Efficacy of Different Vaccination Plans Against Experimental Infection with a Serovar B Variant of Avibacterium paragallinarum from Argentina in Laying Hens.

To assess the efficacy of two inactivated vaccines against Avibacterium paragallinarum serovar B variant infection in laying hens, day-old female chicks were allocated into five experimental groups (G): G1 received a commercial vaccine V1, strains 17756, 0222, and Modesto of serotypes A, B, and C, respectively, at 8 and 12 wk of life; G2 received V1 through an early vaccination plan at 5 and 12 wk of life; G3 received another commercial vaccine V2, strains 083, Spross, 48, and H18 of serotypes A, B, Bvar, and C, respectively, against infectious coryza, which contained a serovar B variant strain in its formulation at 8 and 12 wk of life. Chickens in G4 were not vaccinated and challenged and were used as positive control, and G5 was not vaccinated and not challenged and served as negative control. At 25 wk of life, chickens from G1 to G4 were inoculated (by injection into the infraorbital sinus) with the serovar B variant of Av. paragallinarum, strain INTA H8, into the left infraorbital sinus of each animal. Clinical signs were monitored daily until Day 5 postchallenge. On that day, bacterial isolation was performed on both inoculated and noninoculated sinuses from each animal to determine the presence of Av. paragallinarum. The interpretation of clinical signs involved grading on a scale from 0 to 4, depending on the severity of conjunctivitis and swelling of the periorbital area. On Day 2 postchallenge, the highest number of birds from all inoculated groups exhibiting clinical signs was recorded, gradually decreasing over the following days. The unvaccinated group (G4) had significantly (χ², P < 0.05) more birds with clinical signs compared to vaccinated birds, regardless of the immunization plan or age. Overall, no significant differences were observed between G1 and G3 on any of the postchallenge observation days. However, G1 had significantly fewer birds exhibiting clinical signs compared to G2, the group that received the early vaccination plan. On the other hand, comparing the results of the noninoculated sinuses, no differences were found among vaccinated birds in G1, G2, and G3, whereas significantly more infected sinuses were found in the nonvaccinated birds. In conclusion, both V1 and V2 demonstrated efficacy in significantly reducing clinical signs, both with the conventional vaccination plan at 8 and 12 wk of life (V1 and V2) as well as with early vaccination starting at 5 wk of age (V1), compared to nonvaccinated birds. It also showed effectiveness in reducing the presence of Av. paragallinarum in the infraorbital sinuses following the experimental challenge with the serovar B variant. Furthermore, V1, which does not have a Bvar strain, was proven to be effective against the serovar B variant of Av. paragallinarum, specifically demonstrating its efficacy in protecting against clinical signs associated with this particular serovar.