Four groups of mycoplasma-free commercial broilers were challenged with the R strain of Mycoplasma gallisepticum (MG) at 14 days of age. Groups received feed containing either no medication, or 500 ppm or 1000 ppm oxytetracycline (OTC) beginning at age 13 days, or 1000 ppm OTC beginning at age 15 days. All broilers were vaccinated with a live mild Massachusetts infectious bronchitis vaccine at 17 days of age. Air sac lesions were scored at age 24 days. In two almost identical experiments, all OTC treatment groups had significantly lower mean air sac lesion scores than the unmedicated challenged controls. Groups that were fed 1000 ppm OTC in feed had significantly lower mean air sac lesion scores than groups that were fed 500 ppm OTC in feed. There was no significant difference in mean air sac lesion scores between the groups fed 1000 ppm OTC in feed beginning at 13 days of age and those fed 1000 ppm OTC in feed beginning at 15 days of age.
Serum samples collected from 23 flocks of commercial hens from three different companies were tested by enzyme-linked immunosorbent assay (ELISA) for antibodies against infectious laryngotracheitis (ILT) virus, and data were analyzed statistically. Geometric mean titers (GMTs) were compared from hens that were unvaccinated, once-vaccinated, or twice-vaccinated, from single-age farms or multiple-age farms, from molted or unmolted flocks, and from different companies. There were significant differences among the groups compared by vaccination, between the single-age and multiple-age groups, and between the molted and unmolted groups. The GMT of unvaccinated flocks and the GMT of molted flocks that had been vaccinated once as pullets with a chick-tissue-culture-origin (CTCO) live vaccine could not be differentiated. The ELISA is useful to detect and quantitate ILT vaccine-induced antibody, but it does not reliably identify older flocks that were vaccinated at a young age with CTCO vaccine.
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