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7.1
Clinical signs
7.2
Histopathology
7.3 Detection of the fibrillar form of PRP
There are no routine laboratory diagnostic tests to identify infected cattle before the onset of clinical disease. The diagnosis of BSE therefore depends on the recognition of clinical signs and confirmation by histological examination of the central nervous system. A clinical diagnosis can also be confirmed by EM, biochemical or immunocytochemical detection of SAF or the constituent protein, modified PrP.
The most common presenting sign is nervous behaviour (Fig. 5). This can be seen as a separation from the rest of the herd at pasture, a reluctance to enter the milking parlour and vigorous kicking in response to being milked. The earliest locomotor signs are subtle changes in the hind-limb gait and difficulty in rising from a normal recumbent position.
The early changes in behaviour can be confused with hypomagnesaemia and nervous ketosis. Unresponsiveness to treatment is one way to distinguish these two conditions from BSE. Another is the more insidious onset of the signs of BSE, together with their chronic progression over a period of weeks. The locomotor changes, in particular, progress to an obvious swaying gait, shortened stride and awkwardness in turning.
As mentioned previously, the predominant neurological signs of BSE are apprehension, hyperaesthesia and ataxia (see Fig. 6). Animals exhibiting a combination of these three signs for more than one month should be regarded as likely cases of BSE (Wilesmith et a/., 1 992b).
With the kind of experience gained by many observers in the United Kingdom, BSE can be diagnosed with a high degree of accuracy. Even so, all suspect cases of BSE in the country are examined by routine histopathology. Histological confirmation is essential whenever the BSE status of an individual animal needs to be established with certainty.
FIGURE 6 Frequencies of the neurological signs in 17154 histologically confirmed cases of BSE
The histopathological diagnosis of BSE can be made on the basis of neuroparenchymal vacuolation in the brains of suspected cases. It should be noted that vacuolation in neuronal perikarya is occasionally seen as an incidental feature in cattle (Wells et al., 1987). The frequency of such "normal" vacuolation is usually very low relative to that associated with clinical disease, and it is largely confined to the red nucleus (see Fig. 7). Artefactual vacuolation can also be produced under certain conditions of histological processing, but this principally affects white matter (Wells and Wells, 1989).
The recognition of BSE as a new disease of cattle was based on the routine examination of coronal sections representing the major brain regions (Wells et al., 1987). This would obviously be the required procedure when presented with a possible case of BSE for the first time.
Later in the epidemic. the consistent presence of pathognomonic lesions in the brain stem justified a simplified examination of just four coronal sections which include parts of the medulla oblongata, pons and mesencephalon. Subsequently, the increasing scale of the epidemic and the requirement for histological examination of all suspected cases created the need (and also provided the material) to develop an even simpler procedure.
A study of 684 bovine brains, including 563 confirmed cases, evaluated the histological diagnosis of BSE by examination of a single section of medulla' routinely taken at the obex (Wells et al., 1989). The high frequency of vacuolar changes in two nuclei in the medulla (the solitary tract nucleus and the spinal tract nucleus of the trigeminal nerve) made it possible to identify 99.6 percent of BSE cases that had been confirmed by the more extensive sampling of the brain. This means that other areas of the brain need only be examined when lesions in the medulla are minimal or absent.
7.3 Detection of the fibrillar form of PRP
The convenience and reliability of brain histology obviates the need to base routine diagnoses of BSE on the detection of modified PrP. However, there are situations when it would be useful. The most obvious is when a suspect case of BSE has arisen unexpectedly and it is imperative to be sure of the diagnosis. In these circumstances, the detection of SAF or modified PrP is a useful adjunct to histopathological diagnosis. It would be indispensable if the brain was unsuitable for histology: for example, if removal or fixation was excessively delayed. The protease resistance of SAF makes it possible to isolate them from brain that has undergone significant autolysis. However, SAF can only be purified satisfactorily from fresh or frozen brain, not fixed brain.
Two systematic studies have evaluated the detection of SAF in the routine diagnosis of BSE (Scott et al., 1990b). In the first, 144 histologically positive BSE brains and 23 negative brains were examined by purification of SAF and detection by EM. There were no false positives (out of 23 brains) but a large proportion of false negatives was observed (53 percent of 144 brains). The reason for the false negatives was that the brain areas selected for assay were suboptimal; they had been chosen after the priority areas for histopathology had been allocated.
A second study. of 22 brains, showed that the number of SAF approximated the severity of the vacuolar changes. For this second series, fibrils were detected in samples of all 22 BSE cases when brain stem areas were examined. This study also demonstrated the sensitivity of fibril detection by western blotting (Scott et al., 1990b).
