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

Abstract

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 (T3 d7, T4 d7, and ABd7) and 14 or 15 d (change from 7 to 14 or 15 d was analyzed: ΔT3, ΔT4, and ΔAB). Susceptibility was defined based on mortality, lesions, growth retardation, and eating behavior. There was a significant effect of challenge on T3 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 T3 d7, T4 d7, ΔT3, or ΔT4 or to maternal antibodies (ABd7), but the antibody response tended to increase (decreasing ΔAB) with increasing susceptibility. There were indications of genetic variation in T4 d7, ΔT4, ABd7, and ΔAB, but there was no observed effect of genotype on ΔT3 and ΔT4 during challenge or on the antibody response. Further, there were indications that selection for growth traits has resulted in alterations in ΔT4 due to challenge, as indicated by a lower ΔT4 in the challenge group relative to the control group for more intensively selected genotypes as opposed to a higher ΔT4 for less intensively selected genotypes.