A national-scale picture of U.S. cattle movements obtained from Interstate Certificate of Veterinary Inspection data

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Abstract

We present the first comprehensive description of how shipments of cattle connect the geographic extent and production diversity of the United States cattle industry. We built a network of cattle movement from a state-stratified 10% systematic sample of calendar year 2009 Interstate Certificates of Veterinary Inspection (ICVI) data. ICVIs are required to certify the apparent health of cattle moving across state borders and allow us to examine cattle movements at the county scale. The majority of the ICVI sample consisted of small shipments (<20 head) moved for feeding and beef production. Geographically, the central plains states had the most connections, correlated to feeding infrastructure. The entire nation was closely connected when interstate movements were summarized at the state level. At the county-level, the U.S. is still well connected geographically, but significant heterogeneities in the location and identity of counties central to the network emerge. Overall, the network of interstate movements is described by a hub structure, with a few counties sending or receiving extremely large numbers of shipments and many counties sending and receiving few shipments. The county-level network also has a very low proportion of reciprocal movements, indicating that high-order network properties may be better at describing a county's importance than simple summaries of the number of shipments or animals sent and received. We suggest that summarizing cattle movements at the state level homogenizes the network and a county level approach is most appropriate for examining processes influenced by cattle shipments, such as economic analyses and disease outbreaks.

Introduction

The most widespread data available for tracing large-scale cattle movements in the United States is the Interstate Certificate of Veterinary Inspection (ICVI), although ICVIs are not designed for this purpose. Specifically, when cattle shipments cross state lines, destination states require that most shipments must be accompanied by an ICVI, certifying that an accredited veterinarian has inspected the animals prior to shipment and they are apparently healthy with no signs of communicable diseases and that testing requirements for the destination state are met; cattle going to slaughter are a notable exception in some states. ICVIs list the origin and destination address for the shipment providing a useful source of data on interstate cattle movements. In addition to verifying destination state health requirements are met, ICVIs are the most complete source of national-level movement data. Unfortunately, the use of the ICVI system is limited by storage as paper records, incomplete data fields, and inability to rapidly retrieve the data which limit the ability to use these records for large scale purposes. Although electronic systems are available for entering data on ICVIs, these systems are limited in scope and highly biased (Portacci et al., 2013).

Previous attempts to capture a snapshot of nation-wide cattle movements have tried to circumvent the accessibility issues associated with paper copies of ICVIs and focused on summary information obtained from ICVIs at the state level (Shields and Mathews, 2003). Some states keep track of which states they send and receive cattle from enabling coarse grained models and predictions of cattle movement at the state scale (Shields and Mathews, 2003), although these models are inadequate to capture spatial heterogeneities in cattle shipment patterns that arise at finer resolutions. Unfortunately, this summary information is also incomplete, with less than half of states maintaining summaries about interstate shipping partners, and provides no information on characteristics of individual shipments that could enhance understanding and interpretation of U.S. cattle movement patterns (Forde et al., 1998).

Other attempts to characterize cattle movement in the U.S. have avoided ICVIs entirely. Particularly, data on shipments of animals for feeding have been compiled to identify geographic areas serviced by cattle markets, although these data are limited to shipments from a single video auction and makes no attempt to develop a comprehensive description of national cattle movements (Bailey et al., 1995). Additionally, a study focused on movements from a limited number of counties in California was done by asking farmers directly about their direct and indirect contacts with other cattle facilities, and while the data are illuminating at a local scale, the cost and effort of expanding to a national scale characterization are immense (Bates et al., 2001). Another effort to characterize U.S. cattle movements at finer resolutions (i.e., premises level) has relied almost exclusively on models developed from expert opinion on shipment patterns at highly-local scales (Liu et al., 2012). Without comprehensive movement data to base these models on, management decisions must rely on predictions based on large amounts of unquantifiable uncertainty and unverified assumptions. Based on the limitations of other attempts at characterizing U.S. cattle movements, ICVI data may provide a better understanding of the movement within the U.S. cattle industry as a whole.

