Mean egg output reduction. Mean egg output reduction (95%CI) measured by (a) the McMaster method and (b) the FLOTAC method in isolates 9 days after treatment (44 d.p.i.) with IVM. Calves in group A were infected with a propagated IVM-susceptible laboratory isolate and those in groups B and C with two different field isolates showing phenotypic clinical IVM resistance in field trials. Reductions were similar in all three groups.