Effect of strategic treatments with invermectin on parasitism of set-stocked calves exposed to natural trichostrongyle infection in Lithuania

The effect of strategic treatments with ivermectin in first-season calves e xposed to trichostrongyle nematodes on naturally contaminated pasture was s tudied. Twenty first season heifer calves were divided into 2 groups, accor ding to live weight, and on 22nd May each group was turned out onto a I hec tare pasture. Group A (Plot A) was treated with ivermectin at weeks 3, 8 an d 13 after turn out, while group B (Plot B) served as an untreated control group. The study showed that control calves exhibited increase in trichostr ongyle egg counts in August, while treated calves were excreting low number s of trichostrongyle eggs. Pasture larval counts on Plot B (control animals ) were low during the first part of the grazing season, followed by a steep rise towards the end of July. In contrast, the numbers of infective larvae recovered from Plot A remained low throughout the season. Both groups show ed comparable weight gains from May up to the middle of July. However, from then on, Group B (controls) had lower weight gains than ivermectin treated Group A. From the end of July onwards, most untreated calves (Group B) sho wed clinical signs of parasitic gastroenteritis. It can be concluded that t he strategical ivermectin treatments were successful, and faecal egg counts , pepsinogen levels and herbage larval counts clearly demonstrated that thi s was accomplished through suppression of pasture contamination with nemato de eggs and subsequent reduction of pasture infectivity.