Results
Out of 30 samples screened for ORFV, 20 were positive giving a rate of
67%. Animals <2 years had higher prevalence (76.5%) of ORF
compared to older ages 2-3 years and >3 years with 50% and
66.7%, respectively. More females than males (80% versus 64%) were
infected with ORFV despite having five-fold more of the later in our
study. Logistic analysis showed rate of occurrence of Orf did not differ
significantly with animals’ ages and sexes (P>0.05). Goat
with 71.4% prevalence demonstrated higher susceptibility to ORFV
compared to sheep (55.6%). Hundred percent attack rate was observed
among the Uda and WAD breeds followed by a decreasing susceptibility in
Niger (71.4%), Kano Brown (66.7%), Yankasa (25%) and Balami (0%)
breeds(Table 1). A differential distribution of Orf was observed in
relation to sources of livestock sampled from the market. Animals
sourced from Niger State (100%) were more infected followed by Kano
State (80%), Yobe State (75%), Niger republic (71%) and Katsina State
(40%). Though, current study did not observe any significant influence
of source on the occurrence of ORFV infection (p>0.05).
The ML phylogenetic treeshowed a distinct diversification of the
isolates analyzed into 2 lineages, African and Asian lineage, with
majority of the Nigerian strains clusteringwithin the African lineage, a
few Nigerian stains also clustered within the Asian lineage along with
some South African strains and reference strains from China (Fig. 2).
Time scaled MCC phylogeny of the ORFV sequences indicates that the MRCA
of the African strains was around 2002. The African Viruses clustered
into 2 major clades with both clades containing West African (Nigerian)
and South African viruses (Fig. 3a). There was also a strain from
Botswana which clustered very closely with Nigerian isolates and shared
same TMRCA of around 2014 highest posterior density interval (HPD
2012-2017), while the TMRCA for the entire African sequences was around
2004 (HPD 2000-2006).. The evolutionary rate of the African ORFV B2L
genes was 7.45 × 10-4, 95% HPD (3.46 ×
10-4 to 1.17 × 10-3) substitutions/
site/year. The demographic history of the African ORFV B2L genes showed
a constant viral population expansion from 2005 to 2016, with a slight
rise in population expansion between 2018 to 2020 (Fig. 3b). Amino acid
alignment of the complete B2L protein coding sequence also showed some
non-synonymous mutations (Suppl. Figure 1) which are summarized in Table
2. The sampling dates and Genbank accession numbers of the study
sequences can be assessed in Suppl. Table 1.