Increased level of protection of respiratory tract and kidney by combining different infectious bronchitis virus vaccines against challenge with nephropathogenic Brazilian genotype subcluster 4 strains.

Genotyping of seven infectious bronchitis virus (IBV) strains isolated in Brazil showed that all belonged to the common Brazilian genotype and that these strains were closest to the subcluster of strain IBV/Brazil/2007/USP-19. Pathotyping of four selected Brazilian strains showed that they all caused a considerable level of ciliostasis in the trachea but at a somewhat lower level than did M41 and Brazilian strains 50/96, 57/96, 62/96 and 64/96 representing four different serotypes that had been reported earlier. In contrast to the M41 challenge strain, all Brazilian isolates replicated in kidney tissue in a high percentage of non-vaccinated challenged birds, clearly showing that they are nephropathogenic. As for the tracheal protection, the results using Massachusetts (Mass) vaccination against the recent strains seemed to show protection higher on average than for the strains reported earlier. A single or twofold vaccination with a Mass vaccine resulted in a mean tracheal protection level against the four challenge strains of 92% and 90%, respectively, whereas a single and twofold vaccination with a Mass vaccine halved the percentage of infected kidneys (14% and 13%, respectively, P < .05) compared to that of the unvaccinated birds (27%). The combination of the Mass and the 793B vaccine provided on average a tracheal protection of 99% and a reduction of the percentage of infected kidneys to a mean of 2%. This was a significantly (P < .05) higher protection than that achieved by a single or twofold Mass vaccination, showing the added value of the 793B vaccination following priming with a vaccine of the Mass type.