May 2026
Scientists at the Pirbright Institute have discovered that G57 H9N2 influenza viruses replicate more efficiently and disseminate systemically in chickens than the G1-B lineage, significantly increasing risks to both the poultry industry and public health.
Although often classified as a "low-pathogenicity" virus, H9N2 avian influenza virus (AIV) imposes a substantial global burden, causing major economic losses through respiratory disease, secondary bacterial infections, and reduced egg production. Two major genetic lineages currently predominate worldwide: the G1 lineage (genotype G1-B), common in the Middle East and Africa, and the BJ/94 lineage (genotype G57), which has become the dominant form in China and Vietnam.
The research team compared two prototype strains: Vietnam/315 (G57) and Pakistan/UDL-01 (G1-B). The results revealed a stark contrast in how these viruses behave within the host:
- Lethality and Replication: In chicken embryos, the G57 virus caused approximately 84% mortality within 72 hours, compared to only 33% for the G1-B virus.
- Systemic Spread: While G1-B remained largely localized to the upper respiratory tract, the G57 virus exhibited broad tissue tropism, meaning it could infect and replicate in a wide variety of organs, including the brain, bursa, lungs, spleen, and kidneys.
- Viral Shedding: G57 was found to shed at higher levels from both the oropharynx and the cloaca, increasing the potential for environmental contamination.
Through reverse genetics, the team identified the specific segments responsible for this enhanced performance. The Matrix (M) gene of the G57 virus was found to be a primary driver of increased replication in avian cells. Furthermore, a combination of the PB2, HA, NA, and M genes promoted faster growth and greater cell-to-cell spread in mammalian-origin cells. Technically, the G57 strain also possesses a more acid-stable Hemagglutinin (HA) (pH fusion 5.2) and significantly higher Neuraminidase (NA) activity. This "functional balance" allows the virus to survive better in respiratory droplets and release new progeny more efficiently from infected cells. The study warns that the G57 genotype's high "fitness" makes it a potent donor of internal genes to other dangerous influenza subtypes. With unique mammalian adaptation markers (such as PB2 271I and NP 52N) that help the virus evade human immune restrictions, the continued evolution of G57 poses an imminent threat.
Bhat et al., (2026). A G57 (BJ/94-like) H9N2 avian influenza virus exhibits enhanced replication and tissue dissemination in chickens compared with a G1-B virus. Journal of General Virology, 107(002259).
