**1. Introduction**

Classical swine fever (CSF) is caused by infection with the CSF virus (CSFV), which belongs to the genus *Pestivirus*, family *Flaviviridae*. CSF is described by the World Organisation for Animal Health as a highly contagious febrile disease with potential for high mortality that causes enormous economic loss in the pig industry worldwide [1]. CSFV is a positive-sense, single-stranded RNA virus with a genome of approximately 12.3 kb, comprising one large open reading frame that encodes a polyprotein and flanked by 5'-untranslated region (5'-UTR) and 3'-untranslated region [2]. During virus replication, the polyprotein is processed by cellular and viral proteases into four structural and nine nonstructural proteins [2]. Outbreaks of CSF have been reported over the past decade in Asia (Bhutan, Cambodia, China, India, Indonesia, the Republic of Korea, Lao PDR, Mongolia, Myanmar, Nepal, the Philippines, Thailand, Timor-Leste, and Vietnam), Europe (Latvia, Lithuania, the Russian Federation, Serbia, and

Ukraine), Africa (Madagascar), the Caribbean (the Dominican Republic, Guatemala, and Haiti), and Latin America (Bolivia, Colombia, Ecuador, and Peru) [3]. Based on the amino acid sequence of the 5'-UTR and E2, which is one of the structural region of the protein, CSFVs are classified into three genotypes (1, 2, and 3) and several subgenotypes (1.1–1.4, 2.1–2.3, and 3.1–3.4) [4,5]. The virulence of CSFV is categorized via a clinical score into highly virulent, moderately virulent, low virulent, and avirulent [6,7]. Although the CSFV genotype 2.1b isolated from the Republic of Korea was highly virulent, the same genotype isolated in Mongolia was moderately virulent [8,9]. Moreover, the recently classified CSFV genotype 2.1d from China was moderately virulent compared to different variants and antigenicity from field strains identified in China in the past [10].

No notifications of CSF were reported in Japan since 1992, and the country had an 11-year stretch of CSF-free status defined by the OIE Terrestrial Animal Health Code since 2007. However, CSF reemerged in Japan in September 2018 in Gifu Prefecture, which is located in the central part of the main island of Japan. Phylogenetic analysis revealed that the CSFV strain isolated in Japan in 2018 showed the highest identity in the complete E2 gene sequence with Chinese strains isolated between 2011 and 2015 and in the partial 5'-UTR sequence with strains isolated in China and Mongolia in 2014 and 2015 [11].

By the end of August 2019, a total of 39 CSFV outbreaks on pig farms in four prefectures and 1,071 cases in wild boar in seven prefectures have been reported [12]. Despite the implementation of intensive responses, including movement bans of domestic pigs, surveillance, and oral immunization of wild boar, new notifications of CSF cases in both wild boar and domestic pigs were being reported continually [13]. This might indicate that the pathogenic viruses were widely prevalent and persisted in wildlife around the affected area. As the Eurasian wild boar is also susceptible to CSFV, the circulation and persistence of CSFV among food animals and wildlife makes it difficult to carry out effective control measures for eradicating it in affected areas. Due to contact with infected animals and feeds contaminated with contagious pathogens in garbage dumped on the human sphere, naïve wild boar populations are often infected with CSFV [8,14–24]. Before the 1990s, CSF cases in wild boar were rare concerns as infection was detected rapidly due to the high virulence of circulating strains. However, disease detection appears delayed in the current epidemic due to infection with more moderately virulent strains [25]. As a consequence, there have been serious outbreaks of CSF in the wild boar population in Germany. During an outbreak of CSF in Germany from 1993 to 1998, an epidemiological field investigation confirmed that 59% of the primary cases in domestic pigs could be attributed to either direct or indirect contact with infected wild boar [17]. Virus characteristics and population size can both be considered critical factors for the persistence of CSFV, especially in wild boar populations [25]. It has been suggested that CSFV would be self-limiting within one year in populations of 2000 wild boar, whereas it will persist and become endemic in a larger population [26]. In addition, the population density of wild boar also has been suggested as being a potential factor for the persistence of CSF because more frequent turnover occurs in dense populations, which provides faster renewal of susceptible piglets that increases the chance that the virus will persist in the population [25]. Once the contagious viruses are transmitted to wildlife, specific control measures for wild boar will be needed to eradicate CSF in the affected area and to contain it more effectively.

The present study conducted a spatio-temporal analysis to obtain epidemiological information on current epidemics of CSF in Japan. Based on the official CSF reports on domestic pig farms and wild boar, notified in Gifu Prefecture from September 2018 to June 2019, we assessed the direction of the spread of the disease and identified areas with high densities of notifications. In addition, to identify spatio-temporal aggregation of notifications and to characterize land cover vegetation in areas of disease aggregation, a clustering analysis was conducted, and obtained clusters were then overlapped with quality habitat map. The obtained information can be used to develop more effective disease control measures for application in both domestic pigs and wild boar.
