Trends in age at detection in cases of bovine spongiform encephalopathy in Belgium: an indicator of the epidemic curve.

There were 118 cases of bovine spongiform encephalopathy (BSE) in Belgium before January 1, 2004. Trends in their age at detection were analysed and attempts were made to use this parameter as a predictor of the current status of the BSE epidemic in the country. The following variables were considered: date of birth, breed, date of detection, mode of detection, and the number and age of animals slaughtered and rendered each month. Age at detection as a function of date of birth was a very poor epidemiological indicator. It was concluded that the increasing age of BSE cases when they were detected was due to the depletion of cases, as a result of there being no new infections, and that it is a reliable indicator of a decrease in the epidemic curve in Belgium. By means of a simulation it is shown how age distribution at the time of detection closely follows the epidemic curve and data from Great Britain are used to illustrate the point.