Abstract
Background:
Avian influenza virus H9N2 is a low-pathogenic subtype that causes respiratory disease and economic losses in poultry and carries zoonotic potential. The virus is endemic across Asia, the Middle East, and North Africa, often exacerbated by co-infections with other pathogens. In Libya, H9N2 was first detected in 2006, but data on age-related immune responses in broilers remain limited.

Aim:
This study investigated the age-related serological response to H9N2 in southwest Libyan broiler flocks to provide baseline data for vaccination and control programs.

Methods:
A total of 342 blood samples were collected from six unvaccinated broiler flocks. Antibodies against influenza A were detected by indirect enzyme-linked immunosorbent assay (ELISA), and H9N2-specific antibodies were measured by hemagglutination inhibition (HI) assay.

Results:
The seropositivity for influenza A increased with age (30%, 52%, and 100% at 10, 24, and 38 days, respectively). The positivity rates for H9N2 were 0%, 22%, and 72%. The progressive increase confirms the active field circulation and endemicity of H9N2. The absence of detectable antibodies at 10 days indicates strong maternal antibody interference.

Conclusion:
H9N2 is endemic in Libyan broiler flocks, with continuous exposure reflecting age-dependent seroconversion. Vaccination after the age of 10 days may optimize immune protection by reducing maternal antibody interference. Continuous surveillance and molecular monitoring are essential for guiding effective national control measures and assessing zoonotic risks.

Key words: H9N2; Seroprevalence; ELISA; HI; Libya.