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unpublished data) and Jos21 respectively.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.