Age-dependent immune response of chickens to feeding by Argas persicus larvae

Chickens of the dwarf Velsum breed acquired immunity to Argas persicus larvae during repeated infestations with 100 larval ticks in 2-week intervals, when primarily infested at the age of 10 weeks. This immunity was manifested by 74.6% and 75.9% rejection of larvae during secondary and tertiary infestations (P < 0.01), respectively, in relation to controls. The mean weight reductions observed were 16.5% and 12.2% in secondary and tertiary infestations, respectively (P < 0.05). Weak immunity also occurred after secondary and tertiary infestations in chickens infested first at the age of 6 and 8 weeks, but not in chickens aged 2 or 4 weeks at first infestation. No changes were observed in mean feeding periods of larvae engorged on chickens of all age groups (P > 0.05). A significant increase in the leucocyte counts in peripheral blood (P < 0.01 or P < 0.05), mostly of lymphocytes and heterophil granulocytes, followed each infestation in chickens of all age groups. Specific anti-tick circulating antibodies were detected by ELISA in the sera of 10-week-old chickens after primary infestations, in the 4-8-week chicken groups following secondary infestations when aged 6-10 weeks, and in the 2-week chicken group only after tertiary infestations, when the chickens reached the age of 6 weeks. A 53 kDa protein was recognized in the whole-body extract of larval ticks by immune serum during immunoblotting. This was probably the antigenic protein responsible for eliciting the circulating antibodies. The acquired immunity to ticks seen in chickens aged 6 weeks or more may be connected with maturation of the bursa of Fabricius and the development of complete immunocompetence, which is believed to occur in chickens at the age of 4-5 weeks.