Evaluation of Vaccination Programs in Layer Pullets Using Recombinant Fowl-Pox Mycoplasma gallisepticum Vaccine in Comparison to Commercially Available F-Strain Live Vaccines.

Abstract

Over the last few decades, recombinant vaccines have been increasingly used for disease control in commercial poultry production. This study assesses the efficacy of a recombinant fowl-pox virus-based vector vaccine (rFP-MG) in conferring protection against Mycoplasma gallisepticum (MG) in different prime-boost combinations with or without the live modified F-strain vaccine. Four different vaccination programs were evaluated using Hy-line W-36 layer pullets. Group 1 served as unvaccinated controls (n = 57). Group 2 (n = 30) received a single dose of rFP-MG at 8 wk of age, and group 3 (n = 31) received two doses of rFP-MG, one at 1 day of age and again at 8 wk. Group 4 (n = 30) received a single dose of F strain at 8 wk, and group 5 (n = 32) received rFP-MG at 1 day of age and F strain at 8 wk. Four weeks after the second vaccination, all groups, including the negative control group, were divided into two (challenged and unchallenged) subgroups. All challenged groups were then inoculated with the MG R_low strain at a dose of 3.11 × 10⁸ CCU/bird via the fine spray route. Unchallenged groups included the unvaccinated/unchallenged and four vaccinated/unchallenged control groups. Clinical scoring, air sac scoring, and tracheal mucosal thickness at 21 days postchallenge were used as the main parameters for the evaluation of each vaccination program. No mortalities were observed in any of the study groups after the challenge; however, challenged groups that received only rFP-MG showed clinical signs (ranged from a few snicks to some gurgling sounds) as early as 7 days postchallenge, while the F-strain-vaccinated/challenged groups did not show any clinical signs. Birds from all groups exhibited varying degrees of air sac lesions (ranging from air sacs with suds or small flecks of caseous exudate to cloudy and thickened air sacs), with a higher percentage of birds in the rFP-MG groups being affected. The tracheal mucosal thickness of the F-strain-vaccinated groups was not significantly higher than that of the unvaccinated/unchallenged (negative control) group. However, the tracheal mucosal thickness of the rFP-MG-vaccinated group was significantly lower (P < 0.05) than that of the unvaccinated/ challenged (positive control) group. Results from this trial showed that rFP-MG vaccine provided partial protection against the clinical signs compared to the unvaccinated/challenged positive control; however, it was less protective than the F-strain vaccine.