Control of Newcastle Disease: Do we need to Change Vaccine Candidate

Newcastle disease (ND) is one of the most economically devastating infectious diseases affecting the poultry industry globally. ND is vaccine-preventable but is a persistent threat to the poultry industry. The amino acid sequence of the cleavage site of the F protein has primarily determined the molecular basis for the pathogenicity of NDV. The F cleavage site reversion from avirulent to virulent has been reported for infectious clones after just one passage in chicken brains. Currently, the most widely used ND vaccines belong to early genotypes I and II, which were isolated approximately 70 years ago. Nevertheless, the prevalent NDV strains in poultry belong to late genotypes, including genotype V in America, genotype VII in Asia and Africa, which are genetically and antigenically distinct from traditional vaccines. Live ND vaccines, which include lentogenic and mesogenic vaccines, are commonly used to prevent and control the disease. Current live vaccines’ mismatch with the dominant viruses is a major concern. Although NDV belongs to a single serotype, there are great genetic and antigenic variations between conventional live vaccines and viruses present in the field. Commercial rNDV genotype VII.1.1 vaccines based on the LaSota strain induced a protective immune response; however, GII-based vaccines failed to prevent virus shedding efficiently. A commercial live vaccine composed of recombinant NDV expressing the F and HN genes from genotype V NDV is more effective in reducing viral excretion than the LaSota strain. Genotype VII vaccine had a significantly higher protection rate than the LaSota vaccine against genotype VII NDV. A novel genotype VII-matched vaccine strain, generated by reverse genetics, induces a stronger cell-mediated immune response that blocks the viral shedding and provides full protection to chickens than LaSota in chickens. Looking towards the persistent threat of ND, we need to think about changing the vaccine strain or the use of genotype-matched vaccines for ND outbreaks in India.