Scrapie is found solely in sheep and goats, and there is no evidence that it has ever been transmitted to humans (see Prevention and Control section).
Transmission of scrapie occurs most efficiently following lambing, and rams are not thought to transmit the disease. While the exact mechanisms of transmission from ewes to offspring, or from ewes to other lambs in the same flock, are not fully understood, placental tissues are a well-known source of scrapie infectivity.4
Research indicates that lambs are not infected with scrapie in utero since lambs delivered via Caesarean section from infected mothers do not contract the disease. Instead, the disease is transmitted from mother to offspring or other lambs through contact with placenta or associated fluids.
Mechanisms of transmission other than from infected ewes are unclear. Contrary to the earlier belief that excretions and tissues other than those associated with lambing are free of infectivity, recent research indicates that sheep can be infected with scrapie by blood transfusion.5 Milk6 and urine7 also can carry scrapie prions.
Scrapie also is noted for its persistence in the environment, which may provide an explanation for anecdotal accounts of infection of flocks that were not otherwise exposed to scrapie. One study found that scrapie prions can remain infective for more than three years on unused pastures.8 Still other research has indicated that certain regions of Iceland have retained scrapie infectivity for at least 16 years.9 However, it is difficult to definitively determine if scrapie can be spread by contaminated pastures since scrapie can unknowingly be reintroduced in sheep with preclinical disease.
In contrast to bovine spongiform encephalopathy (BSE) and chronic wasting disease (CWD), scrapie is distinct because sheep prion genetics play a clear role in disease susceptibility.10 The existence of sheep with scrapie-resistant genotypes provide an opportunity to control the disease through breeding programs.
It has been hypothesized that scrapie also exists in a sporadic form. In 1998, a previously unseen form of sheep TSE was identified in Norway, dubbed Nor98. This and other “atypical” forms of scrapie have involved single sheep in flocks that were not known to have been in contact with infected animals.11 Perhaps even more important, these cases occurred in sheep resistant to classical scrapie.2
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