Genetic differences and heritability of antibody response to Escherichia coli vaccination in young broiler chicks.

Broiler chicken lines, selected divergently for high (HC) or low (LC) antibody titer to Escherichia coli vaccination at an early age, were evaluated for antibody response at the S5 and S9 generations of selection. The full-pedigreed populations consisted of about 300 and 400 chicks per line in S5 and S9, respectively. At S5, all chicks were vaccinated at 10 d of age (VA10) and antibody titer was determined twice for each chick, at 8 and 12 d postvaccination (dPV). At S9, each line was divided into two equal groups; in the HC line, one group was vaccinated at 8 d of age (VA8), and the other at 10 d of age (VA10), whereas in the LC line, one group was VA10 and the other was VA12. Antibody titers were determined twice for each chick, 8 and 10 dPV. The effects of line, age at vaccination (VA), and days for antibody development (dPV) were tested, and the heritability of antibody titer was estimated for each line-VA-dPV set of data. The HC and LC lines differed significantly in the maturation process of their immune systems. The percentage of chicks with detectable antibody at 18 d of age (VA10-8 dPV) among HC chicks was significantly higher than among LC chicks (85 vs 48% in S5 and 96 vs 63% in S9). In S9, 90% of the HC chicks had already responded at 16 d of age and 100% at 18 d of age, whereas among the LC chicks, only 62% were positive at 18 d of age, increasing to no more than 98% at 22 d of age. The results demonstrates that selection of antibody titer to E. coli vaccination at 20 d of age actually affects the earliest age of immune response, as the immune system of the HC chicks matures earlier than that of the LC chicks. The HC S9 chicks at 8 dPV exhibited a fourfold higher antibody titer than their LC 8 counterparts. This difference further increased at 10 dPV, indicating that the lines differed not only in the level of antibody at a specific age, but also in their rate of antibody titer development. The highest estimate of heritability was very similar in both lines (0.44 and 0.42 in HC and LC, respectively). However, in the HC line this heritability was exhibited at 18 d of age, and only at 22 d in the LC line. Thus, both lines have a similar amount of genetic variation for early immune response, but in the HC line this variation is fully expressed 4 d earlier than in the LC line. These results suggest that selection for high or low antibody response in young chicks results in early or late antibody production, respectively. To maximize the efficiency of selection for early immune response, one must determine the best vaccination age and timing of antibody evaluation in any given population, and these values must be revalidated and updated as selection proceeds.