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

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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.

Introduction

In 1973, after an absence of 20 years in Great Britain, the mange mite Psoroptes ovis was discovered in a sheep flock in Lancashire (Loxam, 1974). This was followed by a 20 year effort to eradicate the disease, including the reintroduction of compulsory regional and national dipping in approved acaricides and regional clearance programmes. Finally, after a number of policy changes and 4 years of increase in the annual number of outbreaks, the control of sheep scab was deregulated in 1992. The current status of the national flock is unknown, although the disease is now believed to be widespread.

To date, little has been done to explore the spatial and temporal patterns of the disease over this 20 year period. The recent developments in the use of Geographical Information Systems (GIS) and statistical methods for the analysis of spatial and temporal patterns have dramatically improved the tools and techniques available for such analyses. This paper uses some of these methods to examine aspects of the epidemiology of sheep scab during this period.

One of the key features of the epidemiology of sheep scab is the seasonal pattern. It has been known for some time that the majority of outbreaks of sheep scab are noticed in the winter months. There have been many suggested reasons for this pattern including the effects of climate and fleece length on the growth of the mite population (Downing, 1936, Spence, 1949). In this study we examine the seasonal pattern in detail and attempt to interpret the findings in the light of both changes in the national policy for the control of sheep scab in Great Britain, and the underlying patterns of insecticide and acaricide use.

Section snippets

Source of data

Two sources of information were used in this study.

  • 1.

    For the years 1983–1992 information was gathered from files stored at the Ministry of Agriculture, Fisheries and Food (MAFF), State Veterinary Service, Tolworth, UK. Comparable records for the years 1973–1982 were no longer available. The date of parasitological confirmation of the outbreak and the Ordinance Survey (OS) Grid Reference were obtained from these records.

  • 2.

    For the years 1973–1982, the parasitology records at the MAFF Veterinary

Patterns in time

A total of 1480 outbreaks were reported over the 20 year time period. The number of outbreaks per year and the government policy on compulsory dipping is shown in Fig. 1. Major changes in policy appeared to coincide with a subsequent rise or fall in the number of cases. For example, the change from regional compulsory dipping in 1975 to compulsory autumn dipping in 1976 was followed by a marked decline in the number of cases reported. Likewise, the change from a single national summer dip in

Discussion

This study has explored the spatial and temporal patterns of sheep scab over a 20-year period following the reintroduction of the disease into Great Britain in 1973. As far as the authors are aware, this represents the first detailed analysis of these data. The time series revealed a familiar seasonal pattern with an increase of scab in the winter months (Downing, 1936, Spence, 1949). This seasonal component of the cyclicity appeared to change following the removal of the summer dip in 1988 and

Acknowledgements

Many thanks are due to Mr. Peter Bates and Dr. Mike Taylor from the Ministry of Agriculture Fisheries and Food, Veterinary Laboratories Agency, Addlestone, Surrey for their valuable advice and help with obtaining outbreak data. We would also like to thank Joanna Sutherst and the staff at Tolworth for their help in identifying the geographical locations of outbreaks and help in data collection and Dr. Helen Wilson for her comments on the manuscript. This work was funded by MAFF Contract CSA 3503.

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