Heterologous live infectious bronchitis virus vaccination in day-old commercial broiler chicks: clinical signs, ciliary health, immune responses and protection against variant infectious bronchitis viruses

Groups of one-day-old broiler chicks were vaccinated via the oculo-nasal route with different live infectious bronchitis virus (IBV) vaccines: Massachusetts (Mass), 793B, D274 or Arkansas (Ark). Clinical signs and gross lesions were evaluated. Five chicks from each group were humanely killed at intervals and their tracheas collected for ciliary activity assessment and for the detection of CD4+, CD8+ and IgA-bearing B cells by immunohistochemistry (IHC). Blood samples were collected at intervals for the detection of anti-IBV antibodies. At 21 days post-vaccination (dpv), protection conferred by different vaccination regimes against virulent M41, QX and 793B was assessed. All vaccination programmes were able to induce high levels of CD4+, CD8+ and IgA-bearing B cells in the trachea. Significantly higher levels of CD4+ and CD8+ expression were observed in the Mass(2)+793B(2)-vaccinated group compared to the other groups (subscripts indicate different manufacturers). Protection studies showed that the group of chicks vaccinated with Mass(2)+793B(2) produced 92% ciliary protection against QX challenge; compared to 53%, 68% and 73% ciliary protection against the same challenge virus by Mass(1)+D274, Mass(1)+793B(1) and Mass(3)+Ark, respectively. All vaccination programmes produced more than 85% ciliary protection against M41 and 793B challenges. It appears that the variable levels of protection provided by different heterologous live IBV vaccinations are dependent on the levels of local tracheal immunity induced by the respective vaccine combination. The Mass(2)+793B(2) group showed the worst clinical signs, higher mortality and severe lesions following vaccination, but had the highest tracheal immune responses and demonstrated the best protection against all three challenge viruses.