Free-Ranging Pig and Wild Boar Interactions in an Endemic Area of African Swine Fever

Abstract

African swine fever virus (ASFV) is spreading throughout Eurasia and there is no vaccine nor treatment available, so the control is based on the implementation of strict sanitary measures. These measures include depopulation of infected and in-contact animals and export restrictions, which can lead to important economic losses, making currently African swine fever (ASF) the greatest threat to the global swine industry. ASF has been endemic on the island of Sardinia since 1978, the longest persistence of anywhere in Eurasia. In Sardinia, eradication programs have failed, in large part due to the lack of farm professionalism, the high density of wild boar and the presence of non-registered domestic pigs (free-ranging pigs). In order to clarify how the virus is transmitted from domestic to wild swine, we examined the interaction between free-ranging pigs and wild boar in an ASF-endemic area of Sardinia. To this end, a field study was carried out on direct and indirect interactions, using monitoring by camera trapping in different areas and risk points. Critical time windows (CTWs) for the virus to survive in the environment (long window) and remain infectious (short window) were estimated, and based on these, the number of indirect interactions were determined. Free-ranging pigs indirectly interacted often with wild boar (long window = 6.47 interactions/day, short window = 1.31 interactions/day) and these interactions (long window) were mainly at water sources. They also directly interacted 0.37 times per day, especially between 14:00 and 21:00 h, which is much higher than for other interspecific interactions observed in Mediterranean scenarios. The highly frequent interactions at this interspecific interface may help explain the more than four-decade-long endemicity of ASF on the island. Supporting that free-ranging pigs can act as a bridge to transmit ASFV between wild boar and registered domestic pigs. This study contributes broadly to improving the knowledge on the estimation of frequencies of direct and indirect interactions between wild and free-ranging domestic swine. As well as supporting the importance of the analysis of interspecific interactions in shared infectious diseases, especially for guiding disease management. Finally, this work illustrates the power of the camera-trapping method for analyzing interspecific interfaces.

Keywords: African swine fever; camera trapping; critical time window; free-ranging pig; interactions; wild boar.

Figure 1

Example of a camera trapping image showing direct interaction between a free-ranging pig and wild boar.

Figure 2

Example of a camera trapping image showing indirect interaction between a free-ranging pig and wild boar.

Figure 3

Daily activity profile of free-ranging pigs and wild boar, expressed as the percentage of total observations by hour of day and season (spring or summer). The overlap in the profiles for the two subspecies is represented in gray.

Figure 4

Average predicted number of direct interactions between free-ranging pigs and wild boar per animal observed based on statistically significant variables in the best-fit model. Error bars show the 95% confidence interval.

Figure 5

Average predicted number of indirect interactions between free-ranging pigs and wild boar per animal observed assuming a long critical time window of 7 days in spring and 5 days in summer for transmissibility of ASFV, based on statistically significant variables in the best-fit model. Error bars show the 95% confidence interval.