The natural atypical scrapie phenotype is preserved on experimental transmission and sub-passage in PRNP homologous sheep

Abstract

Background: Atypical scrapie was first identified in Norwegian sheep in 1998 and has subsequently been identified in many countries. Retrospective studies have identified cases predating the initial identification of this form of scrapie, and epidemiological studies have indicated that it does not conform to the behaviour of an infectious disease, giving rise to the hypothesis that it represents spontaneous disease.However, atypical scrapie isolates have been shown to be infectious experimentally, through intracerebral inoculation in transgenic mice and sheep. The first successful challenge of a sheep with 'field' atypical scrapie from an homologous donor sheep was reported in 2007.

Results: This study demonstrates that atypical scrapie has distinct clinical, pathological and biochemical characteristics which are maintained on transmission and sub-passage, and which are distinct from other strains of transmissible spongiform encephalopathies in the same host genotype.

Conclusions: Atypical scrapie is consistently transmissible within AHQ homozygous sheep, and the disease phenotype is preserved on sub-passage.

Figure 1

Vacuolation profiles for atypical scrapie, classical scrapie and BSE in AHQ/AHQ sheep. See Table 1 for details of the cases, and Table 3 for area code key, and mean ± SD for each data point. a) The profiles for the classical scrapie (pink, cases 15-17) and ovine BSE (blue, cases 10-14) cases are similar, particularly in terms of brainstem involvement, whereas the atypical scrapie profile (green, cases 1-9) has distinctive peaks in the cerebellum (area 12) and frontal cortex (area 20). b) The vacuolation profiles for experimental atypical scrapie cases subdivided by passage. The profile from the original challenged animals (green, cases 1-4 and 8-9) is very similar to that in the animals that succumbed to disease following sub-passage (red, cases 5-7).

Figure 2

PrP^Sc deposition pattern (PDP) maps for atypical scrapie, classical scrapie, and BSE in AHQ/AHQ sheep. Note that, in comparison to classical scrapie and ovine BSE, the atypical scrapie cases have relatively sparse immunolabelling in the obex and rostral midbrain, widespread and prominent immunolabelling in the cerebellar cortex and white matter, sparse immunolabelling in the hypothalamus, and prominent immunolabelling in the neocortex and cortical white matter.

Figure 3

Immunolabelling in atypical scrapie cases. Immunolabelling characteristics in atypical scrapie are unchanged in natural disease (a-c), and primary (d-f) and secondary (g-i) passage. Note the characteristic three-band pattern in the neocortex (a, d, g); prominent immunolabelling in the molecular and granular cell layers of the cerebellum (b, e, h); prominent fine granular immunolabelling in the basal nuclei (c, f, i).

Figure 4

Western immunoblot of caudal medulla of the atypical scrapie cases. WB using monoclonal antibody Sha31, showing the consistency of blot characteristics between field case, experimental and sub-passaged atypical scrapie. Lanes 1 and 10 Biotinylated markers. Lane 2 Case 4. Lane 3 Case 8 (the sub-passage of Case 4). Lanes 4 and 5 Cases 7 and 5, both sub-passages of Case 1 (not shown). Lane 6 Field case atypical scrapie positive control. Lane 7 Field case classical scrapie control. Lane 8 Bovine BSE positive control. Lane 9 Scrapie negative control.