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

Abstract

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.

Keywords: Broiler; Classical vaccination; Genotype VII.2; Layer; Newcastle disease virus.

Fig. 1

Humoral immune responses in layer birds against APMV-1 before and after infection, measured by HI (A) and indirect ELISA (B). A summary of the obtained data is represented in C. Data are represented at the individual level and as average group value ± standard deviation of log₂ (HI-titer) or log₁₀ (ELISA-titer). The HI geometric mean titers were expressed as reciprocal log₂, and titers ≥ 4 log₂ were considered positive, this cutoff is indicated by the dotted line. Although the challenged and sentinel birds are represented together at the pre-challenge time point, their values were not mixed for statistical comparisons.

Fig. 2

(A) Viral RNA excretion by infected and sentinel layer birds at different points after infection, by the respiratory and gastrointestinal tract. (B) Viral presence in the organs of infected and sentinel birds at time of death or the end of the experiment (14 dpi), determined by RT-qPCR. CLS, cloacal swab; TRS, tracheal swab.

Fig. 3

Representation of mean clinical scores and mortality rates for infected (black) and sentinel (grey) field vaccinated broiler chickens after infection.

Fig. 4

Humoral immune responses in broiler birds against APMV-1 before and after infection, measured by HI (A) and indirect ELISA (B). A summary of the obtained data is represented in C. Data are represented at the individual level and as average group value ± standard deviation of log₂ (HI-titer) or log₁₀ (ELISA-titer). The HI geometric mean titers were expressed as reciprocal log₂, and titers ≥ 4 log₂ were considered positive, this cutoff is indicated by the dotted line. Although the challenged and sentinel birds are represented together at the pre-challenge time point, their values were not mixed for statistical comparisons.

Fig. 5

(A) Viral RNA excretion by infected and sentinel vaccinated commercial field broiler chickens at different time points after infection, by the respiratory and gastrointestinal tract. (B) Viral presence in the feathers over time after infection taken from live birds. (C) Viral presence at time of death or the end of the experiment (14 dpi) in the organs of infected and sentinel birds, determined by RT-qPCR.