Influence of lambing and litter size at birth or weaning on the proportion of Pelibuey ewes treated with an anthelmintic in a targeted selective scheme in the humid tropics

ABSTRACT Lambing ewes are generally assumed to experience peri-parturient relaxation of immunity and therefore, to be more susceptible to gastrointestinal nematodes (GIN) compared to non-pregnant ewes. This study evaluated the effect of lambing and different litter sizes at birth and weaning on the frequency of anthelmintic (AH) treatments in a targeted selective treatment (TST) scheme under hot humid tropical conditions. A prospective observational study was performed during six months of the rainy season (June to December 2016), with 91 pregnant Pelibuey ewes and 40 non-pregnant ewes grazing the same pastures. Pregnant ewes were monitored from lambing to 75 days postpartum, while the non-pregnant ewes were monitored during 180 days. Body condition score (BCS), hematocrit (HT) and fecal egg counts (FEC) of GIN were estimated every 15 days. Animals with BCS   1000 eggs per gramme of faeces (EPG) were treated with an AH, and those events were recorded. Contingency tables were used to analyze the degree of association (relative risk (RR) and 95% confidence intervals (CI95%)) in the frequency of treatments between variables such as physiological status (lambing ewes vs. non-pregnant ewes) and litter size at birth and at weaning (1 vs. 2 or 3 lambs). General linear models were used to compare the effect of AH treatment on litter weight at weaning. Post-partum ewes showed a higher risk of receiving an AH treatment than non-pregnant ewes (62% vs. 28% respectively; RR = 2.23, CI95% = 1.92 – 9.29). Single-rearing ewes had the same risk of being treated as twin-rearing ewes (50.0% vs. 60.7%; RR = 0.82, CI95% = 0.52-1.29), but triple-rearing ewes showed higher risk of receiving AH treatment than ewes with singles (90.9%; RR = 1.81, CI95% = 1.17-2.83). Ewes left without AH treatment in the lactation period did not reduce their litter live-weights at weaning compared to treated ewes (P > 0.05). Thus, the TST scheme could be focused on the lambing ewes, with particular attention to triple-rearing ewes.