Scrapie is an economically important, notifiable disease of sheep that could be masking the presence of Bovine Spongiform Encephalopathy (BSE) in the UK national sheep population. The national scrapie programme (NSP) is a continuing initiative whose aim is to breed a scrapie and BSE resistant national flock. As the prevalence of scrapie goes down, it will become increasingly important to identify risk factors for scrapie, both as a means of identifying potentially infected flocks and to monitor the (presumably decreasing) prevalence of scrapie.
The number of sheep purchased and size of flock are well known to be risk factors for the presence of scrapie on a farm, and the existence of extensive data on the demographics of the sheep industry (via the Animal Movements License Scheme and Annual Agricultural Census) provide the opportunity to examine relationships amongst scrapie-affected flocks over the movement network. In a previous project, analysis of the movement and census data showed evidence that farms with notified scrapie cases were more likely to have traded at the same market on the same day with other affected flocks, were typically more active traders, and were larger, but that flock size and trading activity were negatively correlated.
Though this study has identified important risk associations, there remain a number of further analyses that could clarify the associations. First, the analysis only considered presence or absence of scrapie, and did not consider whether there was any effect associated with having multiple reported cases. Second, the total number of of animals moved was not considered, only number of transactions. Third, the number of potential links between scrapie-affected farms was not considered, only whether a link existed or not. Finally, as there is a significant relationship between scrapie on farms and group lambing in pens, it is likely that sheep that are resident for at least one breeding season are most likely to both acquire and transmit scrapie. The database only contains batch information and so it is not possible to directly distinguish which movements related to breeding, and those that are not.
In this project, we aim to conduct these more detailed analyses, and attempt to identify subclasses of movements that could increase the association amongst scrapied affected farms. Potential subcategories may, for example, be based on time of year (i.e. is the association stronger if we consider the different seasons), the number of sheep traded, or by the trading batch size.