Effects of the In ovo Administration of the 6/85 Mycoplasma gallisepticum Vaccine on Layer Chicken Embryo Hatchability and Early Posthatch Performance

In ovo administration as a possible alternative method of 6/85 MG vaccination was assessed. After 18 days of incubation (doi), the eggs were administered a particular dosage of a live attenuated 6/85 MG vaccine in either the air cell (AC) or amnion (AM). The treatments included non-injected eggs and eggs injected into the AC or AM with diluent alone as controls. Treatments also included eggs injected with diluent, which contained 1.73 × 10², or 1.73 × 10⁴ CFU of 6/85 MG. Hatchability of viable injected eggs (HI) and residual embryonic mortality were determined at 22 doi. At hatch and at three weeks posthatch, one hatched chick per treatment replicate was bled and swabbed for the detection of 6/85 MG in the choanal cleft using PCR, serum plate agglutination (SPA), and ELISA methods. The results show that AC in ovo injection of 6/85 MG had no negative impacts on HI or on the live performance of pullets, but that it failed to provide adequate protection (p ≤ 0.0001) in hatchlings or three-week-old pullets. The 1.73 × 10⁴ 6/85 MG CFU dosage injected into the AM decreased the hatchability of injected eggs containing viable embryos (HI; p = 0.009) and was associated with a significant increase in late dead mortality (p = 0.001). Hatchling and three-week-old chick mortalities (p = 0.008) were significantly greater in the 1.73 × 10⁴ CFU-AM treatment group in comparison with the other treatment groups. In addition, the 1.73 and 1.73 × 10² 6/85 MG-AM treatments had no negative effects on the hatching process or on posthatch growth, and the 1.73 × 10² 6/85 MG-AM treatment was more effective in the protection of pullets against MG (p ≤ 0.0001) as compared with the low dosage and non-injected treatment groups. Further research is needed to examine the influence of the 6/85 MG in ovo vaccine on layer immune competence.