Scrapie

Introduction

Scrapie is one of the Transmissible Spongiform Encephalopathies (TSEs), which are neurodegenerative diseases that have been found in humans, cattle, sheep and other mammals (e.g. cats, goats, deer, antelope). The agent of disease is believed to be an abnormal prion protein (or is prion associated). An abnormal prion is a protein with an abnormal folding structure when compared to the naturally occurring prion found in nervous and lymphoreticular tissues. Various abbreviations are in use to refer to this: Prion Protein (PrP); Abnormal Prion Protein (PrP^ab); Resistant Prion Protein (PrP^res); Scrapie Prion Protein (PrP^sc) and others. In most instances the abnormal prion is resistant to protein kinase digestion, a feature used in diagnostic techniques. Clinical signs include progressive ataxia and pruritus. Histology (immunohistochemistry) usually shows vacuolation and an accumulation of prion proteins in various parts of the CNS (especially the brain and spinal cord).

Three TSEs have been recorded in small ruminants: classical scrapie, atypical scrapie and BSE. In sheep, both resistance to BSE and scrapie infection and the distribution of infectivity through the body is known to be determined genetically. Genetic resistance to TSEs is not defined in goats although research is ongoing.

Classical scrapie (CS) exists at a low prevalence in the UK sheep and goat population and thus has relatively minor animal health implications, however, individual flocks can be affected dramatically. Classical Scrapie was first recorded in 1732. Different strains of scrapie are recognised and a variety of straintyping techniques have been proposed. Most scholars consider that the clinical manifestation is a result of the straintype and its interaction with the genotype of the host. There is no evidence that it is transmissible to man and some epidemiological evidence that it is not. However, it is not possible to rule out this risk completely. The UK Spongiform Encephalopathy Advisory Committee’s (SEAC) advice on this issue has been as follows:

“…reducing the incidence of classical scrapie per se would not directly reduce the risk to public health, since classical scrapie has been evident for over 200 years and there is no evidence it poses a significant risk to human health.” [2006]

“There is no firm evidence for a link between human TSEs and classical scrapie. Although a link cannot be ruled out, even if there is a link, the human health risk from classical scrapie must be very low and result in very few human TSE cases per annum. This is because the incidence of human TSEs is very low and relatively constant world-wide (around one case per million people per year) showing that there must be at least a substantial, if not complete, barrier to transmission of classical scrapie to humans”. [2008]

Atypical scrapie (AS) is considered not transmissible although it has been shown as transmissible experimentally. It was first recorded in the late 90's when new, more sensitive, testing techniques for BSE and scrapie were developed and applied. Atypical scrapie exists at a similarly low prevalence as CS in the UK sheep and goat population and has now been found in most European countries.

BSE - a known human health risk - was confirmed in one goat slaughtered in France in 2002; and probably present in one UK goat born in 1987. Another UK goat killed in 2008 remains under investigation as BSE could not be excluded on standard tests. Naturally occurring BSE has not been recorded in sheep, and based on the results of extensive active surveillance (currently 10,000 fallen sheep, 500 fallen goats and 10,000 sheep intended for human consumption per annum). The UK’s independent Spongiform Encephalopathy Advisory Committee (SEAC) concluded in 2007, that the UK prevalence was probably 0, or at the worst, 10 flocks might be affected.