Scrapie is thought to be spread most commonly from dam to offspring via infected placenta and placental fluids. Genetically-susceptible ewes and lambs in the same lambing environment are considered at-risk for developing the disease. While rams and bucks can get scrapie, they are not believed to be involved in the transmission of the disease. Semen is not known to transmit infectivity.
Other oral routes of transmission are possible. It was recently learned that infective scrapie prions can be transmitted to lambs through the milk. Soil may also serve as a reservoir for the disease. In Iceland, scrapie has re-occurred on farms that were depopulated and disinfected, as long as 16 years after the last sheep left the farm.
Feed is not considered to be a significant factor in the transmission of scrapie. Scrapie is not known to pose a human health risk.
Scrapie of sheep and goats is endemic in most European countries and the Americas, but it is not known to exist in Australia or New Zealand . Most clinical cases of scrapie occur in sheep between 2 and 5 years of age, and unlike other priondiseases, there is evidence that scrapie can be transmitted both vertically, from ewe to lamb before or shortly after birth, and horizontally, from sheep to sheep either directly or via the environment. The genetic makeup (i.e., certain polymorphismswithin the PrP gene) has a strong influence on whether an animal will develop clinical scrapie. There is no direct evidence that the infectious agent of scrapie may be transmitted to humans . Nevertheless, the fact that sheep may have been exposed to BSE prions has opened the possibility that these animals could, in theory, constitute a risk to humans. This has prompted several programs aimed at eliminating scrapie from sheep flocks in the United Kingdom.