Scrapie - Transmission/exposure Pathways

Transmission/exposure Pathways

Various studies have indicated prions (PrPSC) that infect sheep and goats with the fatal transmissible encephalopathy known as scrapie, are able to persist in soil for years without losing their pathogenic activity. Dissemination of prions into the environment can occur from several sources: mainly, infectious placenta or amniotic fluid of sheep and possibly environmental contamination by saliva or excrement.

Confirmatory tesing for scrapie can only be achieved by applying immunohistochemistry (IHC) of disease-associated prion protein (PrPSc) to tissues collected post mortem, including obex, retropharyngeal lymph node and palatine tonsil. A live animal diagnostic, not confirmatory, test was approved in 2008 for immunochemistry testing on rectal biopsy-derived lymphoid tissue by USDA.

Natural transmission of scrapie in the field seems to occur via the alimentary tract in the majority of cases, and scrapie-free sheep flocks can become infected on pastures where outbreaks of scrapie had been observed before. These findings point to a sustained contagion in the environment, and notably the soil.

Prion concentration in birth fluids does not alter the infectivity of the prions. Even when placentas have little PrPSC, kids born to naturally or experimentally infected does become infected themselves. PrPSC is shed at a higher percent, 52%- 72% in in sheep placentas, than goat placenta at 5-10% in study trials at the USDA Agricultural Research Service.

Fecal concentration of PrPSC has been reported in the feces of sheep both in the terminal and the early preclinical stages of the disease, suggesting the prions are likely to be shed into the environment throughout the pathogenesis. Several sources of prions in feces could be postulated, including environmental ingestion and swallowing infected saliva; however, the most likely source is shedding from the gut-associated lymphoid tissue. Ruminants have specialized Peyer's patches that throughout the length of the ileum amount to approximately 100,000 follicles, and all of these could be infected and shedding prions into the lumen. Scrapie prions have been found in the Peyer's patches of naturally infected nonclinical lambs as young as four months of age.