Background & objectives
A three-year-old male child from West Bengal, India, with severe acute respiratory symptoms, was confirmed in the laboratory with LPAI H9N2 virus infection under the Indian Council of Medical Research (ICMR) - Pan India Acute Respiratory Infections (ARI) / Severe Acute Respiratory Infections (SARI) surveillance through the Virus Research and Diagnostic Laboratories network.

Methods
Common respiratory viruses were detected by real-time PCR, followed by subtyping of Influenza A for seasonal and avian viruses. The identified H9N2 virus was isolated and further characterised, including whole genome sequencing. Antibody response was performed in serum samples of the case and family members.

Results
Complete genome sequencing revealed a G1 lineage (Middle East B sub-lineage). Bayesian evolutionary analyses of the HA gene of Indian H9N2 poultry strains showed three clusters of multiple introductions at the estimated node age of 1999 based on the Human strain A/India/NIV/1519/2024(H9N2) and the other poultry viruses from India evolved with 4.49 × 10–3 substitutions per site per year. The isolated H9N2 virus showed a high EID50 titre of 108.25/200 μl with avian-type receptor specificity. The antibodies against the H9N2 virus were only detected in the study case and not in close contacts confirming limited human-to-human transmission. The virus was found to be sensitive to neuraminidase inhibitors oseltamivir and zanamivir.

Interpretation & conclusions
Systematic avian influenza surveillance in both birds and humans is required for the early detection of newly evolved viruses.