Cholinesterase activities of various nematode parasites and their inhibition by the organophosphate anthelmintic Haloxon

Abstract

Homogenates of Dictyocaulus filaria, Haemonchus contortus, Bunostomum trigonocephalum, Trichuris ovis, Oesophagostomum columbianum, Oesophagostomum venulosum, Oesophagostomum dentatum, Chabertia ovina, and Ascaris lumbricoides L. possess the ability to hydrolyze acetylcholine; three of these ( O. venulosum, D. filaria, and B. trigonocephalum) have remarkably high activity (844, 977, and 1,760 μmoles/gm per hour, respectively), the others have activities below 200 μmoles/gm per hour. The organophosphorus anthelmintic Haloxon is a fully effective treatment for a number of the helminth species studied. In them, the cholinesterase activity was inactivated for long periods by low concentrations of inhibitor; this contrasts with the situation found in sheep, where spontaneous reactivation of the cholinesterase is rapid. Of the species, that do not respond to Haloxon treatment under normal dosing conditions, O. columbianum, T. ovis, and C. ovina possess cholinesterases with low inhibition rate constants, and differ only quantitatively from susceptible species. Bunostomum trigonocephalum and D. filaria, by contrast, possess cholinesterases which reactivate spontaneously after inhibition by Haloxon, and thus differ qualitatively from all the other species. These quantitative and qualitative differences and their relationship with the chemotherapeutic effect of Haloxon are discussed.