Relationship between antibody titers from V4-in-cassava vaccination and protection against velogenic virus challenge in laboratory chickens

Unvaccinated laboratory-raised, five week old cockerels were fed V4 newcastle disease vaccine in cassava once, twice or thrice; and tested for hemagglutination inhibition (HI) antibody response. They were subsequently challenged with a local isolate of velogenic newcastle disease virus (NDV). Immune status of the chicken flocks improved with number of vaccinations. Survival from velogenic virus challenge varied significantly with pre-challenge HI antibody titer, protection being better at log2 titer of ≥3 than at log2 titers of ≤ 2 but no difference was observed between titers of 3, 4 and 5. Although HI log2 antibody titers of 3 and 4 were apparently observed to protect vaccinated chickens against ND mortality in this work, 7 out of 8 (87.5%) of the chickens that manifested torticollis (neurological disease symptom) had pre-challenge HI antibody titers within this range. Thus, it is suggested that while log2 HI antibody titer of 3 may be taken as cut-off point for sero-conversion, titers of 5 and above may protect against neurological symptoms. Key words: V4, cassava-based vaccination, velogenic challenge, HI antibody titers, chicken protection.