Abstract

Many vaccines are commercially available to prevent Newcastle Disease (ND) in chickens, but the disease is still endemic in many countries. The present study aimed to evaluate the efficacy of ND vaccines in vivo and to analyze the nucleotide sequence similarity between the vaccines and the wild-type virus encountered in the field, as well as the underlying reasons for vaccination failures against ND in chickens. One hundred laying Isabrown hens, aged 22 weeks with an average weight of 1.75 kg, from two farms (Farm A and B) in Tabanan, Bali, Indonesia, served as experimental animals. All laying hens had previously been vaccinated four times orally with LaSota vaccines (Farm A) and alternately with LaSota and inactive ND vaccines (Farm B). They were then given a booster vaccination orally with live LaSota (Farm A) and intramuscularly with an inactive vaccine (Farm B). Sera samples were collected every week for four weeks in Farm A and for five weeks in Farm B, and the antibody titers were examined by hemagglutination inhibition (HI) test. The genotypes of the field isolates were determined through sequencing and bioinformatics analysis using MEGA 7. The mean log2 antibody titers for hens vaccinated with the LaSota vaccine were 7.2 HI units before vaccination, 7.3 HI units at one week post-vaccination, 8.1 HI units at two weeks post-vaccination, and 7.9 HI units at three weeks post-vaccination. In contrast, the mean log2 antibody titers for those vaccinated with the inactive NDV vaccine were 6.4 HI units at pre-vaccination, 7.3 HI units at one week, 5.5 HI units at two weeks, 5.0 HI units at three weeks, and 5.8 HI units at four weeks post-vaccination. The NDV-LaSota vaccine illustrated 80-81% similarity to a recent ND virus isolate from chickens in Bali (ND/chicken/GAYK01/Penebel Bali/2023), while inactive vaccines exhibited 96-98% similarity. Thus, vaccines were closely related to the Bali virus, whereas the active vaccine was less similar. Based on antibody responses and homology levels, the inactive NDV vaccine induced stronger protective immunity against NDV field isolates than the Active LaSota vaccine.