A descriptive analysis of swine movements in Ontario (Canada) as a contributor to disease spread

Abstract

In recent times, considerable efforts have been made to develop infrastructure and processes of tracing livestock movements. One of common use of this type of data is to assess the potential for spread of infections in source populations. The objectives of this research were to describe Ontario pig movements in 2015, and to understand the potential for disease transmission through animal movement on a weekly and yearly basis. Swine shipments from January to December 2015 represented 224 production facilities and a total of 5398 unique animal movements. This one-mode directed network of animal movements was then analyzed using common descriptive network measures.The maximum yearly (y) weak component (WCy) size and maximum weekly (w) weak component size (WCw) was 224 facilities, and 83 facilities, respectively. The maximum WCw did not change significantly (p > 0.05) over time. The maximum strong component (SC) consisted of two facilities both on a weekly, and on a yearly basis. The size of the maximum ingoing contact chain on a yearly basis (ICCy) was 173 nodes with one abattoir as the end point, and the maximum ICCw consisted of 53 nodes. The size of the maximum outgoing contact chain (OCCy) contained 79 nodes, with one sow herd as a starting point. The maximum OCCw was 6 nodes. Regression models resulted in significant quadratic associations between weekly count of finisher facilities with betweenness >0 (p = 0.02) and weekly count of finisher facilities with in-degree and out-degree >0 (p = 0.01) and week number. Higher weekly counts of nursery and finisher facilities with betweenness >0 and in-degree and out-degree both >0 values occurred during summer months.All study facilities were connected when direction of animal movement was not taken into consideration in the yearly network. As such, yearly networks are potentially representative of infections with long incubation periods, subclinical infections, or endemic infections for which active control measures have not being taken. When the direction of animal movement was considered, such infection could still spread substantially and affect 35% of the study population (79/224).In the study population, finisher sites were proportionally and consistently most represented in WCw (min = 51%, max = 78%), which reflects current Ontario herd demographics. However, abattoirs were over-represented when the number of facilities in the study population was taken into consideration. This, and the size of the maximum ICCw both suggest that abattoirs could be, at least for some infectious diseases, suitable establishments for targeted sampling.