RESULTS

Body weight

No significant differences in body weight were observed among the experimental groups at 21 days of age, indicating that vaccination did not adversely affect early growth performance (Table 2). However, at 26 days of age (5 dpc), birds in the negative control group exhibited significantly greater body weights than those in the vaccinated and positive control groups (p < 0.05), whereas no significant difference was detected between the vaccinated and positive control groups (Table 2). These findings indicate that challenge with the Thai QX-like IBV strain adversely affected growth performance regardless of vaccination status, although vaccination provided protection against other disease-associated parameters. No mortality was observed throughout the experimental period.

Ciliostasis and CPS

At 26 days of age (5 dpc), tracheal samples were collected for ciliostasis assessment and RT-PCR analysis. Birds in the positive control group exhibited significantly higher ciliostasis scores than those in the vaccinated and negative control groups (p < 0.05) (Table 2). Notably, concomitant hatch-day vaccination achieved a CPS of 58.5%, whereas the unvaccinated challenged group exhibited complete ciliostasis (CPS = 0%), demonstrating markedly superior preservation of tracheal ciliary function following vaccination.

The vaccinated group exhibited a substantially higher CPS (58.5%) than the positive control group (0%), indicating significant tracheal protection following heterologous challenge. Furthermore, the BPS was identical in the vaccinated and negative control groups (100%), indicating comparable preservation of ciliary activity in protected birds.

Viral detection

RT-PCR analysis demonstrated that birds in the positive control group had the highest viral detection rate, which was significantly greater than that observed in the negative control group (p < 0.05) (Table 2). Although the vaccinated group showed a lower viral detection rate than the positive control group, the difference was not statistically significant. These findings suggest that concomitant vaccination partially suppressed viral replication but did not completely prevent infection following challenge with the Thai QX-like IBV strain.

Group | Body weight ( g ) at 21 days ( mean ± SD; n = 20 / group ) | Body weight ( g ) at 26 days ( mean ± SD; n = 20 / group ) | Average ciliostasis score / bird ( n = 10 ) | CPS | BPS | RT - PCR detection ( n = 10 )
Vaccinated | 933.75 ± 97.82ᵃ | 1,109.50 ± 113.63ᵃ | 8.3ᵃ | 58.5 | 100 | 7ᵃ
Positive control | 932.75 ± 100.87ᵃ | 1,200.75 ± 64.30ᵃ | 20ᵇ | 0 | 0 | 10ᵃ
Negative control | 984.75 ± 73.10ᵃ | 1,230.50 ± 83.02ᵇ | 0ᶜ | 100 | 100 | 0ᵇ

Serological evaluation

Maternal antibody titers were detected in all experimental groups at 1 day of age (Figure 1). Antibody titers gradually declined in all groups during the first 3 weeks of life as maternally derived antibodies waned. Although the vaccinated group maintained numerically higher antibody titers than the positive and negative control groups at 14 and 21 days of age, these differences were not statistically significant.

Figure 1: Kinetics of serum antibody responses against IBV in broiler chickens vaccinated at hatch with concomitant administration of two live-attenuated IBV vaccines (Cevac IBird and Cevac Vitabron L). Antibody titers were measured by enzyme-linked immunosorbent assay at 1, 7, 14, 21, and 26 days of age.

At 26 days of age (5 dpc), both the vaccinated and positive control groups exhibited significantly higher antibody titers than the negative control group (p < 0.05), indicating antibody responses induced by vaccination and/or challenge infection. The vaccinated group consistently exhibited the highest antibody titers following challenge, suggesting enhanced humoral immunity after concomitant hatch-day vaccination.