3.5. Reports of wild suid presence in surrounding pig farms
Most pig farms were concentrated adjacent to the western (78%) and
south-western (13%) sections of the fence. While some farms were also
located in proximity of the south-eastern fence section (9%), the
eastern section had no adjacent farms. The highest number of domestic
pigs (n=631) was located close to the western section of the fence while
a considerably smaller number (n=172) was found on the south-western
section of the fence (Figure 1).
Among the 254 pig farmers, eight reported bushpigs near their households
(20km radius) while three reported to have seen warthogs. None of them
reported any direct interaction between those wild suid species and
their domestic pigs. Bushpig sightings were reported as far as 19 km
from the fence while for warthogs the furthest was 8 km (Fig 2).
Pig farmers who reported bushpigs near their households were located
adjacent to the western, south-western, and south-eastern section of the
fence, whereas those who had seen warthogs tended to be more evenly
distributed adjacent the western and south-eastern section of the fence.
(Fig 2). None of the farmers reported disease or disease symptoms
compatible with ASF outbreaks.
Discussion
Warthogs and bushpigs are common in the study area. Annual transects
counts in NGR revealed a warthog density of 3-5
individuals/km2, with a total population of 400-500
warthogs in the area. These estimates fall on the lower end of
population densities found by Cumming
(1975) who reported that in Africa warthog densities range from 1-15
individuals/km2. This could be partly explained by
their preference for open savannah
(Deribe, Bekele, & Balakrishnan, 2008),
instead of the bush thicket that is common at NGR. The camera trap
survey indicated that NGR has a significant bushpig population, which
may be similar to warthogs in numbers. This approach was useful in
detecting the abundant bushpig population which had not been recorded
using the diurnal line transects. Most of the bushpig recorded on camera
traps (0.515 animals/ camera day) were found in habitats close to water,
consistent with the observations of different authors who noted that
bushpigs are water-dependent (J. Kingdon,
2014; Seydack, 2017). Warthogs (0.536
animals/ camera day) on the other hand, were also found in sandy
environments. Therefore, NGR provides a good habitat for both wild
suids.
Movements of both wild and domestic suids can facilitate direct or
indirect interactions with domestic pigs as well as ASF transmission at
the interface of a protected area (Arias,
Jurado, Gallardo, Fernández‐Pinero, & Sánchez‐Vizcaíno, 2018).
Both wild suid species regularly crossed the game fence, moving into
adjacent farmland. Standard wire or wire-mesh fencing is not very
efficient to contain suiform species and warthogs are often reported to
escape from other protected areas by digging under fences
(Ferran Jori et al., 2011;
Swanepoel, Schulze, & Cumming, 2016).
Therefore, in cases where it is important to contain wild suids in
conservation areas, we recommend to consider using other kinds of
barriers such as wild boar proof fences
(EFSA, 2014;
Satheeshkumar, Manjusha, Pillai, & Kumar,
2012).
In our study area, crossings were more common during the dry season for
both bushpigs (6.35 bushpigs/day) and warthogs (15.4 warthogs/day). The
high number of fence crossings represents a challenge in the management
of diseases in domestic pigs. This is because, in the farming area (as
in many African rural areas) a considerable population of domestic pigs
is left free-ranging, which increases the chances of direct and indirect
interactions between wild and domestic suids and disease transmission
(F Jori, Payne, Stahl, Nava, & Rossi,
2018). This risk is exacerbated by the fact that most crossings were
observed on the western side of the reserve where the highest number of
pigs farms are located. A possible explanation for more crossings on the
western section could be that this terrain is mountainous and it has a
moister thicket vegetation, providing a more suitable habitat for a shy
species such as the bushpig (Flamand,
Horak, & Boomker, 1991; Ferran Jori &
Bastos, 2009). This habitat also provides the suids with fruit and
bulbs (Nyafu, 2009) particularly during
the dry season when resources in the game reserve are scarce. The farms
therefore have the potential for high levels of direct or indirect
wild-domestic and domestic–domestic pig interactions, facilitating the
transmission of shared pathogens, such as ASF. Despite limited
veterinary research on the pathogenic burden of wild African pigs
compared to the Eurasian wild boar (Sus scrofa ), warthogs and
bushpigs are known to be carriers of ASF as well as other pathogens such
as trichinella, bovine tuberculosis and several porcine viruses that
could be transmitted to domestic pigs sharing the same environment
(F Jori et al., 2018).
Similar to other studies, some farmers (n =11) reported to have
seen at least one of the wild suid species outside the reserve
(Kukielka et al., 2016), but none of the
suids was observed interacting with domestic pigs. While natural
hybridization between domestic pigs and warthogs has not been reported,
there are a number of reports of cross-breeding between bushpigs and
domestic pigs (Ferran Jori & Bastos,
2009; Jonathan Kingdon, 2015). We assume
than while interactions could occur, they are not necessarily observed
due to the elusive and nocturnal behaviour of bushpigs
(Payne et al., 2018). Incursions of
bushpigs in farming areas are likely to occur at night and warthogs
might have human avoidance behaviour
(Kassilly, Tsingalia, & Gossow, 2008).
Therefore, questionnaires alone are not the best method for conclusions
on potential nocturnal interactions and other methodologies such as
radio tracking and setting up camera traps near pigsties should be
considered.
In many African rural areas, a considerable number of domestic pigs are
free-ranging (Nantima et al., 2015;
M.-L. Penrith, Vosloo, Jori, & Bastos,
2013; Quembo et al., 2016), increasing
the chances of direct and indirect interactions between wild and
domestic suids interactions and potential pathogen transmission
(F Jori et al., 2018;
M.-L. Penrith et al., 2013). Even though
the sharing of the same habitat and resources represents an ideal
situation for the transmission of pathogens between wild suids and
domestic pigs (Barth et al., 2018), in our
study we could not find any indication suggesting that NGR currently
harbours a sylvatic cycle that would allow wild suid species to act as
carriers of ASF. We did not find any evidence of Ornithodorostick infestation among our surveyed burrows (n =35), suggesting
that the tick reservoir is currently unlikely to be present in NGR and a
permanent source of ASF virus is not maintained in the reserve despite
the presence of warthogs and bushpigs. These findings are similar to
similar studies in Mkuze Game Reserve (approximately 100 km south of
NGR) who inspected 98 warthog burrows and collected ticks
(Arnot, du Toit, & Bastos, 2009).
The apparent absence of a sylvatic cycle in NGR is consistent with the
fact that none of the farmers interviewed was concerned with severe
disease outbreaks compatible with regular ASF. Given the importance of
the sylvatic cycle in Mozambique and the observations of some ASF
outbreaks on the Mozambican side close to NGR
(M.-L. Penrith et al., 2013), further
research should be undertaken on a larger number of burrows in order to
confirm this suspicion. Another aspect that should be explored further
is a survey of ASF antibodies in wild pigs and domestic pig populations
living at the interface of the NGR. Indeed, an absence of circulating
antibodies in these populations would be a good indicator that ASF virus
is not being maintained in this study area.
The combination of an endemic ASF status in Mozambique (northern
boundary of NGR), the presence of significant numbers of warthog and
bushpig within NGR, the regular movements of both species between NGR
and the surrounding farmland, suggest that strong veterinary
surveillance and management remains necessary to identify potential
infectious disease transmission to the local pig population, and which
can act as a sentinel population.