Comparison between fenbendazole and moxidectin applied in a dose and move system for the control of Dictyocaulus viviparus infections in calves.

A grazing study was performed with the main objective to compare the effect of moxidectin (MDT) and fenbendazole (FBZ) in a 'dose and move' system on nematode infections in calves with special emphasis on Dictyocaulus viviparus. Three groups of six calves were grazed from May to October 1994. All groups grazed together until 9 weeks after turnout when they were moved to separate mowed pastures. One group (MM) was then treated with 0.5 mg kg-1 MDT pour-on, the second group (FM) was treated with 7.5 mg kg-1 FBZ drench and the third group served as untreated pasture control group (PC). Two calves from MM and two from FM were experimentally infected with 20 lungworm larvae at turnout in order to initiate low infections in the herd. Pairs of tracer calves grazing during the first 7, 8 or 9 weeks after turnout acquired mean burdens of 218, 255 and 1156 lungworms, respectively. MDT and FBZ treatment removed adult lungworms from MM and FM. In PC faecal larval counts increased until the end of July, when most animals were suffering from lungworm disease. No lungworm disease occurred in both dose and move groups. In FM larvae reappeared in the faeces of some of the calves from 1 month after treatment and low patent infections remained to be present in FM in some calves until the end of the experiment. No reappearance of larvae after treatment was observed in MM. The mean Optical Density (OD) values of the three groups on pasture closely followed the infection patterns. After housing in October all calves, and also a group of five permanently housed non-infected control calves (HC), were infected experimentally with 5000 D. viviparus larvae to evaluate development of immunity. The worm counts at necropsy showed that groups on pasture had developed immunity. However, the degree of immunity was lower in MM than in FM and PC.