Abstract
AbstractSusceptible host density is a key factor that influences the success of invading pathogens. However, for diseases affecting livestock, there are two aspects of host density: livestock and farm density, which are seldom considered independently. Traditional approaches of simulating disease outbreaks on real‐world farm data make dissecting the relative importance of farm and livestock density difficult owing to their inherent correlation in many farming regions. We took steps to disentangle these densities and study their relative influences on epidemic size by simulating foot‐and‐mouth disease outbreaks on factorial combinations of cattle and farm populations in artificial county areas, resulting in 50 unique cattle/farm density combinations. In these simulations, increasing cattle density always resulted in larger epidemics, regardless of farm density. Alternatively, increasing farm density only led to larger epidemics in scenarios of high cattle density. We compared these results with simulations performed on real‐world farm data from the United States, where we initiated outbreaks in U.S. counties that varied in county‐level cattle density and farm density. We found a similar, but weaker relationship between cattle density and epidemic size in the U.S. simulations. We tested the sensitivity of these outcomes to variation in pathogen dispersal and farm‐level susceptibility model parameters and found that although variation in these parameters quantitatively influenced the size of the epidemic, they did not qualitatively change the relative influence of cattle vs. farm density in factorial simulations. By reducing the correlation between farm and livestock density in factorial simulations, we were able to clearly demonstrate the increase in epidemic size that occurred as farm sizes grew larger (i.e., through increasing county‐level cattle populations), across levels of farm density. These results suggest livestock production trends in many industrialized countries that concentrate livestock on fewer, but larger farms have the potential to facilitate larger livestock epidemics.
Highlights
Pathogen invasions pose global threats to human and animal health (Daszak et al 2000), biodiversity (Mack et al 2000), and food production (Brasier 2008, Hatcher et al 2012) and are a significant burden on the global economy (Thompson et al 2002, Pimentel et al 2005)
Pathogenic avian influenza, classical swine fever, and foot-and-mouth disease (FMD), host density is a key factor that contributes to epidemic risk and can dictate the control measures necessary to contain the epidemic (Ferguson et al 2001, Boender et al 2007, 2008)
The simulations reveal patterns suggesting the occurrence of significant epidemics is more dependent on cattle density than farm density
Summary
Pathogen invasions pose global threats to human and animal health (Daszak et al 2000), biodiversity (Mack et al 2000), and food production (Brasier 2008, Hatcher et al 2012) and are a significant burden on the global economy (Thompson et al 2002, Pimentel et al 2005). A common goal of disease spread models is to estimate epidemic risk and scenario-plan the best control measures, so many simulation studies are performed on real-world farm data (or approximations of real-world data) to estimate epidemic size under varying control regimes (e.g., Tildesley et al 2006, Ward et al 2009, Hayama et al 2015). These studies provide valuable information regarding the epidemic risks and control strategies in those areas and provide a clear link between host density and the severity of livestock epidemics
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