One of the problems in diagnosing transmissible spongiform encephalopathies (TSEs) is that the clinical symptoms often do not manifest themselves until quite late in the progression of the disease. However, as Dell'Omo et al. report in the European Journal of Neuroscience, mice that are infected with different prion strains show significant changes in circadian activity long before any clinical signs appear; moreover, the nature of these changes seems to depend on the prion strain.

The authors infected mice with scrapie prion strain 139A or ME7, or the bovine spongiform encephalopathy (BSE) strain 301C. Using an automated tracking system, the mice were monitored continually from seven weeks after infection. The most pronounced difference between strains was seen during the night, when mice are most active. Mice that were infected with strain 301C or ME7 showed a persistent depression of nocturnal activity, right from the beginning of the monitoring period. Strain-139A-infected mice, on the other hand, initially showed similar levels of activity to controls, but became hyperactive towards the time of onset of clinical symptoms. Also, from week 11 onwards, the 301C-infected mice were significantly less active than those infected with ME7.

The authors believe that their monitoring protocol could be scaled up for use in larger animals, such as sheep, and this might have important implications for agriculture. For example, in the United Kingdom, there is some concern that BSE has spread into sheep. Unfortunately, the symptoms of BSE are difficult to distinguish from those of scrapie, which has been present in sheep for many years and is not known to pose a significant threat to human health. If different prion strains are found to cause distinct early changes in circadian activity in sheep, behavioural monitoring could provide a useful diagnostic tool to distinguish between BSE and scrapie.