Analysis of Swine Movement in Four Canadian Regions: Network Structure and Implications for Disease Spread

Abstract

Direct and indirect contacts among animal holdings are important in the spread of infectious diseases. The objectives of this study were to describe networks of pig movements and the sharing of trucks used for those movements between swine farms in four Canadian regions using network analysis tools and to obtain contact parameters for infectious disease spread simulation models. Four months of swine movement data from a pilot pig traceability programme were used. Two types of networks were created using three time scales (weekly, monthly and the full study period): one-mode networks of farm-to-farm direct contact representing animal shipments and two-mode networks representing the sharing of trucks between farms. Contact patterns among farms were described by estimating a range of relevant network measures. The overall network neglecting the four regions consisted of 145 farms, which were connected by 261 distinct links. A total of 184 trucks were used to transport 2043 shipments of pigs during the study period. The median in- and out-degree for the overall one-mode network was 1 and ranged from 0 to 26 and 0 to 10, respectively. The overall one-mode network had heterogeneous degree distribution, a high clustering coefficient and shorter average path length than would be expected for randomly generated networks of similar size. On average one truck was shared by four farms in the overall network, or by three farms when considered the monthly and weekly networks. Degree distribution of the two-mode overall network demonstrated characteristics of power-law distribution. For more than 50% of shipments on any given day, the same truck was used for at least one other shipment. Findings from this study are in agreement with previous work, which suggested that swine movement networks exhibit small-world and scale-free topologies. Furthermore, trucks used for the shipment of pigs can play an important role in connecting otherwise unconnected farms and may increase the spread of disease.

Simulation of between-farm transmission of porcine reproductive and respiratory syndrome virus in Ontario, Canada using the North American Animal Disease Spread Model

“…The proportion distributed to each of the broad production types was based on a previous pig traceability study (Thakur et al, 2014) or based on experts' judgement, in the case of farrow-to-finish farms. In addition, 60% of nursery and finishing farms were classified as having adopted the practice of all-in-all-out (AIAO, farm types given the suffix "A") with the remaining 40% adopted the continuous flow (CF, farm types with suffix "C") approach.…”

“…To simulate spread by direct and indirect contact, contact parameters were derived from swine movement data collected as part of a pilot study in four Canadian regions (Thakur et al, 2014). Two co-authors of this manuscript (DH, a Professor of swine health management, and ZP, an Associate Professor of Veterinary Epidemiology whose research has a focus on swine disease), have extensive expertise in swine production and management across Canada and provided inputs to establish the model structure.…”

“…However, it should be noted that estimates of contact frequency were based on a relatively small data set and that these data were collected in 2004. Additionally, regions from which the pig movement data were obtained were not identified in this paper, or in the original published report (Thakur et al, 2014), due to confidentiality restrictions. However the report included 50% of the swine-producing regions of Canada and represented both Eastern and Western production system within Canada.…”

“…Similarly, one of the co-authors on this manuscript (ZP) has unpublished data that suggests that the R 0 could be anywhere between 2.5 and 14. In addition, with a mean shipment size of 130 animals, as reported in (Thakur et al, 2014) and with higher than 80% of animal level prevalence of PRRS virus (Nodelijk et al, 2003), it is more likely that more than one infectious animal will be present in a shipment from an infectious farm, therefore suggesting a negligible probability of infection extinction.…”

“…on.ca/biosecurity-resources/transportation) and experts in the swine industry. Recent studies have demonstrated high levels of truck sharing between farms for the shipment of pigs (Bottoms et al, 2012;Thakur et al, 2014) and have documented that only around one third of such trucks are properly cleaned and disinfected between successive shipments (Lambert et al, 2012). However, the likely impact of truck sharing on the spread of PRRS virus has not been explored in previous studies.…”