Scrapie
[7] The name scrapie is derived from one of the clinical signs of the condition, wherein affected animals will compulsively scrape off their fleeces against rocks, trees or fences.
The mechanism of transmission between animals and other aspects of the biology of the disease are only poorly understood, and are active areas of research.
Prions multiply by causing normally folded proteins of the same type to take on their abnormal shape, which then go on to do the same, in a kind of chain reaction.
The experiment did not continue long enough to show if the lambs developed symptoms, but merely that the abnormal prion was present in their bodies.
[23] Eventually, the infection invades the brain, often through the spinal cord or the medulla oblongata by creeping up the sympathetic and parasympathetic nervous system, respectively.
Due to the nerve cell damage caused by the condition, affected animals may exhibit behavioral changes, tremor, pruritus, and locomotor incoordination.
Histology of tissues shows accumulation of prions in the central nervous system, and immunohistochemical staining and ELISA can also be used to demonstrate the protein.
[29][30] Scrapie occurs in Europe and North America, but to date, Australia and New Zealand (both major sheep-producing countries) are scrapie-free.
[31] In 2003, there was pressure from affected Canadian husbandry practitioners on the Chretien government and their CFIA to implement their own national scrapie plan.
Out of fear of BSE-like transmission, many European countries banned some traditional sheep or goat products made without removing the spinal cord, such as smalahove and smokie.
[34] In 2010, a team from New York described detection of PrPSc even when initially present at only one part in a hundred billion (10−11) in brain tissue.
The method combines amplification with a novel technology called surround optical fiber immunoassay and some specific antibodies against PrPSc.
The technique allowed detection of PrPSc after many fewer cycles of conversion than others have achieved, substantially reducing the possibility of artefacts, as well as speeding up the assay.
[35][36] In the United Kingdom, if a sheep is suspected or found to have scrapie in laboratory tests, it will be euthanised to prevent further suffering and to reduce the possibility of transmission.
[38] Various studies have indicated prions (PrPSC) which 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.
[39] 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 testing for scrapie can only be achieved by applying immunohistochemistry of disease-associated prion protein (PrPSC) to tissues collected post mortem, including obex (a brainstem structure), retropharyngeal lymph node and palatine tonsil.
Ruminant animals have specialized Peyer's patches that, throughout the length of the ileum, amount to about 100,000 follicles, and all of these could be infected and shedding prions into the lumen.
[42] Scrapie prions have been found in the Peyer's patches of naturally infected asymptomatic lambs as young as four months of age.
Hamsters fed with contaminated soil exhibited their first scrapie-associated symptoms at two weeks to six months (95% CI) after the first feeding.
[41] Enhanced transmissibility of soil-bound prions may explain the environmental spread of scrapie despite low levels shed into the environment.
Prion binding to Mte or other soil components may partially protect PrPSC from denaturation or proteolysis in the digestive tract, allowing more disease agents to be taken up from the gut.
All animals orally dosed with an equivalent amount of Mte-bound PrPSc manifested disease symptoms in 195 to 637 days.
By contrast, animals orally receiving Mte soil alone or one-tenth as much unbound clarified PrPSc (20 ng) remained asymptomatic throughout the course of the experiment.
An improved understanding of the processes affecting the mobility, persistence and bioavailability of prions in soil is needed for the management of prion-contaminated environments.
