B160 - SCRAPIE

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B160 - SCRAPIE

Nature of the disease
Scrapie is a chronic and fatal neurodegenerative disease of sheep. It is due to a prion and has been recognised for more than 250 years. 
Classification
OIE, List B disease
Susceptible species
In natural condition it is a disease of sheep, goats and mouflon are rarely infected.

Experimentally, scrapie has been transmitted to hamsters, mice, rats, gerbils, mink, cattle, and some species of monkeys. It could have been at the origin of the prion responsible for the Bovine Spongiform Encephalopathy.

Distribution
It is present in Europe and North America. It is not present in the Pacific as Australia and New Zealand eradicated it in 1954.
Clinical signs 
The period of incubation is long, it appears insidiously in 2-5 years old animals. Signs are not constant but the issue of the disease is always fatal. Classically the progression of signs include: 
  • Behaviour changes: they are to many to be all described but some common features include animals that stand apart, lead or trail the flock when driven, animals attacking or collapsing when stressed... 
  • Pruritus is constant but its frequency increases progressively. Animals are seen rubbing and scratching. Progressively consequent external lesions appear: loss of wool, skin lesions, swelling of the face.
  • Muscle tremor, 
  • Emaciation,
  • Incoordination,
  • Convulsions,
  • General hyperexcitability,
  • Change of voice,
  • Inability to swallow, 
  • Blindness
  • Anorexia and extreme emaciation
  • Pregnancy toxemia
  • Death within 1-6 months.
Post-mortem findings 
Characteristic vacuolisation of the neuroparenchyma (histology).
Differential diagnosis 
Specimens required for diagnosis 
There is no serological test.

At post-mortem, histological observation of vacuolisation is pathognomonic. The causative prion, PSc, can also be detected in unfixed brain material by detergent extraction, electrophoresis and immunoblotting or by immunohistochemistry in histological preparation. 

Transmission   
Scrapie can be transmitted from ewe to kid. Horizontal transmission by oral route is also possible. The prion is found in foetal fluids and placenta, NOT in milk, urine and faeces. 

There are some evidences of family linkage in history of scrapie but it is not known if it is due to genetic or maternal transmission factors.  

Risk of introduction   
The recent implications of scrapie prion in the emergence of Bovine Spongiform Encephalopathy and the public health issues consequently raised emphasize the need for completely documented risk analysis. The document developed by the OIE (C.I. Lasmézas & D.B. Adams (eds), Risk analysis of prion diseases in animals, Scientific and Technical Review, Volume 22 (1), April 2003) is strongly recommended.

It should be kept in mind that a infected herd can be apparently free for years before animals show the disease.  

Control / vaccines  
There is no treatment, no vaccine and no screening test. 

Control of the disease is difficult and expensive. If accidentally introduced in a Pacific country all contaminated flocks and in-contact animals should be eliminated.

References
  • GEERING WA, FORMAN AJ, NUNN MJ, Scrapie In Exotic Diseases of Animals, Aust Gov Publishing Service, Canberra, 1995, 288-295p
  • Office International des Epizooties, 2002
  • Office Vétérinaire Fédéral Suisse
  • Scrapie, In Veterinary Medicine, Saunders, Eight ed, 1997, London p. 1112-1116