Estimating roundworm burdens and group sizes in anthelmintic trials with sheep and cattle

Results from controlled anthelmintic trials against roundworms in sheep and cattle have demonstrated that the variability in worm burdens within a group of animals is made up of the variation introduced in the sampling and counting of worms from the gut contents of an individual animal, and the variation in worm burdens from animal to animal within the group. These two factors are examined in detail and a procedure for counting worms from animals in anthelmintic trials proposed. A method is also suggested for determining the number of animals to be used in the untreated (control) and treated groups in anthelmintic tests, depending upon the degree of precision required in the evaluation of anthelmintic activity.