Effect of ewe ovine lentivirus infection on ewe and lamb productivity.

We used a previously described sensitive and specific ovine lentivirus (OLV) recombinant transmembrane (rTM) protein enzyme-linked immunosorbent assay (ELISA) to detect anti-OLV antibodies and define OLV infection in breeding ewes from nine US Meat Animal Research Center (MARC) flocks. We estimated the production impacts of dam rTM ELISA seropositivity on ewe and lamb productivity in the birth-to-weaning interval using production data from 1466 breeding ewes (of which 1242 actually lambed) and their 2452 lambs born in spring 1992 using several multiple linear and logistic regression models. By adjusting for lamb weaning age, gender, type of birth and rearing, birth difficulty, dam age, and flock, the component of ewe or lamb productivity related to ewe OLV infection alone was isolated. The rTM ELISA-negative ewes produced significantly more total weight of weaned lamb per ewe-lambing (3.84 kg) and per ewe ram-exposed (4.95 kg) compared to their OLV-positive flockmates. Negative ewes also weaned 0.11 more lambs per ewe-lambing and 0.09 more lambs per ewe ram-exposed, gave birth to 0.13 more lambs per ewe ram-exposed, and were more likely to lamb after breeding (odds ratio (OR) = 1.9) compared to equivalent OLV-positive ewes. Lambs reared by OLV-negative ewes weighed 0.15 kg more at birth, gained 8 g more per day through weaning, and weighed 0.59 kg more at 56-day weaning. Preweaning mortality was lower (OR = 0.8) among lambs born to OLV-negative compared to OLV-positive ewes, although this difference was not significant. Our results suggest that subclinical OLV infection has important detrimental effects on sheep production which occur in cumulative fashion from breeding through weaning and that OLV control efforts may be financially justified in some sheep flocks.