Abstract
The interactions of host and infecting strain in ovine transmissible spongiform encephalopathies are known to be complex, and have a profound effect on the resulting phenotype of disease. In contrast to classical scrapie, the pathology in naturally-occurring cases of atypical scrapie appears more consistent, regardless of genotype, and is preserved on transmission within sheep homologous for the prion protein (PRNP) gene. However, the stability of transmissible spongiform encephalopathy phenotypes on passage across and within species is not absolute, and there are reports in the literature where experimental transmissions of particular isolates have resulted in a phenotype consistent with a different strain. In this study, intracerebral inoculation of atypical scrapie between two genotypes both associated with susceptibility to atypical forms of disease resulted in one sheep displaying an altered phenotype with clinical, pathological, biochemical and murine bioassay characteristics all consistent with the classical scrapie strain CH1641, and distinct from the atypical scrapie donor, while the second sheep did not succumb to challenge. One of two sheep orally challenged with the same inoculum developed atypical scrapie indistinguishable from the donor. This study adds to the range of transmissible spongiform encephalopathy phenotype changes that have been reported following various different experimental donor-recipient combinations. While these circumstances may not arise through natural exposure to disease in the field, there is the potential for iatrogenic exposure should current disease surveillance and feed controls be relaxed. Future sheep to sheep transmission of atypical scrapie might lead to instances of disease with an alternative phenotype and onward transmission potential which may have adverse implications for both public health and animal disease control policies.
Highlights
Scrapie belongs to a group of diseases known as the transmissible spongiform encephalopathies (TSEs) or prion diseases
For the six animals reported in this paper, inoculum was prepared from the brain of a single naturally occurring case of atypical scrapie in an ARR/ARR sheep which was characterized initially by immunopathology and Western blot and subsequently strain typed in an ovine transgenic mouse model as described [14]
Mapping of PrPSc types in the mouse brain was in agreement with the incubation period and lesion profile data (Fig. 5). It is clear from all the different aspects of phenotype investigated in this study that one recipient sheep in this experimental transmission expressed a phenotype that was CH1641-like, and different from that of the donor which was diagnosed with atypical scrapie
Summary
Scrapie belongs to a group of diseases known as the transmissible spongiform encephalopathies (TSEs) or prion diseases. Despite naturally-occurring atypical scrapie being observed in a range of genotypes, successful experimental transmissions of clinical disease have so far only been reported within a particular homologous donor-recipient genotype model using sheep which are AHQ/AHQ homozygous [8,15,16]. These published transmissions represent part of a large study at APHA which has been running since 2004, investigating the potential transmissibility of atypical scrapie in a range of both homologous and cross-genotype combinations. We describe an unexpected and interesting finding from that study where one experimental challenge in which atypical scrapie from an ARR/ARR donor was inoculated intracerebrally into two AHQ/AHQ recipient sheep, and in one of them the resulting disease had a phenotype that was indistinguishable from CH1641 [29], a classical scrapie strain which has some BSE-like Western blot properties
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