Targeted screening of high-risk cattle populations for BSE to augment mandatory reporting of clinical suspects

In Switzerland, the first case of bovine spongiform encephalopathy (BSE) wa s diagnosed in November 1990. Case numbers peaked in 1995, with a total of 352 BSE cases identified by 30 April 2000. Reporting of clinically suspect cattle is currently the most commonly used method worldwide to detect BSE c ases. The effectiveness of mandatory reporting depends on a variety of fact ors; for other diseases passive surveillance underestimates the incidence o f clinical cases. The efficiency of passive surveillance systems for BSE wi ll remain unknown until screening tests able to identify clinically affecte d cattle have been applied in several countries. This paper provides the fi rst detailed description of a targeted screening programme for BSE. Two pop ulations of cows >24 months of age were included in the targeted screening: (i) cows found dead or culled on site where the carcass was submitted to r endering (fallen stock) and (ii) cows with health-related problems unfit fo r routine slaughter that were slaughtered under emergency procedures (emerg ency slaughter). Between 1992 and 1999, on average 81 clinical BSE suspects per year were reported to the veterinary authorities (passive surveillance ), of which 43% were confirmed with BSE. A total of 30 clinical cases were captured by passive surveillance and an additional 20 BSE cases detected by targeted screening between May 1999 and April 2000. The odds of finding a BSE case was 49 times higher in the fallen stock and 58 times higher in eme rgency-slaughtered cattle when compared to passive surveillance. The target ed screening of fallen stock and emergency-slaughtered cattle considerably increased the number of detected cases in this 12-month period. Targeted-sc reening cases were on average 4 months younger than the clinical suspect ca ses. In conclusion, post-mortem testing of fallen stock and emergency-slaughtere d cows >24 months for BSE is an important active surveillance element withi n a total surveillance system that principally is based on mandatory report ing of clinical suspect ca.,es. Without ante-mortem screening tests to dete ct BSE-infected cattle during the incubation period, a combination of effec tively functioning passive and active BSE surveillance strategies might be the only approach to assess the BSE situation reliably in a given country o r region - and it is necessary to substantiate claims of freedom from the d isease. (C) 2001 Elsevier Science B.V. All rights reserved.