Discussion
This is the first comprehensive study on the possible role of LPMs in
spread of ASFV in Nigeria. Field reports revealed that ASF outbreaks
occur throughout the year with many unanswered questions on how the ASFV
persists and continue to spread in the pig populations in the country.
Findings from this study suggest that LPMs in Nigeria may play a key
role in the unabated ASF outbreaks in the country. During the duration
of this study which was conducted over a period of 13 months involving
34 visits. ASFV was detected by PCR in samples collected during at least
one visit in the 4 selected LPMs. In addition, at one sample collected
during 28 out of 34 visits was positive and ASFV was also detected in 12
out 13 months that samples were collected within the study period (Table
3). Although, visits to the markets were disrupted by COVID-19 lockdown
restrictions in Nigeria between March 2020 and September 2020, however,
samples were collected for the first 11 calendar months. The total
number of samples collected was 613, with ASFV detected by PCR in
10.72% (66) of samples collected. In addition, ASFV was detected in
3.13% -23.81% of pig samples collected in all selected live pigs
markets. Finding from this study indicates that ASF-infected pigs are
sold at all the selected pig markets in Nigeria. This is worrisome
because pigs from these markets are supplied to several cities and towns
in Nigeria (Fig 2) both for slaughtering and restocking. Previously,
ASFV has been detected in Katsit pig market, however, this is the first
report of detection of ASFV in Dawaki, Numan, and Pandam LPMs (Luka et
al., 2016; Owolodun et al., 2010; Owolodun, Yakubu, et al., 2010b).
Phylogenetic analysis of characterized sequences B646L andE183L genes selected positive samples collected from the 4 LPMs
revealed ASFV genotype I and Ia respectively (Fig 4 & 5). While,
analysis of the sequence CD2v gene revealed Nigerian ASFV samples
clustered in serogroup 4 (Fig 6). However, analysis of CVR sequences
showed 8 TRS variants were recovered from the markets within the study
period (Table 4). Six variants (Tet-10, Tet-13, Tet-18, Tet-26, Tet-33,
Tet-41) are being reported for the first time in this study. These
findings suggest that live pig markets are the most likely locations to
recover the current circulating ASFV TRS variants in Nigeria. In
addition, adequate attention and regulation of activities at live pig
markets are important in the control of the disease in the country.
Previously, 5 TRS variants (Tet-15, Tet-17a, Tet-20a, 20b, Tet-48) were
recovered at Katsit market, the same variants were detected in outbreak
cases in other parts of the country. Particularly, Tet-20b which was
widely circulating in Nigeria between 2009 and 2015 (Luka et al., 2016;
Owolodun, et al., 2010). In addition to TRS variants recovered in this
study, 21 variants have now been are identified in Nigeria, 13 variants
of which have been recovered from LPMs in the country. The live animal
market trade system plays a crucial role in the introduction of
pathogens into new areas particularly long-distance disease spread
(Martin et al., 2011; Zhou et al., 2015). Likewise, previous studies
have described the important role of the live bird market in the
epidemiology of avian influenza in Nigeria, but no report on the role of
LPMs in the epidemiology of ASF in the country (Coker et al., 2014;
Sulaiman et al., 2021). This study has demonstrated that ASFV is
circulating in LPMs in Nigeria. These markets are possibly hotspots for
the long-distance spread of the virus via the movement of ASF-infected
pigs to several locations in the country from LPMs. Though most of the
pigs traded in these markets are for slaughter, however, farmers also
buy pigs at these markets for restocking on their farms. In addition,
unsold pigs are returned to owners’ farms after the market day because
of the unavailability of withholding facilities at these markets. Such
practice further complicates the situations of ASF in Nigeria. It is
important to note all the markets where this study was carried out,
there is limited government regulation of market operators’ activities.
The apparent absence of antemortem inspection at market gates, lack of
holding pens, close contact of pigs from different locations, and
selling of pork within market premises enhances the ease of ASFV spread
between pigs at market premises. Other poor biosecurity practices
observed were free access to market sites by free-roaming pigs and dogs
and unrestricted access of slaughter slab operators. Field reports in
Nigeria and previous studies in Zambia have reported outbreaks following
the introduction of pigs bought at LPMs (Siamupa et al., 2018; Simulundu
et al., 2018). This study also demonstrated the widespread free movement
of pigs from surrounding communities to the four live pig markets and
the movement of pigs from markets to various destinations in Nigeria
(Fig 1 and Fig 2). In total, pigs are sourced from over 70
villages/towns located in 7 States of Nigeria and villages in Cameroon
for trading at the market sites (Table 2, Fig 2), while pigs traded at
the markets are supplied to 42 towns and cities in 13 States of Nigeria
(Table 2, Fig 2). It is interesting to note that all these pig movements
are done without movement permits or health certificates issued by the
appropriate government agencies. In other countries like Zambia,
Indonesia, and China, movement permits or health certificates are
required before the movement of pigs between farms, live pig markets,
districts, and regions (Gao et al., 2021; Leslie et al., 2016; Siamupa
et al., 2018). Movement permits and certificates are important tools to
track pig movements and reduce the risk of disease spread, but in
Nigeria these movement permits/health certificates were not in use in
all the live pig markets selected for this study. Results from this
study estimate that nearly 500,000-700,000 pigs transit through these
markets yearly, without appropriate restrictions or regulation to
mitigate the spread of important transboundary diseases such as ASF. The
pigs are transported to live pig markets using motorcycles, cars, buses,
trucks, and on foot. The vehicles visit multiple farms, villages or
towns, and livestock markets to collect and deliver the pigs to the
markets without proper sanitary measures to prevent disease
transmission. Also after delivery, the vehicles were washed at market
premises leading to possible environmental contamination with the virus.