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ABSTRACT: The F strain of Mycoplasma gallisepticum (MG-F) protects chickens against mycoplasma infections, in which monitoring is made by serology and histopathology of trachea. This trial used 90 chickens, being 30 unvaccinated (G1 group), 30 eye-drop vaccinated at 8 weeks of age with MG-F (Ceva Animal Health, São Paulo, SP, Brazil) (G2), and 30 immunized at 8 and 11 weeks of age (G3). Samples were obtained from chickens on the 8, 12, 15, 18, 20 and 24th weeks of age for the enzyme-linked immunosorbent assay (ELISA) test. Tracheal fragments were collected after necropsies on the 15 and 24th weeks of age. Up to 12 weeks, the ELISA reactions in optical density (OD) were 0.165 (G1), 0.151 (G2) and 0.151(G3), all below 0.20 and with no significant difference among groups (p &gt; 0.05). After the 15th week, the ELISA reactions rose, yielding the following group averages by collecting dates: G1 (0.18, 0.19, 0.18, and 0.16), G2 (0.36, 0.49, 0.47, and 0.44) and G3 (0.41, 0.52, 0.59, 0.60), being the means in G2 and G3 not significantly different between than, but significantly different from G1. The initial weight (592.71, 621.33, and 594.40), the final weight (1,932.58, 1,987.59, and 1,875.20) and the weekly weight gain (11.65, 11.90, and 11.14) were not significantly different among groups. At necropsy the gross tracheal score means by group and dates were: 15th week (0.25, 0.61, and 0.54) and 24th week (0.54, 0.58, and 0.67), being these difference not significantly (p &gt; 0.05). On microscopy, the tracheal score averages by groups G1, G2 and G3, respectively, were: 15th week (0.25, 0.32, and 0.47) and 24th week (0.07, 0.75, and 0.08). G2 yielded higher score average than G1 and G3 on the 24th week. Higher tracheal changes for G2 and G3 as compared to G1 could be ascribed to MG-F infection. There were no evident prejudicial effects on live weight, weight gain and tissue changes by applying one or two vaccination doses.

Performance, serologic and tracheal responses of laying hens exposed to F strain of Mycoplasma gallisepticum