Scrapie, a Prion Disease of Sheep

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Trotting disease, Nibbling disease

MP Gupta

• It is a non-febrile, fatal chronic degenerative neurologic disease of sheep and goat characterized by pruritis, long incubation period, progressive debilitation and death. • Goats are far less commonly affected than sheep.

• They are generally thought to be prions, although a minority opinion suggests that TSEs may be caused by virinos or retroviruses. Prions are infectious proteins that seem to replicate by converting a normal cellular protein into copies of the prion. This cellular protein, which is called PrPc, is found on the surface of neurons. The scrapie prion, which is a pathogenic isoform of PrPc, is designated PrPSc.

• Scrapie is caused by infective particles- PRIONS, which is a small infectious particle capable to withstand the usual inactivation process known to destroy the virus.

• Prions contain no or little nuclear materials. The agents are not antigenic and produce no detectable immune response in the host.

• As a result of scrapie surveillance, an atypical form of this disease has recently been detected in many countries in Europe, as well as in the United States. This novel form of scrapie, which was first reported from Norway in 1998, is called Nor98.

• These particles are very resistant and can withstand boiling for 30 mins. or exposure to 20% formaline. However, 2% NaOH solution is recommended for inactivation.

• The incubation period is usually 2 to 5 years in sheep; cases are rare in sheep less than a year old. • Cases of scrapie have been reported in 2 to 8 year old goats. The incubation period in experimentally infected goats is less than three years, with a range of 30 to 146 weeks.

• In majority of caprine scrapie, there is a history of direct contact with infected sheep, though the contact with infected placenta may also transmit the disease.

• At present there is considerable evidence to suggest that scrapie is transmitted by(a) contact or (b)ingestion of infected materials eg, placenta or is (c)vertically transmitted from dam to offspring

• Sheep scrapie, , has been present in Europe and England for over 100 years and now has a world-wide distribution (with the exception of Australia and New Zealand).

• Australia and New Zealand have remained free of scrapie; although outbreaks occurred in these two countries, the disease was eradicated by slaughtering the imported sheep and their flockmates soon after they were released from quarantine.

• -Sheep scrapie is believed by many scientists to be the • source of BSE infection as well as CWD.

• -Replication of prions occurs first in lymphoid tissues especially in spleen before invading other tissues including the brain.

• Onset of clinical signs depends upon the concentration of infectious agents in the brain and on the site of infection. • Transfer of scrapie agent to brain from the peripheral site is as yet unexplained but believed to be via the tissues of sympathetic nervous system.

• -In the brain scrapie particles causes vacuolation of neurons in the spinal cord, medulla, mid brain and progressive neuronal degeneration of the brain tissues

Clinical findings
• Progressive wt. loss and cessation of milk production • Intense pruritis which is constant finding in sheep and is less intense & infrequent in goats. • • -Hyperesthesia and incordination are predominant signs in goats.

• -Difficulty in rising; grinding of teeth; salivation; regurgitation of rumen contents and impaired...
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