Sheep scab outbreaks in Great Britain between 1973 and 1992: spatial and temporal patterns

Abstract

In January 1973, the mange mite Psoroptes ovis was found in a sheep flock in Lancashire, UK after a 20 year period of eradication. Over the next 20 years, a total of 1480 flock outbreaks were reported, until June 1992 when the control of sheep scab was deregulated. The spatial and temporal patterns of the number of outbreaks in each month over this period were analysed using time series methods, a geographical information system (GIS) and second order approaches to the exploration of space-time interaction (K-function analysis). The monthly time series displays clear evidence of seasonal cyclicity, with most outbreaks being reported in the winter months. After removal of the summer compulsory dip in 1988 however, there is evidence of a change in the seasonal pattern, with more cases being reported in the autumn months. There is also evidence of a contribution from a sinusoidal wave with a period of 8 years, which roughly corresponds to the major changes in the national control policy. The spatial pattern also appears to expand and contract on a roughly 4 year cycle, again reflecting the oscillating periods of success and failure. Examination of the spatial and temporal pattern revealed strong evidence of space-time clustering (p < 0.001) and a major contribution to the pattern from very local (< 12 km) spread within 5 months of an arbitrary outbreak.