Previous - Next
The original clinical description of BSE was based on the first six cases to be recorded (Wells et al., 1987). Subsequent accounts show that the frequency of different clinical signs has remained constant during the course of the epidemic (Cranwell et al., 1988; Gilmour et al., 1988; Wilesmith et al., 1988; 1992b; Scott et al., 1988; 1989; Winteretal., 1989; Wilesmith and Wells, 1991). The following account is based on the signs recorded in a comprehensive study of nearly 200 cases of BSE (Wilesmith et al., 1988). Supplementary clinical data have now been obtained from the reports of over 17 000 confirmed (Wilesmith et al., 1992b).
Cases of BSE show a combination of neurological and general signs of disease. The neurological signs fall into three categories:
• Changes in mental state were observed, most commonly
seen as apprehension, frenzy and nervousness when confronted by
doorways and other entrances. About 98 percent of all cases
showed altered behaviour in this category.
• Abnormalities of posture and movement occurred in 93
percent of cases. The most common manifestations were hind-limb
ataxia, tremors and falling.
• Changes in sensation were a feature of about 95 percent of
all cases. This was exhibited in many different ways, but the
most striking was hyperaesthesia, to both touch and sound.
A large majority of cases (87 percent) exhibited signs that fell into all three neurological categories. This is consistent with a diffuse central nervous system disorder. There are many points of similarity in the clinical signs of' scrapie and BSE. The most obvious difference is that pruritus was only seen occasionally in BSE.
In addition, there were some general clinical signs associated with BSE of which the most frequent were loss of body condition (78 percent), live weight loss (73 percent) and reduced milk yield (70 percent). A good appetite was maintained in the great majority of cases. At some stage in the clinical course, about 79 percent of all cases showed one of the above general signs along with signs in each of the three neurological categories. No pathologically confirmed cases of BSE exhibited only general signs.
There is considerable day-to-day variation in the presence and severity of individual signs. Keeping animals in a quiet and familiar environment reduces the severity of some signs, particularly hyperaesthesia. But over a period of weeks, the clinical signs are progressive, leading to recumbency and death. However, the slaughter of the great majority of affected animals becomes necessary at an earlier stage because of unmanageable behaviour and injury from repeated falling.
The duration of the clinical disease, from the earliest signs to death or slaughter, can range from under two weeks to as long as a year. The average period is about one to two months.