A Porcine Epidemic Diarrhea Virus Outbreak in One Geographic Region of the United States: Descriptive Epidemiology and Investigation of the Possibility of Airborne Virus Spread

Beam, Andrea; Goede, Dane; Fox, Andrew; McCool, Mary Jane; Wall, Goldlin; Haley, Charles; Morrison, Robert

This study described an outbreak of PEDv that occurred in four contiguous states over a 31-day period in spring 2013. The description of this outbreak may be useful in understanding factors that influence the spread of PEDv. Several spatial methods were used to describe the outbreak. The hotspot analysis showed that, overall, the disease was clustered rather than randomly distributed. This result suggested that sites in proximity to a positive site were at increased risk of acquiring PEDv. One limitation of the data in this study was that negative We also examined the possibility of airborne spread of PEDv. If airborne dissemination of PEDv played a role in this outbreak, we would expect the direction of disease spread to be correlated with the predominant wind direction. We used two methods to determine the direction of disease spread, and both had similar results: one indicated spread was south to slightly southwest, and the other indicated spread was to the southeast. The slight difference in results is attributable to the different underlying methodologies. The predominant wind direction during the outbreak was toward the south with some southeast and southwest winds; the strongest wind gusts were toward the southwest. Therefore, our results provide support to the hypothesis of airborne PEDv spread, since the direction of disease spread correlated qualitatively with wind direction. The results, however, should be interpreted cautiously because our analysis has a number of limitations. First, we did not have information on direct and indirect contacts among sites, such as movement of trucks, feed, pigs, people, and equipment. These contacts should be considered before pathogen spread is attributed to airborne mechanisms. For example, if truck movement predominately occurs in a southern direction, this could explain the southern movement of PEDv. In addition, swine farm density in the area needs to be considered because unequal distribution of swine farms could influence the observed direction of disease spread. To analyze the distribution of swine farms in the area, we calculated the number of swine farms at risk (uninfected swine sites in our dataset) in the eight primary compass directions relative to the index (first) case. The results of this analysis show that 76 percent of the population at risk was located south of the index case (Fig 6) , which could also be a factor in the apparent southern movement of PEDv. Furthermore, we did not consider other meteorological variables such as temperature and humidity, which may also play a role in airborne pathogen transmission.