The effect of strategic treatments with ivermectin in first-season calves exposed to trichostrongyle nematodes on naturally contaminated pasture was studied. Twenty first season heifer calves were divided into 2 groups, according to live weight, and on 22nd May each group was turned out onto a 1 hectare pasture. Group A (Plot A) was treated with ivermectin at weeks 3, 8 and 13 after turn out, while group B (Plot B) served as an untreated control group. The study showed that control calves exhibited increase in trichostrongyle egg counts in August, while treated calves were excreting low numbers 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 showed 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) showed clinical signs of parasitic gastroenteritis. It can be concluded that the strategical ivermectin treatments were successful, and faecal egg counts, pepsinogen levels and herbage larval counts clearly demonstrated that this was accomplished through suppression of pasture contamination with nematode eggs and subsequent reduction of pasture infectivity.
Undersøgelsen beskriver effekten af strategiske ivermectinbehandlinger mod trichostrongylider hos førstegangsgræssende kalve på naturligt smittede græsmarker. I alt 20 kalve blev fordelt på 2 hold efter vægt. Holdene blev udbundet den 22. maj på sammenlignelige forsøgsarealer, hver på 1 hektar. Hold A (mark A) blev behandlet med ivermectin 3, 8 og 13 uger efter udbindingen, mens hold B (mark B) fungerede som et ubehandlet kontrolhold. Efter første behandling faldt hold A’s fækale ægudskillelse til lave værdier, som forblev lave gennem behandlingsperioden og sæsonen ud. Kontrolholdets ægudskillelse var indledningsvist moderat og steg til høje værdier. Græssets larveindhold på mark B (kontrol) steg til høje værdier i juli, i modsætning til larveindholdet på mark A (ivermectin), som forblev på lavt niveau hele sæsonen. De to hold havde sammenlignelig tilvækst frem til midten af juli, men fra dette tidspunkt voksede de behandlede kalve bedre end kontrollerne, som tillige viste kliniske tegn på parasitær gastroenteritis hen mod sæsonens slutning. Det konkluderes, at de strategiske ivermectinbehandlinger havde særdeles god effect. Fækal ægudskillelse, antal larver i græs samt serumpepsinogen niveauer viste, at denne effekt primært skyldtes en suppression af græskontamination.