Protective efficacy of classical vaccines and vaccination protocols against an exotic Newcastle disease virus genotype VII.2 in Belgian layer and broiler chickens.

Vaccination against Newcastle disease (ND) has been routinely implemented in the Belgian professional poultry sector since 1993, using genotype I and II vaccines. Despite this, an outbreak of genotype VII.2 avian paramyx-ovirus 1 (APMV-1) occurred in 2018, with 20 reported cases over the course of 3 months. Although the economic impact on the professional poultry sector was limited, this epizootic raised questions regarding the efficacy of implemented classical genotype I and II vaccines against phylogenetically distant exotic velogenic strains. The present study provides insights into the protective efficacy of standard vaccination programs applied in layer and broiler flocks against the introduction and transmission of this velogenic APMV-1 VII.2 strain. For fully field-vaccinated 26-week-old layer chickens, high levels of specific antibodies were measured at the time of the velogenic APMV-1 challenge, resulting in good clinical protection. However, despite the observed humoral immunity, viral excretion was not prevented, leading to transmission of the virus to non-infected sentinel birds. In fully field-vaccinated 4-week-old broiler chickens, assessment of vaccine uptake and coverage revealed low levels of ND specific antibodies despite double vaccination at day 1 and day 14. Consequently, poor protection against velogenic APMV-1 infection was observed, with both clinical signs and viral excretion occurring in both infected and sentinel birds. This study demonstrates that the introduction of velogenic APMV-1 VII.2 can lead to its dissemination among the Belgian avian poultry population despite the implementation of standard vaccination.