Effect of strategic helminthosis control on mortality of communally grazed Menz lambs of smallholders in the cool central Ethiopian highlands

A 3-year longitudinal study was conducted to evaluate the effects of strategic anthelmintic dosing on communally grazed village sheep in Gera Keya district in the central highlands of Ethiopia. Ewes were stratified by weight and randomly allocated to three treatment groups: untreated control (TG1), twice-dosed per year (TG2; for both nematodes and trematodes in mid-January and mid-June) and four-time-dosed per year (TG3; in June for nematodes, in August–September for nematodes and adult Fasciola, in November–December and January–February for immature liver flukes). The fixed effect of treatment, lamb sex, dam parity, season and year of birth on mortality from birth to 90, 180, 270 and 365 days was analyzed. Least square means of lamb mortality from birth to 90, 180, 270 and 365 days were 11.3, 16.8, 18.0 and 19.5%, respectively. Anthelmintic treatment had significant effect (P < 0.05) on mortality of lambs at all ages: lambs in TG3 had consistently higher mortality than lambs in TG2. The lambs in TG3 were worse in terms of survival from birth to 180 days even when compared with the control group (TG1). Mortality rate in male lambs was twice as high compared to their female contemporaries at all ages (P < 0.001). Season of birth had significant (P < 0.01) effect on lamb mortality at all ages. Lambs born during rainy season had the lowest mortality in the first 6 months (0–90 and 0–180 days) of age than those born during short rainy or dry seasons. Instead of frequent mass drenching, discriminatory drenching on a case-by-case basis should be considered to improve lamb survival.