Scrapie

Scrapie

R BRADLEY AND D W VERWOERD

Introduction

Scrapie is the oldest known transmissible spongiform encephalopathy (TSE). The clinical signs were well known in Europe and accurately described in the mid-eighteenth century.90 Some authors claim that it has been known since Roman times. A historical primer on scrapie has recently been published by Brown and Bradley.16 Major events leading to our current knowledge of scrapie occurred as follows:

  • 1898: Description of the microscopic neuropathology and neuronal vacuolation notably of motor neurons in the spinal cord.12
  • 1936: Scrapie shown to be an experimentally transmissible disease in sheep.31
  • 1936 to 1938: Iatrogenic scrapie occurs in hundreds of sheep in Great Britain via louping-ill vaccine contaminated accidentally with scrapie agent from brains and spleens of sheep used for its manufacture. This was first reported by Gordon, Brownlee and Wilson at the Third International Congress for Microbiology in 1939,57 and subsequently by Gordon in 194655 and Greig in 1950.58 The transmissible nature of the scrapie agent was thus confirmed beyond reasonable doubt.
  • 1957 to 1959: Zigas and Gajdusek reported the occurrence of kuru (also known as the laughing death), a rare and unusual neurological disease in the Fore-speaking people of Papua New Guinea.146 They believed this was propagated by endocannibalistic consumption of the whole bodies of dead relatives during funeral rites. Dr W.J. Hadlow, an American veterinary pathologist, made the important pathological connection between scrapie of sheep and kuru of humans by observing that ‘the overall resemblance between kuru and scrapie is too impressive to be ignored’.60 He went on to suggest the experimental transmission of kuru to a laboratory primate. This was subsequently achieved and was followed by successful transmission of human Creutzfeldt-Jakob disease (CJD) to primates thus establishing that these rare human disorders were, like scrapie, transmissible spongiform encephalopathies.
  • 1961: Experimental transmission of scrapie to mice thus establishing a model system in which the disease, and the agent that caused it, could be studied in the laboratory. 23
  • 1966 to 1967: Report of the very small size of the scrapie agent, its almost complete resistance to destruction by massive germicidal doses of ionizing radiation supporting the view that the agent may have a mode of replication independent of the integrity of a nucleic acid moiety.4, 5
  • 1979: Report of the scrapie replication site hypothesis by Dickinson and Outram 35 and the results of studies on the genetics of scrapie in sheep and mice by Dickinson and Fraser.34 The latter report showed that murine genes influenced the phenotype of scrapie, notably its incubation period and lesion profile, when a small number of important factors were controlled. This work was the basis for identifying and distinguishing strains of scrapie agent and led to further opportunities to study the pathogenesis of scrapie notably by Bruce, Dickinson, Fraser and Kimberlin. The scrapie strain typing methods were to become vitally important after the discovery of bovine spongiform encephalopathy (see Bovine spongiform encephalopathy).
  • 1979, 1980 and 1982: Reports on the tissue distribution of scrapie infectivity in goats and Suffolk sheep and in some other breeds of sheep with natural scrapie, including that during the incubation period of Suffolk sheep from high scrapie-risk families.62, 63, 64
  • 1982: Establishment of the prion definition (small, proteinaceous infectious particles that resist inactivation by procedures that modify nucleic acids) and prion hypothesis.109

An excellent, concise, textbook account of the natural disease as it was then known is given by Kimberlin in 1981,83 and this was followed with an updated essay on the disease in 1986.84

Since that time considerable, but still incomplete, progress has been made in understanding the disease. The new knowledge acquired in the following six years or so was reviewed by Detwiler in 1992.33 Information acquired, and subsequently interpreted, is given in the following pages.

Some substantial epidemiological studies have revealed no connection between CJD in humans and scrapie (e.g. Brown et al. 1987).15 Indeed, scrapie in sheep, goats and moufflon (Ovis ammon musimon) appears not to be naturally transmissible to any other species, including cattle (but, see Bovine spongiform encephalopathy).

Scrapie has been found in most of the sheep-raising countries of the world at some time or another, often following importation of infected animals, but in some of these countries, including Australia, New Zealand and South Africa, the disease was eradicated before it could spread to indigenous sheep. For a treatise on the historical aspects of the dissemination of sheep and of scrapie, see Parry,104 and in regard to Australia specifically, Parsonson.105 The first case in South Africa was in 1966 in a Hampshire Down sheep imported from England.134 This incident occurred in KwaZulu- Natal and was confirmed by microscopic examination of the brain. The disease was subsequently found in 11 sheep, all first- or second-generation progeny of the English imports of Hampshire Downs. The cases occurred on nine farms in Kwazulu-Natal, Eastern Cape and the Free State provinces. A strict slaughter policy was applied, and no case has been diagnosed in...

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