Published by Australian Academy of Science
Activity 1 | Prions – morphing agents of disease

Graphing and extrapolating the incidence of BSE in Britain

The following table shows the approximate figures for confirmed cases of BSE in Britain from 1986 to the first 5 months of 1994.

1. Extrapolating from the January to May figures, estimate the total number of cases for 1994, then represent the data (1986-1994) graphically.

2. BSE is not spread directly from cow to cow, so what accounts for the growth in the number of cases?

3. How do you explain the recent trend depicted in the graph?

Teachers notes

1. Extrapolation implies an estimate of 22 800 cases. (It would be acceptable if students suggested a smaller number than this since numbers of diseased animals appear to be decreasing since 1993.)

   Students may need help in devising a suitable scale for the numbers of cases, and some may need guidance to put time on the horizontal axis. This activity provides a good opportunity to discuss the sort of graph that would be the most suitable. After discussion, students will probably decide that a bar graph (histogram) is best for these data.

2. The spread of BSE apparently occurred because cows were fed protein supplements containing sheep offal (and also cattle offal) that had not been sufficiently treated with very high temperatures or strong chemical solvents to destroy the scrapie agent. The scrapie agent in the offal survived the procedure and was present in the supplements given to cows and they acquired BSE.

3. The number of BSE-infected cattle continued to increase after the 1988 ban on supplementing cattle feed with offal in Britain. This is because prion diseases have very long incubation periods so cattle infected before 1988 would only show symptoms some years later.