Role of animal movement and indirect contact among farms in transmission of porcine epidemic diarrhea virus

Abstract

Epidemiological models of the spread of pathogens in livestock populations primarily focus on direct contact between farms based on animal movement data, and in some cases, local spatial spread based on proximity between premises. The roles of other types of indirect contact among farms is rarely accounted for. In addition, data on animal movements is seldom available in the United States. However, the spread of porcine epidemic diarrhea virus (PEDv) in U.S. swine represents one of the best documented emergences of a highly infectious pathogen in the U.S. livestock industry, providing an opportunity to parameterize models of pathogen spread via direct and indirect transmission mechanisms in swine. Using observed data on pig movements during the initial phase of the PEDv epidemic, we developed a network-based and spatially explicit epidemiological model that simulates the spread of PEDv via both indirect and direct movement-related contact in order to answer unresolved questions concerning factors facilitating between-farm transmission. By modifying the likelihood of each transmission mechanism and fitting this model to observed epidemiological dynamics, our results suggest that between-farm transmission was primarily driven by direct mechanisms related to animal movement and indirect mechanisms related to local spatial spread based on geographic proximity. However, other forms of indirect transmission among farms, including contact via contaminated vehicles and feed, were responsible for high consequence transmission events resulting in the introduction of the virus into new geographic areas. This research is among the first reports of farm-level animal movements in the U.S. swine industry and, to our knowledge, represents the first epidemiological model of commercial U.S. swine using actual data on farm-level animal movement.

Identifying outbreaks of Porcine Epidemic Diarrhea virus through animal movements and spatial neighborhoods

“…The effect of animal movements on risk reported here is consistent with general principles of disease spread in networks [42][43][44][45][46] ; livestock movements and network connectivity have been shown to be important for PEDV infection elsewhere 16,37 . In contrast to these studies, the uniqueness of our approach is that we moved beyond simply accounting for the focal farm's position in the movement network, and instead quantified the influence of movements into spatial neighbors.…”

“…A number of studies have examined swine movement networks as a main transmission route for disease spread 16,[35][36][37][38][39] , network analysis and epidemiological data have recently been used to assess risk of local areas being exposed to pathogens as a consequence of animal introduction 40 . The approach presented here provides a realistic understanding of PEDV dynamics by capturing long distance spread via animal movements and linking it with epidemiological factors related to local area spread between neighboring farms.…”

Illustration of Different Disease Transmission Routes in a Pig Trade Network by Monopartite and Bipartite Representation

“…Thus, disease can be spread over different shipments and can, in this way, infect farms that do not directly trade with each other [ 17 , 18 , 19 , 20 ]. The same is true for the veterinarian or the feed supplier [ 7 , 11 , 21 ]. Over these vectors, more transmission routes can be analyzed instead of the pure direct animal movements between farms.…”

“…Due to the fact that the inclusion of different transmission routes offers the more detailed illustration of the real probability of the disease spread, more recent studies also included indirect transmission routes into the network analysis and evaluated their impact on potential disease spread [ 5 , 11 , 13 , 18 , 19 , 21 , 22 , 23 , 24 , 25 ]. However, most of these studies focused on the inclusion of indirect disease transmission via livestock trucks.…”

Estimation of swine movement network at farm level in the US from the Census of Agriculture data

“…However, similar programs are yet to be mandated for the US. In the US, a comprehensive livestock tracking system has not been implemented because of a cultural preference for privacy and competition between producers 4 . The United State Department of Agriculture collects movement information when livestock shipments cross state boundaries.…”