Maternal transmission of Small Ruminant Lentivirus has no epidemiological importance

The relative importance of maternal and horizontal transmission of small ruminant lentivirus (SRLV), the causative organism in maedi-visna, is poorly understood. Review of the literature shows that maternal transmission is inefficient, infecting only about 10-25 % of the lambs of infected ewes. Theory proves that maternal transmission alone cannot achieve the rates of transmission that would be required to start or maintain an outbreak. Maternal and horizontal transmission are additive in effect, and we use modelling to show that maternal transmission does not amplify or enhance prevalence in the presence of horizontal transmission. Taking steps to avoid maternal transmission by rearing lambs without infected maternal colostrum does have a role in producing a clean flock, but has no significance for the control of a disease outbreak if the conditions for horizontal transmission are present. Efforts to prevent disease by reducing the spread of SRLV must be focussed on minimising horizontal transmission.