Evaluation of the Newcastle disease virus genotype VII–mismatched vaccines in SPF chickens: A challenge efficacy study

Newcastle disease virus (NDV) strains, while falling under a single serotype, are classified into distinct genotypes. Genotype VII virulent NDVs pose a significant threat to poultry due to their association with high mortality rates and economic losses. This study aimed to evaluate the efficacy of three commercial live vaccines based on genotype II against genotype VII virulent NDV (vNDV) in specific pathogen-free (SPF) chickens. Forty one-day-old chickens were randomly divided into four groups (n = 10) and inoculated with one dose of each ND pneumotropic vaccine—B1, Clone.12IR, and La Sota—or received phosphate-buffered saline (PBS) as a control at eight days of age via eye drop. At 28 days of age (20th post-vaccination days), chickens were intramuscularly challenged with genotype VII virulent NDV (≥ 10⁵ LD₅₀). Serum samples were collected at 28 days of age (challenge day), 7 and 14 post-challenge days to measure NDV antibodies via the hemagglutination inhibition (HI) test. Cloacal and oropharyngeal swabs were taken on the 3rd, 5th, 7th, and 10th post-challenge days to evaluate virus shedding. Vaccinated groups exhibited significantly higher antibody titers and greater protection levels compared to the control group (P≤ 0.001). While HI antibody titer was not different at 28 and 35 days of age between vaccinated chickens, the Clone.12IR groups showed higher HI antibody titer compared to B1 at day 42 of age (9.43 vs. 7.42; P≤ 0.002). La Sota and Clone.12IR vaccines demonstrated superior protection against mortality compared to the B1 vaccine (90 %, 80% vs. 60 %, respectively) with 6.0 and 2.67 odds ratio of survivability. All three mismatched vaccines effectively curbed the shedding of virulent genotype VII NDV, with 0 % to 11 % positive cloacal samples up to the 3rd post-challenge day. These findings demonstrate that in the experimental setting, the administration of mismatched ND vaccines, particularly La Sota and Clone.12IR, confer protection against genotype VII virulent NDV and control viral shedding, which can help to develop effective vaccination strategies to mitigate the impact of vNDV outbreaks in the poultry farms.