Scrapie

Scrapie is a fatal, degenerative disease that affects 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 1732 and does not appear to be transmissible to humans. Scrapie is a fatal, degenerative disease that affects 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 1732 and does not appear to be transmissible to humans. 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 apparently 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 these 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. The disease has been notifiable in the EU since 1993 but unlike BSE 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 said that while scrapie shows up every year on Canadian farms, 'We've had a lot of experience with scrapie and there's never been a link between scrapie and human illness.' As of 2004, the USDA made no mention of scrapie in its Sheep and Goats Death Loss circular. Historically, scrapie had been considered to be an animal health issue. However, between 1996 and 1999 the UK SEAC had been considering the control and eradication of scrapie in the UK also with public health in mind because of concern over five issues: The cause of scrapie, as with other transmissible spongiform encephalopathies, is unknown and is a matter of debate; the agent is very difficult to destroy with heat, radiation and disinfectants, does not evoke any detectable immune response, and has a long incubation period of between 18 months and 5 years. The agent is thought to be much smaller than the smallest currently known virus and the prevailing theory is that it is caused by a misfolded protein called a prion. 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. 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.