THE EFFECT OF CONTINUOUS DRUG EXPOSURE ON THE IMMUNE-RESPONSE OF LAMBS CHALLENGED WITH DRUG-SUSCEPTIBLE OR DRUG-RESISTANT NEMATODE LARVAE

Groups of lambs either with or without controlled-release albendazole (ABZ) capsules (CRCs) were challenged twice weekly for 6 weeks with ei ther drug-susceptible or drug-resistant Ostertagia circumcincta and Tr ichostrongylus colubriformis. Groups with and without CRCs remained un challenged as controls. There was minimal establishment of drug-suscep tible parasites of either species in those lambs with CRCs. However, d rug-resistant parasites of both species established adult worm burdens in the presence of the capsules. The humoral immune response, as meas ured by the serum antilarval (L-3) antibody (Ab) titre, was pooled for weeks 4-6 and compared for each group. With the exception of anti-T c olubriformis Ab in group 2, anti-L-3 Ab titres were significantly high er in all the parasite-challenged groups as compared to the control an imals. Also, with the exception of anti-resistant O. circumcincta Ab l evels in the CRC-treated animals (group 5), no significant difference was observed in Ab titres between the four groups challenged with eith er resistant or susceptible larvae. The results demonstrate the inabil ity of CRCs to prevent establishment of drug-resistant parasites and t hat immune stimulation is not inhibited by the capsules.