THE COMBINED EFFECT OF FENBENDAZOLE TREATMENT AND A MOVE TO AFTERMATH7 OR 9 WEEKS AFTER TURNOUT ON DICTYOCAULUS-VIVIPARUS INFECTIONS IN CALVES

A grazing study was performed with the main objective of examining the effect of 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 1993. One g roup (DM7) was treated with FBZ and moved to aftermath (pasture which had only been mown) 7 weeks after turnout. The second group (DM9) was similarly treated and moved 9 weeks after turnout and the third group served as untreated pasture control group (PC) and was moved to afterm ath 9 weeks after turnout. FBZ treatment removed adult lungworms from DM7 and DM9. Tracer calves grazed during the first 7 or the first 9 we eks after turnout acquired mean burdens of 18 and 125 lungworms, respe ctively. In PC faecal larval counts increased until the end of August. Most of the animals in this group were then suffering from lungworm d isease and emergency treatment with ivermectin was given. In both FBZ- treated groups, larvae reappeared in the faeces of some of the calves 4-5 weeks after treatment. Subsequent reinfection resulted in higher m ean faecal larval counts in both groups 2 months after treatment, alth ough variation in faecal larval counts was high. In DM7 values tended to be higher than in DM9, These higher larval counts were associated w ith mild signs of parasitic bronchitis in some calves of DM7, whereas no signs were seen in DM9. At the end of the experiment, all calves, a nd also a group of six permanently housed non-infected control calves (HC), were infected experimentally with 5000 D. viviparus larvae to ev aluate development of immunity. The worm counts at necropsy showed tha t all calves on pasture had developed immunity.