Early challenge with LPAI-H9N2 followed by vNDV-VII.1.1 in vaccinated and unvaccinated broilers: a comprehensive study

Sixteen commercial broiler groups, G (n = 18 birds/group), were used to investigate the early co-infection with LPAI-H9N2 and vNDV-VII.1.1 following three vaccination regimens: inactivated ND + H9N2 followed by two doses of live V4 (G1, G2, G3, G14), inactivated ND + H9N2 only (G4, G5, G6, G15) and live V4 only (G7, G8, G9, G16), and unvaccinated controls (G10, G11, G12 and G13). Subsequently, G1, G4, G7 and G10 were challenged with LPAI-H9N2 at 18 days old and vNDV-VII.1.1 at 22 days old, while G2, G5, G8 and G11 received LPAI-H9N2 only and Gs G3, G6, G9 and G12 received vNDV-VII.1.1 only, at the same ages. G13–16 were not challenged. Mortality occurred only in G4 and G6 (40%) and in G10 and G12 (100%), with faster mortality and more clinical disease especially regarding respiratory and enteric manifestations in G10. Significantly, the highest LPAI-H9N2 shedding was detected in G11, G7, G10 and G8, respectively, whereas the highest vND-VII.1.1 shedding appeared in G12, G10 and G4, respectively at 3 days post-challenge. In conclusion, vaccination of broilers against ND [using bivalent inactivated ND+H9N2-P, plus double live (V4) or double live (V4) ND vaccine only] or LPAI-H9N2 [using inactivated ND + H9N2-P] protected them from clinical disease, mortality rates, postmortem lesions, histopathology and significantly decreased, but did not stop, their shedding of virus after early co-infection. Finally, strict biosecurity measures are essential tools parallel to vaccination to control vND-VII.1.1 and LPAI-H9N2 infections (especially at early age).RESEARCH HIGHLIGHTS Vaccinated broilers were protected against mortality from early H9N2-vNDV-VII.1.1 co-infections.Vaccination of broilers significantly decreased but did not stop shedding of either virus.H9N2 infection prior to vNDV infection in nonvaccinated broilers induced faster mortality.