Maternal immunity to fowl adenovirus in chicks derived from vaccinated hens

Fowl adenoviruses (FAdVs) cause various diseases, including gizzard erosion, hepatitis-hydropericardium syndrome, and inclusion body hepatitis, resulting in significant economic losses in the poultry industry. In this study, as a preliminary step toward the development of a trivalent inactivated FAdV vaccine against serotypes 1, 2, and 8b, protective maternal antibody titers in chicks and maternal antibody transfer efficiency following vaccination of hens were investigated. In challenge trials using virulent FAdV-1/2/8b strains, chicks with maternal antibody titers <1:8 showed mortality rates ranging from 21% to 51%, whereas chicks with maternal antibody titers of 1:8 were protected from mortality, although some birds exhibited clinical signs of FAdV-associated disease. In contrast, chicks with maternal antibody titers of ≥1:16 showed no clinical signs or gross pathological lesions. Analysis of chick antibody titers derived from vaccinated hens with various antibody levels demonstrated that maternal antibody transfer rates against each viral serotype ranged from 25% to 40%, indicating that vaccinated hens require antibody titers of at least 1:64 to confer protective maternal antibody titers of 1:16 in chicks. In a study evaluating immunity onset using a trivalent FAdV-1/2/8b prototype inactivated vaccine, at least 90% of vaccinated hens showed antibody titers of ≥1:64 against each serotype by 21 days after vaccination, and all hens reached this titer by day 28. In a separate duration-of-immunity study, 100% of vaccinated hens maintained antibody titers of ≥1:64 against all serotypes for at least 15 months after vaccination.