*Article* **Impact of a Live Attenuated Classical Swine Fever Virus Introduced to Jeju Island, a CSF-Free Area**

**SeEun Choe 1, Jae-Hoon Kim 2, Ki-Sun Kim 1, Sok Song 1, Wan-Choul Kang 3, Hyeon-Ju Kim 3, Gyu-Nam Park 1, Ra Mi Cha 1, In-Soo Cho 1, Bang-Hun Hyun 1, Bong-Kyun Park 1,4 and Dong-Jun An 1,\***


Received: 13 October 2019; Accepted: 14 November 2019; Published: 20 November 2019

**Abstract:** Here, we examine the effects of LOM(Low virulence of Miyagi) strains isolated from pigs (Jeju LOM strains) of Jeju Island, where vaccination with a live attenuated classical swine fever (CSF) LOM vaccine strain was stopped. The circulation of the Jeju LOM strains was mainly caused by a commercial swine erysipelas (*Erysipelothrix rhusiopathiae*) vaccine mixed with a LOM vaccine strain, which was inoculated into pregnant sows of 20 pig farms in 2014. The Jeju LOM strain was transmitted to 91 pig farms from 2015 to 2018. A histopathogenic investigation was performed for 25 farms among 111 farms affected by the Jeju LOM strain and revealed pigs infected with the Jeju LOM strain in combination with other pathogens, which resulted in the abortion of fetuses and mortality in suckling piglets. Histopathologic examination and immunohistochemical staining identified CSF-like lesions. Our results also confirm that the main transmission factor for the Jeju LOM strain circulation is the vehicles entering/exiting farms and slaughterhouses. Probability estimates of transmission between cohabiting pigs and pigs harboring the Jeju LOM strain JJ16LOM-YJK08 revealed that immunocompromised pigs showed horizontal transmission (r = 1.22). In a full genome analysis, we did not find genetic mutation on the site that is known to relate to pathogenicity between Jeju LOM strains (2014–2018) and the commercial LOM vaccine strain. However, we were not able to determine whether the Jeju LOM strain (2014–2018) is genetically the same virus as those of the commercial LOM vaccine due to several genetic variations in structure and non-structure proteins. Therefore, further studies are needed to evaluate the pathogenicity of the Jeju LOM strain in pregnant sow and SPF pigs and to clarify the characteristics of Jeju LOM and commercial LOM vaccine strains.

**Keywords:** classical swine fever virus (CSFV); LOM vaccine strain; Jeju LOM strain; omega value; transmission
