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Role of Thyroid Hormones, Maternal Antibodies, and Antibody Response in the Susceptibility to Colibacillosis of Broiler Genotypes

B. Ask^*,,1, E. Decuypere and E. H. van der Waaij

^* Department of Farm Animal Health, Utrecht University, 3508 TD, The Netherlands; Animal Breeding and Genetics Group, Wageningen University, 6700 AH, The Netherlands; and Laboratory for Physiology of Domestic Animals, Department of Animal Production, Katholieke Universiteit Leuven, 3001, Belgium

¹ Corresponding author: birgitte.ask{at}wur.nl

The purpose of this study was to investigate whether differences in susceptibility to colibacillosis are associated with maternal antibodies, antibody response, and alterations in thyroid hormones [triiodothyronine (T₃) and thyroxine (T₄)] and to investigate the effect of genotype on the changes in T₃ and T₄ during challenge and antibody response. A challenge experiment was executed in 2 trials. Per trial, 24 chicks per genotype were challenged, and 20 chicks per genotype were controls. At 7 d of age, challenged chicks were intratracheally inoculated with 0.3 mL of Escherichia coli O78K80 and controls with 0.3 mL of PBS. All chicks were euthanized at 14 or 15 d. Thyroid hormone plasma concentrations and E. coli-specific antibody titers (AB) were measured at 7 d (T_{3 d7}, T_{4 d7}, and AB_d7) and 14 or 15 d (change from 7 to 14 or 15 d was analyzed: T₃, T₄, and AB). Susceptibility was defined based on mortality, lesions, growth retardation, and eating behavior. There was a significant effect of challenge on T_{3 d7}; probably due to eating pattern in association with circadian rhythm. The challenge group was suggested to have functional hypothyroidism relative to the control group, indicating metabolic changes due to the challenge, and it was indicated that an antibody response was elicited. Differences in susceptibility were not significantly related to differences in T_{3 d7}, T_{4 d7}, T₃, or T₄ or to maternal antibodies (AB_d7), but the antibody response tended to increase (decreasing AB) with increasing susceptibility. There were indications of genetic variation in T_{4 d7}, T₄, AB_d7, and AB, but there was no observed effect of genotype on T₃ and T₄ during challenge or on the antibody response. Further, there were indications that selection for growth traits has resulted in alterations in T₄ due to challenge, as indicated by a lower T₄ in the challenge group relative to the control group for more intensively selected genotypes as opposed to a higher T₄ for less intensively selected genotypes.

Key Words: triiodothyronine • thyroxine • antibody response • broiler • susceptibility