Projects per year
Several studies have shown that transmission of natural scrapie can occur vertically and horizontally, and that variations in scrapie incidence between and within infected flocks are mostly due to differences in the proportion of sheep of susceptible and resistant Prnp genotypes. In this report we present the results of a 12-year period of scrapie monitoring in a closed flock of Suffolk sheep, in which only animals of the ARQ/ARQ genotype developed disease. Amongst a total of 120 of these, scrapie attack rates varied between birth cohorts from 62.5% (5/8) to 100% (9/9), and incidence of clinical disease amongst infected sheep from 88.9% (8/9) to 100% (in five birth cohorts) . Susceptible sheep born to scrapie infected ewes showed a slightly higher risk of becoming infected (97.2%), provided earlier biopsy positive results (average 354 days) and developed disease at a younger age (median 736 days), than did those born to non-infected dams (80.3%, 451 days and 782 days, respectively). All this together is interpreted as evidence of maternal transmission. However, it was also observed that for the birth cohorts with the highest incidence of scrapie (90-100%), sheep born to infected and non-infected dams had a similar risk of developing scrapie (97.1% and 95.3%, respectively). Compared to moderate attack rate cohorts (62.5-66.7%), high incidence cohorts had greater numbers of susceptible lambs being born to infected ewes, suggesting that increased rates of horizontal transmission in those cohorts could have been due to high levels of environmental contamination caused by infected placentas.