The value of ICVIs in characterizing cattle movement through the development of a network approach has not been thoroughly explored. Network models have been successful in describing cattle and sheep movements in other countries (Webb, 2005, Bigras-Poulin et al., 2006, Kiss et al., 2006, Robinson and Christley, 2007, Brennan et al., 2008, Natale et al., 2009, Vernon and Keeling, 2009, Volkova et al., 2010, Bajardi et al., 2011) and are likely to be helpful in characterizing animal movement in the United States. Also, using networks for post hoc analysis has proven useful in identifying determinants of disease spread and the importance of animal movements in disease outbreaks, most notably the 2001 outbreak of Foot-and-Mouth disease in the United Kingdom (Green et al., 2006, Ortiz-Pelaez et al., 2006, Kao et al., 2006). However, integrating movement data into disease spread models should not be limited to after-the-fact analysis but should also be used to develop predictions of risk based on animal movements. Predictions derived from these types of animal movement and disease simulation models can then be used to efficiently structure limited surveillance resources as well as determine the economic consequences of disease control strategies during an outbreak (Kao et al., 2007, Velthuis and Mourits, 2007). In addition, economic analyses of cattle shipment patterns can increase efficiency of the U.S. cattle industry as a whole through an understanding of market interactions (Bailey et al., 1995) and livestock hauling behavior and limitations (Hoffman et al., 1975).

In this study, we use ICVIs to characterize cattle shipments and to build network models that provide the first comprehensive quantitative characterization of interstate national cattle movement in the U.S. This demonstrates the utility of an ICVI beyond tracing individual animals and ensuring destination state health requirements are met. We note that ICVIs are limited to shipments that travel interstate and, thus, ICVI based inference about the cattle movement in the U.S. is also restricted to interstate cattle transport patterns. However, we evaluate the robustness of the ICVI-based networks using several independent data sources to identify any potential systematic bias that may be introduced by sampling interstate data. In addition, attributes derived from the cattle movement network can then be tied to the underlying infrastructure of the cattle industry to help guide national scale strategies on disease risk prevention, surveillance, and control, as well as economic analyses.

Section snippets

Data collection

Because ICVIs are maintained by both the destination and origin states, we requested that all states send a 10% sample (either photocopies or scans) of their calendar year 2009 cattle ICVIs for cattle shipments originating in the state by taking a systematic sample of every tenth cattle ICVI. This enabled us to obtain estimates of the total number of ICVIs for 2009 and avoid potential temporal and spatial biases that could arise by sampling the first 10% of records. We requested ICVI records

Shipment characteristics

Interstate shipments of cattle not going to slaughter in the U.S. were primarily small shipments consisting of less than 10 animals (Fig. 1). These shipments were dominated by animals going to feed (45.1%), although breeding (17.1%), sale (11.4%), and show (6.8%) movements were also represented (Appendix B, Fig. B1). The ICVI sample contained both local and long distance cattle movements, including rare extreme long-distance movements (>3000 km; Appendix B, Fig. B2). There were no apparent

Discussion

ICVIs, to this point, have primarily been used for to ensure state animal health requirements are met. Here, we co-opted the utility of ICVIs for tracing individual shipments to obtain a sample of the interstate cattle movement network in the U.S. This work represents one of the first truly comprehensive studies of national-level cattle shipment contents and spatial patterns in the U.S. by describing shipment patterns at a county resolution; enhancing previous efforts at summarizing interstate

Conclusions

Understanding cattle movements at the national scale can have far reaching implications for disease response planners and is one of the most important applications of the network analysis of this novel data set. Identification of nodes that are highly connected can be used to develop targeted surveillance programs, and these nodes may also indicate locations where movement controls can be implemented in the event of a disease outbreak. Viewing the U.S. interstate cattle movement network at the

Conflicts of interest

None declared.

Acknowledgements

Funding provided by the Research and Policy for Infectious Disease Dynamics (RAPIDD) Program, Science and Technology Directorate, U.S. Department of Homeland Security, and Fogarty International Center, National Institutes of Health; Foreign Animal Disease Modeling Program, Science and Technology Directorate, U.S. Department of Homeland Security (Grant ST-108-000017); and USDA Cooperative Agreement 11-9208-0269-CA 11-1 and 09-9208-0235-CA. Data was provided by the U.S. Department of Agriculture,

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    Present address: Department of Ecology and Evolutionary Biology, University of California – Los Angeles, Los Angeles, CA, USA.

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