Arginine metabolism in the sheep abomasal nematode parasites Haemonchus contortus and Teladorsagia circumcincta

► Neither ornithine urea cycle (except arginase) nor arginine deiminase/dihydrolase enzyme activities in L3 and adult H. contortus and T. circumcincta. ► Nitric oxide synthase (NOS) activities not detected in homogenates but shown histochemically in nerves. ► Arginase and agmatinase activities may be due to a single enzyme in T. circumcincta but two enzymes in H. contortus. ► Putrescine can be synthesised either by arginase and ornithine decarboxylase (ODC) or by arginine decarboxylase (ADC) and agmatinase in H. contortus, whereas ADC activity was not detected in than in T. circumcincta. ► The flexibility in synthesis of putrescine in H. contortus may make it less useful as a target for parasite control than in T. circumcincta.