Abstract
Success of the recently developed precocious vaccine for controlling coccidiosis in chicken largely depends on the extent of immunological heterogeneity among strains. In this study, seven single-oocyst-derived strains of Eimeria tenella isolated from Dhaka (D91/1), Mymensingh (M91/1, M91/2 and M91/3), Chittagong (Ch91/1 and Ch91/2) and Sylhet (S91/1) districts of Bangladesh were compared for pathogenicity and cross-protection in vivo. Strain M91/1 was found to be more pathogenic compared to M91/2 and M91/3, and caused a significant (P<0.05) depression in weight gain and blood haematocrit values, and a higher mortality. None of these three strains were different from other strains. Two immunising doses produced a solid homologous immunity. Following heterologous challenge with strain Ch91/1, there was an increased oocyst output and significant (P<0.05) depression in weight gain in chicks immunised with S91/1. The results suggest that S91/1 was less immunogenic than strain Ch91/1, although neither of these two strains were different from other strains. It is suggested that this difference was not due to a difference in fecundity or pathogenicity, rather it was more likely to be a fundamental difference in antigenic repertoire.
