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.