Introduction
The aetiologic agent for African swine fever (ASF) is the African swine fever virus (ASFV), a double-stranded DNA virus with a diameter of 70–100 nm and the sole member of the family Asfaviridae and the genus Asfivirus (Dixon et al, 2012). According to the first description of ASF, ASFV was transmitted from wild African suids in Kenya to domestic pigs, resulting in 100% mortality (Montgomery, 1921). Currently, the disease is present on the continents of Africa, Europe and Asia (World Organisation for Animal Health, 2019).
The first outbreak of ASF in Vietnam was reported in February 2019 in Hung Yen province located in the northern area of Vietnam, 250 km from the China border (Le et al, 2019). A year after the first outbreak, ASFV was widespread within all 63 provinces in the country, with 4126 ongoing outbreaks reported by March 2020 (World Organisation for Animal Health, 2020). It has been reported that as a result of national disease control efforts, outbreaks were showing signs of slowing down by the end of 2019. However, in some provinces where the ASF epidemic was declared to be over (i.e., when there are no further ASF cases detected after a 30-day period), the ASF re-appeared (The United States Department of Agriculture, 2019).
The ASF clinical forms—peracute, acute, subacute and chronic manifestation of infected pigs—depend on the strain of the virus, route of transmission, dose and the host characteristics (Sánchez-Vizcaíno et al, 2015). The ASFV strains can also be classified as highly virulent, moderately virulent or mildly (low) virulent (Pan & Hess 1984), and there are 24 genotypes of ASFV strains (Quembo et al, 2008). After the onset of the clinical signs of ASFV-infected pigs, the virus is shed at high levels in all secretions and excretions, and surviving pigs show long-term viremia, becoming an important ASFV reservoir in domestic pigs (Sanchez-Vizcaino et al, 2019).
ASFV is an exotic disease in the Southeast Asia region. It is a real threat to the swine industry in developing countries, especially where the majority of pig farmers are smallholders or practice backyard farming with limited biosecurity measures. As the description of histopathological lesions observed in the natural cases of ASF was limited in the literature, this study focused on the description of the histopathological lesions and viral antigen distribution in pigs naturally infected with ASFV to enhance our understanding of the pathological imaging and advance our insight onto the pathogenesis of ASFV during an ongoing outbreak.