Table 1.

Characteristics of classical scrapie and atypical scrapie (including disease caused by the Nor98 agent) (from Parry, 1983; Benestad and Bratberg, 2006; Hörnlimann et al., 2006; Ulvund, 2006, 2008; Benestad et al., 2008; Wemheuer et al., 2011; Fast and Groschup, 2013).

Characters	Classical Scrapie	Atypical scrapie (including Nor98 scrapie)
Ovine genotypes affected*	VRQ and ARQ haplotypes most susceptible. ARR most resistant.	Nor 98 can occur in ARR genotypes. High incidence of atypical scrapie in AHQ genotype and low incidence in ARQ genotype.
Age of onset of natural disease	Between 2 and 5 years of age with average age 3.5 years	On average 6.5 years old.
Duration of disease	2 weeks to 6 months to death, rarely longer	6 weeks to 8 months to death
Symptomatology	Behavioural change, tremour, pruritus and locomotor disorder.	Weight loss, behavioural change (nervousness, anxiety), tremour infrequent, pruritus not observed, and locomotor disorder (ataxia, circling).
Nature and tissue distribution of PrPSc and infectivity	PrPSc and infectivity occurs as constant feature in peripheral lymphoid tissue.	PrPSc not detected outside the central nervous system or in the peripheral lymphoid tissue. Infectivity detected in lymphoid tissue, nerves, and muscles by bioassay with transgenic mice.
Nature and distribution of lesions in the central nervous system	Lesions in grey matter include vacuolation, neuronal loss and occasional astrocytosis. Lesions predominate in brainstem and occur at the level of the obex. Distribution of lesions in more rostral areas of the brain varies widely.	Vacuolated grey matter predominates in the molecular layer of the cerebellar cortex and also occurs more rostrally in the basal ganglia and cerebral cortex, Brainstem lesions are insubstantial and no lesions are observed at the obex.
Nature and distribution of PrPSc deposits in central nervous system	PrPSc occurs intra- and perineuronally, in association with glia and elsewhere. PrPSc deposits are a constant feature in caudal brainstem, mainly at the level of the obex and the dorsal motor nucleus of the vagus nerve (DMNV). Extent of PrPSc deposits depends on progression of disease and occurs throughout the CNS at clinical endpoint.	No PrPSc in neurons. PrPSc deposits mainly in the cerebellar cortex (especially in the molecular layer), cerebral cortex and basal ganglia, in the spinal tract nucleus of the trigeminal nerve, and within white matter throughout the brain. Dorsal motor nucleus of the vagus nerve (DMNV) without PrPSc.
Immunoblot pattern	Three bands indicating unglycosylated, monoglycosylated and diglycosylated PrPSc with molecular masses between 18 and 30 kDa.	Four bands: Three indicate unglycosylated, monoglycosylated and diglycosylated PrPSc with molecular masses between 18 and 30 kDa. A fourth band occurs below 15 kDa.
Transmissibility to transgenic mice	Yes	Yes

^*Refers to variations in amino acid substitutions at codons 136, 154 and 171 of the ovine PRNP gene and haplotypes that are concerned with genetic resistance to classical scrapie.