Spatial dynamics of transmission and persistence of African swine fever in wild boar populations: insight for control
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1
National Wildlife Research Center, Animal and Plant Health Inspection Service, United States
ASF is a devastating disease for swine producers that is hypothesized to persist in wild boar populations in Eastern Europe through contact with contaminated carcasses and/or continual re-introductions, depending on host density and spatial ecology. However, the role of different transmission and persistence mechanisms remain poorly understood despite their importance for planning allocation of surveillance and control resources across space. We predicted that carcass-based transmission would be important for persistence, especially in low-density host populations because carcasses can increase transmission probability when contact rates are low. We also predicted that host density and social structure would be important for predicting the amount of environmental transmission.
We developed a spatially-explicit model of African swine fever (ASF) in wild boar, and fit the model to surveillance data from Eastern Poland (2014-16) using Approximate Bayesian Computation. A total of 4625 wild boar samples were collected through hunters and 271 samples were through reports of dead carcasses. Thus, hunter harvests made up most of the surveillance samples (94.5%). We estimated new introduction rates, and daily probabilities of direct and carcass-based transmission that best explained the data. Our prior distributions were vague such that new introduction rates ranged from a single event to 60 introductions per year (i.e., continuous spillover at the border). Likewise for transmission probabilities by both mechanisms, we considered prior distributions where the probabilities ranged from no transmission to transmission to all neighbors daily. This design allowed us to quantify the relative role of these three persistence mechanisms at explaining the data. We also evaluated three different spatial models of contact structure, a process that is poorly understood, to identify which spatial processes are important for prediction and to account for their uncertainty. Using estimated parameters we derived the relative number of direct and carcass-based transmission events that most likely occurred to generate the observed data. To understand the potential role of carcass-based transmission beyond this particular outbreak, we conducted sensitivity analyses on the three parameters that affect persistence: introduction frequency, direct transmission probability, and carcass-based transmission probability; over a range of host population densities. The purpose of this latter analysis was to understand how host density drives mechanisms of persistence.
We inferred that between 50-68% of transmission events occurred through contact with a contaminated carcass, depending on host density and contact structure. Carcass-based transmission was higher at low host density and when social grouping was considered in the contact structure. Carcass-based transmission was necessary for autonomous persistence under realistic parameters for this system. Autonomous persistence by direct transmission alone required higher host densities. Persistence also became possible with direct transmission alone when re-introductions occurred more than once per year and direct transmission probabilities were moderately high.
We showed how the relative role of different persistence mechanisms can differ based on small changes in host population density and assumptions about spatial contact processes. Social structure was an important consideration to capture along with standard spatial spreading processes in order to best explain the data, and thus infer of the role of different transmission mechanisms. Management policies should emphasize removal of carcasses and consider how reductions in host densities may affect carcass-based transmission. Understanding the ecological conditions that determine the frequency of different transmission mechanisms is important for choosing optimal management strategies across different ecological contexts.
Acknowledgements
KMP was funded by the USDA-APHIS National Feral Swine Damage Management Program. TP was supported by the National Science Centre, Poland (grant number 2014/15/B/NZ9/01933). We thank M. Łyjak, A. Kowalczyk, K. Śmietanka, and G. Woźniakowski from Department of Swine Diseases, National Veterinary Research Institute in Pulawy, Poland, for surveillance data. N. Selva provided valuable information on carcass persistence time.
Keywords:
African Swine Fever,
wild boar,
Approximate Bayesian Computation,
Spatially-explicit transmission model,
Contact structure,
environmental transmission,
Disease persistence
Conference:
GeoVet 2019. Novel spatio-temporal approaches in the era of Big Data, Davis, United States, 8 Oct - 10 Oct, 2019.
Presentation Type:
Regular oral presentation
Topic:
Special topic on African Swine Fever (ASF)
Citation:
Pepin
KM
(2019). Spatial dynamics of transmission and persistence of African swine fever in wild boar populations: insight for control.
Front. Vet. Sci.
Conference Abstract:
GeoVet 2019. Novel spatio-temporal approaches in the era of Big Data.
doi: 10.3389/conf.fvets.2019.05.00009
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Received:
20 Jun 2019;
Published Online:
27 Sep 2019.
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Correspondence:
Mx. Kim M Pepin, National Wildlife Research Center, Animal and Plant Health Inspection Service, Fort Collins, United States, kim.m.pepin@usda.gov