Introduction
African swine fever (ASF) is a highly fatal haemorrhagic disease of
domestic pigs caused by the ASF virus (ASFV), resulting in severe
morbidity and mortality (Odemuyiwa et al., 2000; Penrith & Vosloo,
2009). ASFV is a unique double-stranded DNA virus, and the sole member
of the genus Asfivirus and family Asfarviridae (Alonso et
al., 2018). ASF was first reported in Kenya in 1921, after which the
disease was reported in other parts of Africa (Brown et al., 2017;
Montgomery, 1921; Mulumba‐Mfumu et al., 2019). The disease was initially
restricted to Africa, but now has spread to Europe, Asia, and Oceania
(Dellicour et al., 2020; Dixon et al., 2020). ASFV is transmitted by
direct and indirect contact between pigs in the ASFV domestic cycle,
soft ticks of the genus: Ornithodoros spp in the tick
transmission cycle, and via warthogs in the sylvatic cycle (Costard et
al., 2013; Jori et al., 2013). Currently, genetic typing of ASF viruses
is carried out through partial nucleotide sequencing of the p72capsid protein of the B646L gene and full-length of envelope
protein p54 of the E183L gene (Bastos et al., 2003; Lubisi
et al., 2005). Further differentiation of ASFV genotypes can be done
using the central variable region (CVR) of the B602L gene
(Gallardo et al., 2009). Based on the sequence analysis of thep72 capsid protein of the B646L gene, 24 genotypes of ASFV
have been identified (Blome et al., 2020). However, multiple genotypes
in a geographical region are associated with tick and sylvatic
transmission cycles of ASF mostly reported in East and Southern Africa
regions (Costard et al., 2013; Quembo et al., 2018). Molecular
characterization of B646L, E183L, and B602L of the ASFV
can be used for investigating the source and extent of outbreaks and
possible genetic diversity of circulating viral strains (Lubisi et al.,
2005; Malogolovkin, Burmakina, Titov, et al., 2015). ASFV is a
relatively stable DNA virus with low mutation rates and coupled with
lack of closely related viruses which reduces the risk of high genetic
variation (Dixon et al., 2020; Gaudreault et al., 2020). However,
certain regions of the virus such as the CVR are prone to mutations
leading to the creation of new ASFV variants (Luka et al., 2016). These
new variants might have implications for tracing and tracking the rate
of ASF infections across time and space. In the absence of a vaccine or
any other therapy, it is very important to mitigate the spread of the
disease by tracking sources of outbreaks and instituting swift control
measures particularly in endemic countries. Following the introduction
of ASF into Nigeria in 1997, the disease is now endemic in the country
with frequent reports of outbreaks in pig producing areas of the country
(Odemuyiwa et al., 2000; Owolodun et al., 2010). Previous studies have
identified genotype I as the only ASFV circulating in Nigeria and other
West African countries before ASFV genotype II was reported in Nigeria
recently (Adedeji et al., 2021; Couacy‐Hymann et al., 2019; Tizhe et
al., 2020). However, different variants of the ASFV genotype I based on
the CVR within the B602L gene have been recovered from outbreaks
due to possible mutation in that genetic region of the virus. Between
years 1997 and 2015, eleven (11) ASFV genotype I variants have been
recovered from outbreaks in Nigeria (Couacy‐Hymann et al., 2019; Luka et
al., 2016; Owolodun et al., 2010). The ASF disease situation in the
country has been complicated by poor husbandry system, unrestricted live
pig movement, and a weak veterinary system. Despite several studies, the
epidemiology and probable drivers of the disease in Nigeria are poorly
understood (Awosanya et al., 2015; Fasina et al., 2012). Between 2016
and 2018, there was an upsurge in reported cases of ASF in Nigeria
affecting eight States of the country. Epidemiological investigations
revealed limited understanding of how ASFV spread into and within
farming communities. Therefore, this study carried out the genetic
characterization of circulating ASFV to shed light on possible insights
on the course and characteristics of these outbreaks.