Patterns of parasitic nematode infection and immunity in dairy heifers treated with ivermectin in a sustained-release bolus formulation either at turnout or in the middle of the grazing season

Twenty-eight Holstein-Friesian heifers, born the previous year and weighing between 130 and 310 kg, were allocated to one of two treatment groups by r estricted randomisation, based on their initial weight. The heifers in grou p 1 were each treated with ivermectin in a sustained-release bolus formulat ion at turnout in April, and those in group 2 were each given an ivermectin bolus on July 10, 84 days after turnout. On that day the mean geometric wo rm egg counts of groups 1 and 2 were 0 . 4/g and 38 . 8/g, respectively, an d they both had a mean plasma pepsinogen concentration of 0 . 59 iu/litre; in group 1, two of 14 faecal samples were positive for Dictyocaulus vivipar us larvae, and in group 2 all 13 samples were positive; in group 1 eight ca lves were positive and three inconclusive for the presence of antibodies to D viviparus, and in group 2 the corresponding figures were 10 positive and two inconclusive; the mean liveweights of groups 1 and 2 were 274 . 4 kg a nd 262 . 8 kg, respectively. By December 4, 231 days after turnout, the cor responding results were: mean geometric worm egg counts of 2 . 2/g and 0 . 5/g; one of 13 and none of 14 faecal samples positive for D viviparus larva e; 12 positive and two inconclusive and none positive and 10 inconclusive f or the presence of antibodies to D viviparus; 214 days after turnout their mean liveweights were 361 . 1 kg and 358 . 3 kg. Although the patterns of p arasitic nematode infection were different in the two groups during the gra zing season, by the time they were housed both groups had achieved similar liveweights and showed evidence of an immune response to both D viviparus a nd gastrointestinal nematodes.