Network analysis of pig movements: Loyalty patterns and contact chains of different holding types in Denmark

Understanding animal movements is an important factor for the development of meaningful surveillance and control programs, but also for the development of disease spread models. We analysed the Danish pig movement network using static and temporal network analysis tools to provide deeper insight in the connection between holdings dealing with pigs, such as breeding and multiplier herds, production herds, slaughterhouses or traders. Pig movements, which occurred between 1st January 2006 and 31st December 2015 in Denmark, were summarized to investigate temporal trends such as the number of active holdings, the number of registered movements and the number of pigs moved. To identify holdings and holding types with potentially higher risk for introduction or spread of diseases via pig movements, we determined loyalty patterns, annual network components and contact chains for the 24 registered holding types. The total number of active holdings as well as the number of pig movements decreased during the study period while the holding sizes increased. Around 60–90% of connections between two pig holdings were present in two consecutive years and around one third of the connections persisted within the considered time period. Weaner herds showed the highest level of in-loyalty, whereas we observed an intermediate level of in-loyalty for all breeding sites and for production herds. Boar stations, production herds and trade herds showed a high level of out-loyalty. Production herds constituted the highest proportion of holdings in the largest strongly connected component. All production sites showed low levels of in-going contact chains and we observed a high level of out-going contact chain for breeding and multiplier herds. Except for livestock auctions, all transit sites also showed low levels of out-going contact chains. Our results reflect the pyramidal structure of the underlying network. Based on the considered disease, the time frame for the calculation of network measurements needs to be adapted. Using these adapted values for loyalty and contact chains might help to identify holdings with high potential of spreading diseases and thus limit the outbreak size or support control or eradication of the considered pathogen.

解析动物移动模式，是制定科学有效的监测与防控方案、构建疾病传播模型的关键基础。本研究采用静态网络分析（static network analysis）与时序网络分析（temporal network analysis）工具，对丹麦生猪移动网络（pig movement network）展开分析，以期深入解析各类生猪相关养殖主体（holding）间的关联模式，涵盖原种猪场（breeding herd）、扩繁场（multiplier herd）、商品代猪场（production herd）、屠宰场（slaughterhouse）及生猪贸易商（trader）等。本研究汇总了2006年1月1日至2015年12月31日期间丹麦境内所有生猪移动数据，以探究时序变化趋势，包括活跃养殖主体（active holding）数量、已登记生猪移动次数及生猪跨场转运总量等指标。为识别出那些通过生猪移动更易引入或传播疫病的养殖主体及主体类型，本研究针对24种已登记的养殖主体类别，分析了其关联忠诚度模式（loyalty pattern）、年度网络组分（annual network component）及接触链（contact chain）特征。观测周期内，活跃养殖主体总量与生猪移动次数均呈下降趋势，而单主体养殖规模则逐步扩大。约60%至90%的生猪养殖主体间关联会在连续两年中持续存在，且约三分之一的关联在整个观测时段内始终保持。保育猪场（weaner herd）的入向关联忠诚度最高，而所有原种猪场与商品代猪场的入向关联忠诚度则处于中等水平。公猪站（boar station）、商品代猪场及生猪贸易主体的出向关联忠诚度较高。在最大强连通分量（strongly connected component）中，商品代猪场的占比最高。所有商品代猪场的入向接触链（in-going contact chain）水平均较低，而原种猪场与扩繁场的出向接触链（out-going contact chain）水平则较高。除牲畜拍卖场（livestock auctions）外，所有中转型主体（transit site）的出向接触链水平均较低。本研究结果揭示了该生猪移动网络的金字塔式层级结构。针对不同疫病场景，需适配网络指标计算的时间窗口。通过使用适配后的关联忠诚度与接触链指标，可有效识别出具备高疫病传播潜力的养殖主体，从而助力限制疫病暴发规模，或为目标病原的防控与根除提供技术支撑。