Estimation of the Basic Reproduction Number of Classical Swine Fever in Wild Boar During the Early Phase of an Outbreak

Abstract

In Japan, classical swine fever re-emerged in September 2018, after an absence of 26 years, and spread among wild boar populations. This study was conducted to understand the transmissibility of classical swine fever virus in wild boar populations, limited by the impossibility of a correct case count because of the nature of wildlife. Wildlife PCR test results between September 2018 and February 2019 were collected from the homepages of Gifu, Aichi, and Mie Prefectures. In descriptive epidemiology, the geographical spread in the three prefectures, and temporal patterns of PCR test positivity in Gifu Prefecture were observed. Using the Susceptible (S ) ‐ Exposed (E ) ‐ Infectious (I ) ‐ Recovery (R ) model, assuming the weekly test positivity follows beta distribution, parameters were estimated in a Markov Chain Monte Carlo simulation, and the basic reproduction number (R₀) was calculated. As a result, the mean infectious period was as long as 100 ‐ 145 days, and R₀ was estimated to be 4.2 ‐ 5.1. In this analysis, however, population dynamics considering births and deaths and population density were not considered. In the future, a more detailed study based on wild boar field data is necessary.