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Skin test responses to avian PPD (□) and bovine PPD (▪) at 11 weeks after vaccination (A) and 13 weeks after challenge (B). The data are expressed as the mean increase in skin thickness (mm) ± SEM between the time of inoculation and 72 h later. *, the mean was significantly different compared to that for the nonvaccinated group (P < 0.05); **, the mean was significantly different compared to that for the nonvaccinated group (P < 0.01).
