Abomasal pH and serum pepsinogen and gastrin were increased in parasite-naive sheep by infection with either larval or adult H. contortus. Four sheep received 10 000 larvae intraruminally and 9000 adult worms were given directly via an abomasal cannula to another 4 sheep. The latter animals were dosed orally with 0.4 mg kg−1 ivermectin 4 days after worm transfer and their recovery was monitored for a further 8 days. Whereas the presence of adult worms rapidly induced changes in the secretory activity of the abomasum, the early larval stages had minimal effects up to the 4th day post-infection. After either larval or adult infection, the initial hypergastrinaemia began at the same time as the increase in abomasal pH, but serum gastrin continued to increase after abomasal pH had reached a maximum and returned more slowly to normal values after drenching. The increase in serum pepsinogen did not precede those in the other parameters, unlike the earlier hyperpepsinogenaemia which occurs with Ostertagia infection. Three of the 8 infected sheep had no detectable serum pepsinogen increases during the parasitism while showing typical effects on abomasal pH and serum gastrin. The rapidity of the responses to the transfer of adult worms and to their removal by treatment with anthelmintic supports a role for worm excretory/secretory (ES) products which possible are not produced by the early larval stages. The similarity of responses to H. contortus and O. circumcincta infection suggests the involvement of the same or very similar ES products
