Release assessment
The probability of release (Pre ) means that at least one ASFV infected animal is introduced into the importing country. In our case, it reflects the probability of importing an ASFV infected but undetected pig during the HRP from the country of origin into South Korea at a particular month. Pre was estimated with four conditional probabilities: (P1 ) the probability of ASFV infection in the country of origin; (P2 ) the probability of selecting an ASFV infected pig from the country of origin to be exported to South Korea before detection of ASFV infection in the country of origin (i.e. during the HRP); (P3 ) the probability that an ASFV infected pig survives the ASFV infection; (P4 ) the probability that the ASFV infected pig survives the transportation from the country of origin to the country of destination.
The probability of ASFV infection in the country of origin (P1 ) was estimated using a beta distribution (α1, α2), where the α1 was the number of months with at least one undetected ASF outbreak in the country and α2 was the number of months considered for the analysis (OIE, 2019).
The probability that an ASFV infected pig is selected from the country of origin in a specific month m (P2) was also modelled using a beta distribution. The shape of this distribution was determined by the number of expected ASFV infected pigs before ASF detection in the country of origin (NI ) and the total number of pigs in the country of origin (No ). To estimate NI , four independent parameters were multiplied: (Ou ) Undetected outbreaks during the HRP for each country/continent which were parameterized using Pert (min, most likely, max) (OIE, 2019); (To ) Average size of pig farms in the country, which was calculated with the total number of pig farms and total number of pigs in the country of origin using a normal distribution expressed as an average number (μ ) and standard deviation (σ ) (OIE, 2018b); (Hp ) ASF intraherd prevalence, which was calculated using the minimum, maximum and mean of the proportions of cases vs susceptible animals obtained from the OIE (2019) and was modelled by a Pert distribution for each country. The probability that an ASFV infected pig survives the ASFV infection (P3 ) and the transportation (P4 ) were modelled using a Pert distribution based on Spickler and Roth (2006) and Murray and Johnson (1998), respectively.