Spatial and temporal epidemiology of porcine epidemic diarrhea (PED) in the Midwest and Southeast regions of the United States

Emergence of porcine epidemic diarrhea virus (PEDV) in the US in 2013 caused a major impact in the swine industry due to its high mortality and rapid spread through the country. Even though the role of potential sources of infection in the epidemiology of the disease at the farm level (feed, fomites) has been extensively investigated, there is a lack of knowledge about the dynamics of disease spread at the regional level. Here, we investigated the dissemination of PEDV infection in two areas located in the regions with the highest swine density in the country (Southeast and Midwest) including more than 2400 farms. Location and date of outbreaks were used to assess the spatial and temporal clustering of cases using global (Cuzick-Edwards, Knox and directional tests) and local (Bernouilli model of the spatial scan statistic) techniques in the first 10 months of the epidemic. A strong spatio-temporal pattern was detected in both areas of study, with an increased risk of disease at <2 km distances of recently (<7 days) infected farms, although extent of clustering was higher in the Southeast. Results, computed for two different locations in the first months of the epidemic, suggest that local transmission from infected farms into neighboring PEDV-free sites is a likely explanation for a substantial proportion of the reported PEDV-positive farms and consistent with the rapid spread of a highly infectious disease in the absence of immunity.