Paraffin-embedded tissue blot as a sensitive method for discrimination between classical scrapie and experimental bovine spongiform encephalopathy in sheep

Conventional and State-of-the-Art Detection Methods of Bovine Spongiform Encephalopathy (BSE)

Bovine spongiform encephalopathy (BSE) is a fatal neurodegenerative disease that belongs to a group of diseases known as transmissible spongiform encephalopathies (TSEs). It is believed that the infectious agent responsible for prion diseases is abnormally folded prion protein (PrP^Sc), which derives from a normal cellular protein (PrP^C), which is a cell surface glycoprotein predominantly expressed in neurons. There are three different types of BSE, the classical BSE (C-type) strain and two atypical strains (H-type and L-type). BSE is primarily a disease of cattle; however, sheep and goats also can be infected with BSE strains and develop a disease clinically and pathogenically indistinguishable from scrapie. Therefore, TSE cases in cattle and small ruminants require discriminatory testing to determine whether the TSE is BSE or scrapie and to discriminate classical BSE from the atypical H- or L-type strains. Many methods have been developed for the detection of BSE and have been reported in numerous studies. Detection of BSE is mainly based on the identification of characteristic lesions or detection of the PrP^Sc in the brain, often by use of their partial proteinase K resistance properties. The objective of this paper was to summarize the currently available methods, highlight their diagnostic performance, and emphasize the advantages and drawbacks of the application of individual tests.

Pathology of Animal Transmissible Spongiform Encephalopathies (TSEs)

Pathology is the study of the structural and functional changes produced by diseases or - more specifically - the lesions they cause. To achieve this pathologists employ various approaches. These include description of lesions that are visible to the naked eye which are the subject of anatomic pathology and changes at the cellular level that are visible under the microscope, the subject of histopathology. Changes at the molecular level which are identified by probes that target specific molecules - mainly proteins that are detected using immunohistochemistry (IHC). As transmissible spongiform encephalopathies (TSEs) do not cause visible lesions anatomic pathology is not applicable to their study. For decades the application of histopathology to detect vacuoles or plaques was the only means of confirming TSE disease. The subsequent discovery of the cellular prion protein (PrP^C) and its pathogenic isoform, PrP^Sc, which is a ubiquitous marker of TSEs, led to the production of anti-PrP antibodies, and enabled the development of PrP^Sc detection techniques such as immunohistochemistry, Histoblot and PET-blot that have evolved in parallel with similar biochemical methods such as Western blot and ELISA. These methods offer greater sensitivity than histopathology in TSE diagnosis and crucially they can be applied to analyze various phenotypic aspects of single TSE sources increasing the amount of data and offering higher discriminatory power. The above principles are applied to diagnose and define TSE phenotypes which form the basis of strain characterisation.

The interpretation of disease phenotypes to identify TSE strains in mice: characterisation of BSE using PrPSc distribution patterns in the brain

In individual animals affected by transmissible spongiform encephalopathies, different disease phenotypes can be identified which are attributed to different strains of the agent. In the absence of reliable technology to fully characterise the agent, classification of disease phenotype has been used as a strain typing tool which can be applied in any host. This approach uses standardised data on biological parameters, established for a single host, to allow comparison of different prion sources. Traditionally prion strain characterisation in wild type mice is based on incubation periods and lesion profiles after the stabilisation of the agent into the new host which requires serial passages. Such analysis can take many years, due to prolonged incubation periods. The current study demonstrates that the PrPSc patterns produced by one serial passage in wild type mice of bovine or ovine BSE were consistent, stable and showed minimal and predictable differences from mouse-stabilised reference strains. This biological property makes PrPSc deposition pattern mapping a powerful tool in the identification and definition of TSE strains on primary isolation, making the process of characterisation faster and cheaper than a serial passage protocol. It can be applied to individual mice and therefore it is better suited to identify strain diversity within single inocula in case of co-infections or identify strains in cases where insufficient mice succumb to disease for robust lesion profiles to be constructed. The detailed description presented in this study provides a reference document for identifying BSE in wild type mice.

The interpretation of disease phenotypes to identify TSE strains following murine bioassay: characterisation of classical scrapie

Mouse bioassay can be readily employed for strain typing of naturally occurring transmissible spongiform encephalopathy cases. Classical scrapie strains have been characterised historically based on the established methodology of assessing incubation period of disease and the distribution of disease-specific vacuolation across the brain following strain stabilisation in a given mouse line. More recent research has shown that additional methods could be used to characterise strains and thereby expand the definition of strain “phenotype”. Here we present the phenotypic characteristics of classical scrapie strains isolated from 24 UK ovine field cases through the wild-type mouse bioassay. PrP^Sc immunohistochemistry (IHC), paraffin embedded tissue blots (PET-blot) and Western blotting approaches were used to determine the neuroanatomical distribution and molecular profile of PrP^Sc associated with each strain, in conjunction with traditional methodologies. Results revealed three strains isolated through each mouse line, including a previously unidentified strain. Moreover IHC and PET-blot methodologies were effective in characterising the strain-associated types and neuroanatomical locations of PrP^Sc. The use of Western blotting as a parameter to define classical scrapie strains was limited. These data provide a comprehensive description of classical scrapie strain phenotypes on isolation through the mouse bioassay that can provide a reference for further scrapie strain identification.