Between-herd movements of cattle as a tool for evaluating the risk of introducing infected animals

Abstract

The spread of pathogens between cattle herds is closely related to the between-herd contact structure. This structure is based on neighbourhood relationships, animal movements and other infectious contacts. The aim of this study was to qualify and quantify cattle movements between herds and their heterogeneity in order to evaluate the theoretical risk of introducing infected animals in herds. Data collected from 1998 to 2002 were studied for every cattle herd located in Brittany. Herd-year types were based on herd size, cow numbers per breed, and fattening units (veal-calf, bull fattening). Herd-years were either isolated (no introduction, no transfer to another herd), closed source (no introduction, at least one transfer), open source (introduction and transfer) or strictly destination (introduction but no transfer) herds. A third of the herds were open source and 16% were isolated. The proportions varied with the herd-year type and the fattening unit. Open herds had almost as many source herds as yearly introduced animals. The contact structure between cattle herds was complex and heterogeneous. Almost all the herd-year types were related to each other with a varying number of transfers and introductions. More than 90% of the pairs between two herds were not renewed the next year. Assuming that movements were not controlled, within-herd prevalence was beta distributed averaging 15%, and herd-level prevalence was 30%, the theoretical risk of introducing an infected animal in herds without any fattening unit varied from 32 to 81%. If herd testing was implemented and able to detect herds with a prevalence above 10% and if movements were not allowed from detected herds, the herd- and animal-level prevalences in the remaining herds became respectively 9% and 2% on average. The risk varied from 2 to 6% for herds without any fattening unit. Animal movements control may reduce the spread of pathogens. © INRA, EDP Sciences, 2006.