Modeling potential windborne spread of Foot and Mouth Disease from an infected U. S. feedlot

Abstract

Foot and Mouth Disease (FMD) causes severe production and economic losses in many parts of the world. It is highly communicable, and transmitted via direct contact, indirect contact, and occasionally windborne transmission. Windborne transmission requires specific epidemiological and meteorological conditions. It can allow the FMD virus to escape traditional epidemiological controls, such as quarantines and biosecurity measures. While multiple studies have modeled the risk of windborne transmission of FMD, to our knowledge none of these studies modeled cattle as the source of the outbreak. The large cattle feedlots found in the U.S. present a risk of windborne transmission that has not yet been quantified. The objective of this research was to investigate the potential risk of windborne transmission of FMD from an infected U.S. feedlot using an integrated modeling approach. To do this, we integrated a within-herd epidemiological model, an advanced atmospheric dispersion model, and calculation of infection risk dependent on exposed herd size. A previously developed epidemiological model was used to simulate the transmission of FMD through a typical US feedlot, while the National Oceanic and Atmospheric Administration’s HYSPLIT atmospheric dispersion model, which has been validated for FMD modeling, was used to model virus dispersion. Infection risk for exposed herds was calculated as a binomial probability accounting for dose and exposed herd size. We modeled risk of windborne transmission from a typical 4,000 head feedlot in IA, and a typical 48,000 head feedlot in KS during winter and summer seasons. Risk of windborne transmission of FMD varied based on weather/season conditions, estimated average per head viral shedding rate, size of infected herd, and size of exposed herd. In the baseline winter scenario at peak shedding day for the infected feedlot, the median of the maximum daily risk for a 1,000-head exposed herd located downwind of a KS feedlot ranged from 89.88% at 3km to 8.37% at 10km, and from 48.38% at 3km to 1.13% at 10km for an IA feedlot. Risk for larger exposed herds was greater. Overall, our results indicate that an infected feedlot would have a variable, but non-zero risk of infecting nearby herds via windborne transmission. In some situations, significant risk of windborne spread may extend beyond 10km, which is the minimum outbreak control area recommended by USDA APHIS. This could be a concern, particularly in areas with large feedlots in relatively close proximity. Our model may be useful as a research tool in the absence of an outbreak. It also could be used to help target surveillance and response efforts in the event of an outbreak.