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 (T(3 d7), T(4 d7), and AB(d7)) and 14 or 15 d (change from 7 to 14 or 15 d was analyzed DeltaT(3), DeltaT(4), and DeltaAB). 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), DeltaT(3), or DeltaT(4) or to maternal antibodies (AB(d7)), but the antibody response tended to increase (decreasing DeltaAB) with increasing susceptibility. There were indications of genetic variation in T(4 d7), DeltaT(4), AB(d7), and DeltaAB, but there was no observed effect of genotype on DeltaT(3) and DeltaT(4) during challenge or on the antibody response. Further, there were indications that selection for growth traits has resulted in alterations in DeltaT(4) due to challenge, as indicated by a lower DeltaT(4) in the challenge group relative to the control group for more intensively selected genotypes as opposed to a higher DeltaT(4) for less intensively selected genotypes.