Incomplete Protection by Maternally Derived Antibody Against H9N2 Low-Pathogenic Avian Influenza in Commercial Broiler Chickens

H9N2 low-pathogenic avian influenza remains endemic in Korean poultry production, where commercial broilers rely largely on maternally derived antibodies (MDA) transferred from vaccinated breeder hens. This study quantified MDA transfer and decay and evaluated protection against homologous Y280-lineage H9N2 challenge. Paired sera were collected from breeder hens and day-old progeny on six commercial farms. MDA decay was monitored in 90 broilers from 1 to 22 days of age, and 32 birds with individual HI titers of 0, 2, 4, or 6 log2 were challenged intranasally at 7 days of age. Transfer efficiency averaged 18.2 ± 11.5%, and MDA had an estimated half-life of 3.2 days. Clinical signs occurred in 8/8, 8/8, 7/8, and 0/8 birds in the MDA 0, 2, 4, and 6 groups, respectively. Cecal tonsil virus detection decreased from 87.5% in the MDA 0 group to 37.5% in the MDA 6 group, although the pairwise difference was not statistically significant (p = 0.119). Oropharyngeal shedding occurred in all challenged birds at comparable titers. These findings suggest that higher MDA levels are associated with reduced clinical disease and lower cecal tonsil virus detection but do not prevent oropharyngeal shedding under the conditions tested.