Scrapie () is a fatal, degenerative disease affecting the nervous systems of sheep and goats. It is one of several transmissible spongiform encephalopathies (TSEs), and as such it is thought to be caused by a prion. Scrapie has been known since at least 1732 and does not appear to be transmissible to humans. However, it has been found to be experimentally transmissible to humanised transgenic mice and non-human primates. 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 disease apparently causes an itching sensation in the animals. Other clinical signs include excessive lip smacking, altered gaits and convulsive collapse. Scrapie is infectious and transmissible among conspecifics, so one of the most common ways to contain it (since it is incurable) is to quarantine and kill those affected. However, scrapie tends to persist in flocks and can also arise spontaneously in flocks that have not previously had cases of the disease. 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. Recent studies suggest prions may be spread through urine and persist in the environment for decades.
Scrapie usually affects sheep around three to five years of age. The potential for transmission at birth and from contact with placental tissues is apparent.
Regulation The disease has been notifiable in the EU since 1993, but unlike bovine spongiform encephalopathy (BSE, commonly known as mad cow disease), there was no evidence as of 1999 to suggest that scrapie is a risk to human health. In July 2003, a Canadian Food Inspection Agency officer echoed that in his assessment of the danger to Canadian animals. As of 2004, the USDA made no mention of scrapie in its Sheep and Goats Death Loss circular. Historically, scrapie was considered to be an animal health issue. However, between 1996 and 1999, the UK Spongiform Encephalopathy Advisory Committee considered the control and eradication of scrapie in the UK also with public health in mind because of concern over five issues:
Meat and bone meal (MBM), which was suspected to be the source of the BSE in cattle outbreak in the late 1990s, had also been fed to sheep and goats. BSE had been transmitted to sheep following experimental oral exposure. Transmissible spongiform encephalopathy (TSE) infection was widespread through the carcasses of the sheep, unlike cattle infection which is limited to neural tissues. Scrapie in sheep was underreported and it might be masking BSE, were it present in sheep. Specified risk material (SRM) measures may not be adequate to control human exposure.
Cause Scrapie and other transmissible spongiform encephalopathies are caused by prions. 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. These abnormal proteins are gradually accumulated in the body, especially in nerve cells, which subsequently die.
Transmission and pathogenesis The primary mode of transmission is from mother to lamb through ingestion of placental or allantoic fluids. The agent can also enter through cuts in the skin. An experiment has shown lambs risk being infected through milk from infected ewes, but the lambs in the experiment also infected each other, making the risk of infection difficult to assess. 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. The pathogenesis of scrapie involves the lymphatic system. Once the agent is absorbed through the intestines, misfolded prions first appear and accumulate in the lymph nodes, especially in Peyer's patches at the small intestine. 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.
Clinical signs and diagnosis Changes are mild at first; slight behavioral changes and an increase in chewing movements may occur. Ataxia and neurological signs then develop, and affected sheep struggle to keep up with the flock. The signs and effects of scrapie typically appear 2–5 years after infection but may appear later. Once the onset of clinical signs has occurred, sheep typically live for 1–6 months. In some cases, they may live longer, but death is an inevitable consequence of the condition. Signs of scrapie vary between infected individual animals and develop slowly. Due to the nerve cell damage caused by the condition, affected animals may exhibit behavioral changes, tremor, pruritus, and locomotor incoordination. Some sheep scratch excessively and show patches of wool loss and lesions on the skin. Scratching sheep over the rump area may lead to a nibbling reflex, which is characteristic for the condition. Signs of a chronic systemic disease appear later, with weight loss, anorexia, lethargy, and death. Post mortem examination is important for the diagnosis of scrapie. 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.
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