Methods in the evaluation of antiparasitic drugs in the horse.

The critical test is the primary method used for the efficacy evaluation of drugs against the major internal parasites (bots, ascarids, large strongyles, small strongyles, and pinworms) of the horse. The critical test determines: (1) spectrum of activity, (2) effectiveness of removal, (3) pattern of discharge, and (4) physical condition of each species of these parasites. General characteristics of the major parasitisms of the horse are discussed briefly. Criteria of the critical test also are considered including: (1) number of tests, (2) strain variation and drug resistance, (3) selection of test horses, (4) diagnosis of parasitic species, (5) numbers of parasites, (6) minimal efficacy requirements, and (7) other parasitic species. The controlled test principally is used on a selected basis for the small nematodes in the proximal portion of the digestive tract which cannot be properly evaluated by the critical test, or for other limited objectives. Clinical trials are discussed briefly but are invaluable supplements to the critical and controlled tests in the total assessment of a drug as a new product or for continued effectiveness in clinical use. Experimental procedures used in the conduct of drug evaluations should not be rigidly prescribed but should reflect input by the individual investigatior.