Effect of a single treatment with doramectin on gastrointestinal nematode infections of sheep grazing on alpine pastures

The persistent effect of doramectin injectable against gastrointestinal nem atodes was investigated in a controlled field study with 70 sheep kept on a lpine pastures in Switzerland. After grazing on home pastures for four week s 50 lambs and 20 ewes were allocated to two equal groups according to age and body weight. At turnout to alpine pasture in June doramectin (0.3 mg/kg ) was administered by intramuscular injection to 25 lambs and 10 ewes (Grou p D), whereas control sheep (Group K) remained untreated. Animals of both g roups were kept on separate pastures (altitude: 1100 m) and were rotated be tween three paddocks during a total grazing period of 13 weeks. After doram ectin treatment faecal examinations of Group D sheep showed a marked reduct ion of the trichostrongyle egg output which remained close to zero for eigh t weeks. During this period serum pepsinogen levels did not indicate the pr esence of a substantial immature worm burden in the abomasal mucosa. In the last five weeks of the alpine grazing period the trichostrongyle egg count s increased markedly in the lambs of Group D. Lambs of the control group de veloped mode-rate-to-high Haemochus-infections, and eight animals of this g roup had to be treated with anthelminthics. Lambs of Group D had gained sig nificantly (P<0.05) more weight eight weeks after turnout whereas the mean bodyweight stagnated during the last five weeks of the grazing period. The results indicate, that the single administration of 0.3 mg/kg doramectin to lambs and ewes provided good protection against severe infections with gas trointestinal nematodes for a period of approximately eight weeks.