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.