Managing Maedi-Visna in sheep flocks infected by Small Ruminant Lentivirus (SRLV) by limiting production losses and reducing transmission could be a more cost-effective approach than eliminating the disease. We quantified the progressive physiological costs of SRLV infection by analysing performance measures as readily accessible criteria of disease severity in data from a five-year study of a Polish flock with bi-annual tests of serostatus. SRLV infection had no effect on ewes’ pre-natal reproductive traits but caused increased mortality and reduced growth rate in their lambs, and reduced ewes’ productive lifespan and output. These performance deficits were negligible soon after seroconversion but increased significantly with time since seroconversion, such that they became substantial in older ewes with longer durations of infection. We argue that infectiousness is likely to be correlated with disease severity and therefore tends to increase with the duration of infection, implying that the oldest animals pose the greatest infection risk. Selective culling of under-performing older ewes would limit ongoing production losses and may also reduce disease transmission in affected sheep flocks.