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.