Discussion
In this study, high prevalence of Orf occurred among animals of all ages which reinforces the endemicity which may jeopardize the animal productivity in the area. The greater incidence of ORF in ages <2years indicates increased susceptibility due to lower immunity in younger ages which has been corroborated in other studies18,19 . The absence of statistical significance despite higher prevalence of ORF among female animals support reports from the eastern Sudan which posited that sex is not a significant predictor of Orf19 . Higher rate of susceptibility in goats compared to ovine species has been reported20 . The naturally aggressive behavior of the former compared to the latter predisposes them to injury, thereby increases the susceptibility to ORFV transmitted via direct contact. Most small livestock holders in Nigeria do not practice dehorning as observed in this study and this may increase the risk of deep wounds or injuries which serve as a predisposing factor for virus penetration via the skin wounds. Current study observed 100% morbidity among the Uda and WAD breeds, similar to previous observation in Ibadan (Onoja et al., unpublished data) and Jos21 respectively. These findings suggest the influence of intrinsic genetic factors in certain breed susceptibility to Orf which needs to be fully investigated.
Over 70% morbidity rates were observed among animals imported from Niger republic and the ones sourced from Kano, Niger and Yobe States, raising the concern of trans-boundary and national sporadic spread to susceptible new herds which could be the starting point of con­tagious ecthyma outbreaks often associated with production loss in livestock and reduction in the market value22 . The high morbidity observed in this study can be traceable to inadequate hygienic20 and quarantine procedures as most of the sick animals are not isolated before mixing up with previously apparently healthy sheep and goats. Climatic change over the years has made more ruminants susceptible to this hitherto low pathogenic but highly contagious viral disease. In addition, Abuja been a commercial centre and the nation’s capital receives a high influx of both human and livestock. With a burgeoning population, consumption of livestock is on the increase consequently increasing importation of sheep and goat from other African countries. It is noteworthy that all samples were collected from live markets where sheep and goats are ready to be sold. In the process of handling and bargain between the potential buyer and seller, transmission and infection with contagious ecthyma may occur. The highly contagious nature of contagious ecthyma affects production. Animals refuse or reduce intake of food which leads to poor market value consequently leading to low protein availability and loss to the farmers.
The phylogenetic analysis revealed that majority of the isolated Nigerian strains clustered within the African lineage, with a few stains isolated from goats sold within the FCT, clustering within the Asian lineage along with South African strains and reference strains isolated from China (Figure 2). This observation suggests that the goats sampled from Nigeria infected with the Asian Lineage, confirms Orf virus as a transboudary animal disease. This confirms that some of the Nigeria goats were breed outside the country in neighboring Countries such as Niger republic or Chad and might have had contact with other ruminants from North Africa or Arabia, where the Asian virus might have been introduced. Another theory for this observation is that the Asian virus might have been circulating within the African continent for decades undetected because of poor surveillance and weak diagnostic infrastructure. The MCC tree of African Orf B2L sequences was also clustered into a similar topology with majority of the African sequences falling in to the African Lineage and FCT, South African and Egyptian strains falling within the Asian lineage (Figure 3a). This clustering of sequences further buttresses our earlier observation that the sheep and goats may have been infected with the same strain during transit. Since majority of sheep and goats are from neighboring African countries especially Chad and Niger republic, sharing borders with Northern Nigeria, it is not unlikely that they came from a common source and were transported together across the border to Nigeria. Moreover, the stress of trans-border transportation makes the animals more susceptible to contagious ecthyma virus. Unchecked and indiscriminate trans-border activity of both humans and animals is a very important factor in the dissemination of ORFV. The time to most recent common ancestor (TMRCA) for the Nigeria African lineage was around 2014, highest posterior density interval (HPD 2012-2017), while the TMRCA for the entire African sequences was around 2004 (HPD 2000-2006). This is quite recent and might not actually represent the true picture of when the virus was introduced into circulation because of the paucity of sequence information and regency of the data presented. However it shows the gap in knowledge posed by insufficient molecular data on this virus in Africa. In addition the calculated evolutionary rate of the African Orf virus B2L genes was 7.45 × 10-4, 95% HPD (3.46 × 10-4 to 1.17 × 10-3) substitutions/ site/year. This is higher than the calculated evolutionary mean rate of 1.32 × 10-6 derived for whole Orf gene sequences from a recent study23 . This observation might be due to the small genome size analyzed, as well as the limited number of sequences, but reelects the relatively slower evolutionary rate for DNA viruses in comparism to RNA viruses. The viral demographic history portrays a steady population demography throughout the years past, except for a slight increase towards the year 2020, around 2016 (Fig. 3b). This observation should be taken with caution as it may not represent the true picture because of the lack of sequence data from Africa before the year 2000, however the rise in viral population, towards the more recent years shows an increase in detectable cases reflected by more sequence information being made available in various databases.
Table 2 shows a summary of major amino acid mutations observed among our B2L ORFV sequences in relation to the prototype vaccine strain (Accession number AY278208.1). The mutations observed are concomitant with that of a previously isolated strain reported in Sokoto State. The phenotypic properties of these mutations have not been well described, but some of them may affect the pathogenicity of the isolates24 . Limitations of this study are paucity of African ORFV genomic data, and the small sample size.
In conclusion, we report the genetic diversity and molecular evolution of ORFV in Africa based on B2L sequences. Further, we highlighted the epidemiology of ORFV among goats and sheep sold in Northern Nigeria. The impact of trans-border movement of farm animals such as sheep and goats on the transmission pattern of trans-boundary disease such as ORFV, and have illuminated the information gap particularly genetic information of this virus in Africa. We hereby recommend a regional approach to molecular surveillance of ORFV and other zoonotic trans-boundary animal diseases to help in prevention and control effort.