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A retrospective cohort study used records from a large flock of sheep in Illinois, USA in logistic regression models to identify risk factors for respiratory disease mortality (RM) in lambs born from 1986 to 1991 (n = 6806). The flock of several purebred and crossbred lines was intensively managed in open-front barns from birth to weaning (56 days) and fattened in slotted-floor barns. Annual postnatal mortality ranged from 12% to 19% of lambs born. Potential risk factors examined were gender, lamb breed, birth weight, lambing location, number of siblings at birth and rearing, birth vigor, and the dam's parity, milk supply and udder condition. RM occurred in 4.3% of lambs that survived the first 24 h after birth, and this rate was fairly stable over all years of the study (3.3–5.0%). One model compared RM cases with all lambs surviving the first day of age and a second model compared RM cases with all lambs surviving to weaning (56 days of age). In the first model, there was increased risk for RM in ram lambs (OR = 1.9), purebred Suffolks (OR = 2.0), lambs from first parity dams (OR = 1.9), and lambs from dams with poor milk production (OR = 2.2). Lambs with average birth weight were at slightly lower risk for RM than light or heavy weight lambs (OR = 0.8). The second model identified similar factors, with ram lambs, Suffolks, and lambs from dams at first parity or with poor milk supply at increased RM risk (OR = 2.0, 2.3, 2.0, and 2.3, respectively) and average weight lambs at lower risk for RM (OR = 0.7). A strong birth vigor reduced risk for RM (OR = 0.7). Variables not included in final models included lambing location, number of siblings at birth/rearing, and the dam's udder condition. Although factors such as breed and gender may not generally be manipulated to decrease RM, other factors such as birth weight and the dam's milking ability can be improved to reduce respiratory disease mortality. Awareness of variations in RM risk within a population can enhance early response to potential outbreaks, reducing losses. |
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