The residual effect of treatment with ivermectin after experimental reinfection with nematodes in calves

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Abstract
The residual effect of treatment with ivermectin after experimental reinfection in calves was tested. Twenty‐four calves were divided into 6 groups of 4 calves each. All calves received a primary infection of 50,000 larvae of both Ostertagia ostertagi and Cooperia oncophora and 1000 Dictyocaulus viviparus larvae. Calves of group 1 remained untreated, and all other calves were treated 21 days after primary infection (0.2 mg/kg injected subcutaneously). Calves of groups 1 and 2 were slaughtered 7 days later. Calves of groups 3–6 were reinfected with the same number of larvae 3 days, I, 3 and 6 weeks after treatment respectively. Slaughter was 21 days after reinfection. Based on post‐mortem worm counts the efficacy of ivermectin after primary infection was 99.7% for O. ostertagi, 95.1% for C. oncophora and 100% for D. viviparus. A residual effect was present for at least one week, but could not be observed 3 weeks after treatment.