Age-related protective immunity after vaccination with Haemonchus contortus excretory/secretory proteins

Protection against an experimental challenge infection by immunization with excretory/secretory products (ES) from Haemonchus contortus, containing pr edominantly proteins of 15 and 24 kDa, depends on the age of the sheep. Vac cinated sheep 9 and 6 months of age had reduced final worm burdens of 82 an d 77, respectively. No reduction in worm burden was found in 3-month-old la mbs. Nine-month-old sheep had significantly higher ES-specific serum immuno globulin (Ig)G1 and IgA during immunizations and after challenge infection than 3-month-old lambs. There was no correlation within the 9-month-old she ep between ES-specific IgA or IgG1 levels and protection, measured as worm burden. However, when the different age groups were combined, negative corr elations between percentage protection and ES-specific IgA and IgG1 levels after challenge were found. At the end of the experiment, peripheral blood eosinophils and mast cell counts in abomasal tissue were also significantly higher in the vaccinated and challenged 9-month-old sheep than in the vacc inated and challenged 3-month-old or than in the 9-month-old sheep with cha llenge, but without vaccination. The responses measured in young lambs were similar to the responses in sheep, but the height of these responses was i n general of a lower magnitude.