Time- and dose-dependent development of humoral immune responses to Ascaridia galli in experimentally and naturally infected chickens

The plasma antibody (PAB) and egg yolk antibody (EAB) levels of experimentally infected birds were compared to those of control and tracer birds throughout the study period, and PAB levels were found to depend initially on the infection dose but thereafter mainly on reinfections. Starting at wpi 2, 1000+ had consistently higher PAB levels than CON did (P < 0.05). With exceptions at wpi 0, 2 and 12, PAB levels were also higher (P < 0.05) or tended to be higher (P < 0.10) in 100+ than in CON. An earlier and higher increase was observed in the PAB levels of T1000+ than in those of T100+, implying that (re-)infection occurrence depended on the infection dose. Although 1000+ showed higher (P < 0.05) EAB levels than CON did at both wpi 14 and 18, EAB levels were higher in 100+ than in CON only at wpi 28 (P < 0.05). The total worm burdens in the initial experimentally infected birds were similar (P = 0.257); they were also highly comparable between experimentally and naturally infected birds, indicating that final worm burden was mainly determined by the naturally occurring infections resulting from continuous exposure. When all available information on the retrospective plasma and egg yolk IgY levels was collectively evaluated to estimate the larval or total worm burdens of the experimentally infected birds, both PAB and EAB levels at particular wpi were significantly associated with worm burden, especially with larval count. In conclusion, our data support the hypothesis that the number of larvae, rather than the number of mature worms, affects the antibody levels in both plasma and egg yolk. Despite the increased levels of A. galli-specific antibodies in plasma and egg yolk throughout the study period, only a weak indication was found that antibodies might be directly associated with protection.