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

Abstract

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.

Drivers and Dynamics of Methicillin-Resistant Livestock-Associated Staphylococcus aureus CC398 in Pigs and Humans in Denmark

“…Based on simulation models and network analyses, previous work has shown that pig movements and herd types are likely to play important roles in the spread of swine diseases in Denmark ( 4 , 8 ). In addition, several small-scale studies from Denmark and the Netherlands have found that the LA-MRSA CC398 status of a given pig farm depends largely on the LA-MRSA CC398 status of its suppliers ( 10 , 15 , 16 ).…”

“…This suggests that the rapid spread of LA-MRSA CC398 within the Danish pig production system was caused by clonal expansion of the three lineages. Breeding herds are considered a critical factor in the spread of swine diseases because they produce and supply hybrid breeding pigs to a large number of production holdings ( 4 ). Thus, it is possible that the emergence of the same lineages in breeding farms somewhere between 2008 and 2014 has accelerated the spread of LA-MRSA CC398 within the Danish pig production system.…”

“…In humans, LA-MRSA CC398 is primarily associated with skin and soft tissue infections (SSTIs) but has also caused invasive disease, such as bloodstream infections (BSIs), and occasionally death ( 1 – 3 ). The Danish pig industry has a pyramidal structure in which nucleus and multiplier herds (here collectively referred to as breeding farms) at the apex produce and sell sows to production and weaner herds (here collectively referred to as production farms) at the bottom ( 4 ). In the first nationwide survey conducted in 2008 in Denmark, the prevalence of LA-MRSA CC398 was 0% and 3.5% in breeding and production farms, respectively, both of which increased to more than 60% by 2014 ( 5 – 7 ).…”

“…Because its transmission pathways remain poorly understood, the causes of this epidemic are keenly debated, with various possible explanations for the observed farm-to-farm transmission, including movement of positive animals, inadequate control measures, and spread by humans, contaminated fomites, wind, insects, rodents, and other alternative hosts. Animal movements have long been considered a critical factor in the spread of livestock diseases ( 8 , 9 ), and it has been suggested that animal movements may play a similar role in the dissemination of LA-MRSA CC398 ( 4 , 10 , 11 ).…”

Network analysis of pig movements in Argentina: Identification of key farms in the spread of infectious diseases and their biosecurity levels

“…Network analysis has been described as a useful methodology to characterize pigs movements, and the impact that the contact structure has on the spread of diseases (Lentz et al, ; Schulz, Boklund, Halasa, Toft, & Lentz, ; Sterchi et al, ) in countries such as Canada (Dorjee et al, ; Thakur et al, ), the USA (Lee et al, ), Germany (Büttner, Krieter, & Traulsen, ), France (Salines et al, ) and Kenya and Uganda (Lichoti et al, ). When comparing these studies to our results, we realize that Argentinean networks have a smaller diameter (2–6) than networks in the USA, Canada, France, Germany, Kenya and Uganda: 9, 3.25, 6 to 20 and 7, respectively.…”