The role of lambs in louping-ill virus amplification

In some areas of Scotland, the prevalence of louping-ill virus has not decr eased despite the vaccination of replacement ewes for over 30 years. The ro le of unvaccinated lambs in viral persistence was examined through a combin ation of an empirical study of infection rates of lambs and mathematical mo delling. Serological sampling revealed that most lambs were protected by co lostral immunity at turnout in May/June but were fully susceptible by the e nd of September. Between 8 and 83 % of lambs were infected over the first s eason, with seroconversion rates greater in late rather than early summer. The proportion of lambs that could have amplified the louping-ill virus was low, however, because high initial titres of colostral antibody on farms w ith a high force of infection gave protection for several months. A simple mathematical model suggested that the relationship between the force of inf ection and the percentage of lambs that became viraemic was not linear and that the maximum percentage of viraemic lambs occurred at moderately high i nfection rates. Examination of the conditions required for louping-ill pers istence suggested that the virus could theoretically persist in a sheep flo ck with over 370 lambs, if the grazing season was longer than 130 days. In practice, however, lamb viraemia is not a general explanation for louping-i ll virus persistence as these conditions are not met in most management sys tems and because the widespread use of acaracides in most tick-affected hil l farming systems reduces the number of ticks feeding successfully.