Understanding the early dynamics of the 2014 porcine epidemic diarrhea virus (PEDV) outbreak in Ontario using the incidence decay and exponential adjustment (IDEA) model

Greer, Amy L.; Spence, Kelsey; Gardner, Emma

We used a previously described, 2-parameter model to evaluate the early epidemic dynamics of PEDV on Ontario swine farms. The "incidence decay and exponential adjustment" (IDEA) model was first described in 2013 and has since been used to examine the epidemic dynamics of the ongoing Ebola outbreak in West Africa and Middle East Respiratory Syndrome Coronavirus (MERS-CoV) in Saudi Arabia [20] [21] [22] . The first model parameter is an exponential growth term. Exponential growth is a function of the basic reproductive number (R 0 ) of the pathogen, which is defined as the (average) number of successful transmissions per infected individual within an entirely susceptible population, and the average serial interval [24] . In the case of PEDV and the farm-level nature of the OMAFRA dataset, we modify the definition slightly to consider the farm as the unit of observation. In this case, we define R 0 as the (average) number of successful farm-to-farm transmissions per infected farm within an entirely susceptible population of swine farms in Ontario. The average serial interval is defined as the time between laboratory confirmation in an index case/farm and laboratory confirmation in a secondary case/farm. The model also includes simultaneous exponential decay in the form of a control parameter (d). This parameter describes the slowing of the disease transmission process that is expected to occur as a result of interventions such as behavior change, increased immunity within the population or enhanced biosecurity. The model is not mechanistic and therefore is unable to identify between different prevention and control measures but rather, as a descriptive tool, permits us to identify the time point at which the epidemic begins to slow.