Protection against Dictyocaulus viviparus in second year cattle after first year treatment with doramectin or an ivermectin bolus.

Forty-five calves were assigned randomly to three groups of 15 at turnout (day 0) in the spring of their first grazing season on to separate paddocks on a pasture known to carry infective larvae of Dictyocaulus viviparus. Group 1 calves were left as untreated controls, group 2 were injected with doramectin at 200 micrograms/kg at turnout and again on day 56, and group 3 were treated with an intraruminal bolus containing ivermectin. From day 42 onwards group 1 calves developed parasitic bronchitis which required repeated treatment with levamisole. The two endectocide regimes controlled lungworm infection, although some calves in group 2 developed some coughing during the week before the second dose of doramectin. After the end of the grazing season and again in May of the following year, five cattle from each group were infected experimentally with lungworm larvae and slaughtered 28 days later for lungworm counts and measurements of length to be made. At both times group 1 calves were found to be largely resistant to reinfection; group 2 were slightly more susceptible although the differences from group 1 were not statistically significant. Group 3 calves were more susceptible with no significant difference in worm counts from naive infection controls.