Abstract
BackgroundThe majority of atypical bovine spongiform encephalopathy (BSE) cases so far identified worldwide have been detected by active surveillance. Consequently the volume and quality of material available for detailed characterisation is very limiting. Here we report on a small transmission study of both atypical forms, H- and L-type BSE, in cattle to provide tissue for test evaluation and research, and to generate clinical, molecular and pathological data in a standardised way to enable more robust comparison of the two variants with particular reference to those aspects most relevant to case ascertainment and confirmatory diagnosis within existing regulated surveillance programmes.ResultsTwo groups of four cattle, intracerebrally inoculated with L-type or H-type BSE, all presented with a nervous disease form with some similarities to classical BSE, which progressed to a more dull form in one animal from each group. Difficulty rising was a consistent feature of both disease forms and not seen in two BSE-free, non-inoculated cattle that served as controls. The pathology and molecular characteristics were distinct from classical BSE, and broadly consistent with published data, but with some variation in the pathological characteristics. Both atypical BSE types were readily detectable as BSE by current confirmatory methods using the medulla brain region at the obex, but making a clear diagnostic distinction between the forms was not consistently straightforward in this brain region. Cerebellum proved a more reliable sample for discrimination when using immunohistochemistry.ConclusionsThe prominent feature of difficulty rising in atypical BSE cases may explain the detection of naturally occurring cases in emergency slaughter cattle and fallen stock. Current confirmatory diagnostic methods are effective for the detection of such atypical cases, but consistently and correctly identifying the variant forms may require modifications to the sampling regimes and methods that are currently in use.
Highlights
The majority of atypical bovine spongiform encephalopathy (BSE) cases so far identified worldwide have been detected by active surveillance
Contrary to the control cattle that displayed this behaviour around that time, the BSE-infected cattle continued to display this sign on subsequent examinations, which was accompanied by sensory changes, such as increased over-reactivity to tactile facial stimuli or visual stimuli during assessment of cranial nerve function [compare signs ‘head shyness’ and ‘over-reactivity to tactile stimuli’ between H/L-type BSE cases and controls in Figure 1; see Additional file 3: ‘Control (CO1)’, which shows the lack of over-reactivity in this control animal prior to cull]
There was a noticeable change in the behaviour when the animals were released in the corridor: they became more reluctant to move in the corridor, stopping at grooves on the floor [see Additional file 1: ‘H-type BSE (H1)’ of this steer stopping at grooves on the floor], whereas they had previously been curious and active in the corridor like the control cattle [see Additional file 3: ‘control (CO1)’ showing this control animal running along the corridor and stopping to sniff objects]
Summary
The majority of atypical bovine spongiform encephalopathy (BSE) cases so far identified worldwide have been detected by active surveillance. Based on the prion protein fragments visualised on a Western immunoblot after proteinase digestion of brain tissue from affected animals, the molecular mass was either lower (L-type BSE, known as BASE) or higher (H-type BSE) than in the classical form of BSE (C-type BSE). Both forms of atypical BSE were subsequently identified in the United Kingdom (UK) [17,18]. Transmission studies in cattle produced a disease characterised by signs of dullness and difficulty in rising that maintained its molecular phenotype difference to C-type BSE [19,20]
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