ASFV and PPV3 are highly contagious viral pathogens that can cause devastating diseases in domestic and wild pigs. ASFV, in particular, is associated with high mortality rates in affected pigs, while PPV3 can lead to reproductive disorders and decreased productivity in breeding sows. Coinfection with both viruses can exacerbate the disease burden on pig farms, increasing the swine population's morbidity and mortality rates. The detection of ASFV and PPV3 infections at pig farms in the North Kivu province suggests a complex epidemiological situation with concurrent infections with two important veterinary viral pathogens.
The coinfection of ASFV genotype X and PPV3 could probably be due to the immunosuppressive potential of the PPV3 that exposes pigs to the risk of multiple infections [15]. For this reason, the infection with PPV3 most probably occurred before ASFV infection. This may lead to severe socio-economic losses due to increased mortality, reduced reproductive performance, and decreased growth rates in surviving pigs. Additionally, outbreaks of ASFV can lead to trade restrictions on pork products, further impacting the economic viability of pig farming operations in DRC.
The proportion of coinfections indicates that 25% of the pigs that tested positive for PPV3 were also positive for ASFV. These results corroborate with a previous study in South Kivu province, DRC [9] and China [16] where ASFV was found in pigs infected with PPV .This is further supported by reports of a survey on the Namibian warthog ecosystem showing a concurrent infection between ASFV and PPV type 1 (PPV1) [17]. High infection rates of ASFV and PPV3 were detected from Misisi and Goma territories, and this was mainly due to the increased displacement of pig breeders and their belongings from rural areas to towns as people fled heightened rebel insurgency in rural villages. The viruses could quickly move from rural areas and beyond due to the transboundary nature of ASFV and PPV3 infections. This scenario may necessitate stringent biosecurity measures, quarantine protocols, and targeted control strategies to contain the spread of these viruses and prevent further transmission within the swine population.
The proportion of ASFV-PCR positivity rates obtained from this study is lower than the previous study in the South Kivu province of DRC, where 22.8% of apparently healthy pigs were positive [8]. This result agreed with previous findings [18], where the ASFV genome was detected in asymptomatic Nigerian indigenous pigs. In contrast, the study conducted in Uganda did not detect ASFV DNA despite the high apparent disease incidence [19]. However, only 13.6% of PPV3 (originally named porcine hokovirus) was detected, confirming the observation by a recent study conducted in South Kivu province, where 17.5% of PPV3 strain was detected in pigs [8]. Equally, previous studies have shown variations of PPV3 positivity in different countries and regions. This could be attributed to different sampling criteria, health status, and sample size, directly influencing research outcomes [20] [21]. For instance, this study involved analysis of pig farms without symptoms related to reproductive disorders, while the previous study conducted in South Africa by Afolabi et al.. [21] targeted only confirmed porcine circovirus type 2 (PCV2) infected pigs.
Logistic regression analysis revealed that meat consumption from dead pigs, breeding pigs from the same farm, and not testing pigs before buying and mixing pigs of different ages in the same pen were the most prominent risk factors associated with ASFV infection in North Kivu province. Mixing pigs of different ages within the same pen as a factor associated with ASFV transmission may be likely attributed to direct pig-to-pig contact, especially on confined farms with poor biosecurity measures. Similar results were reported in South Kivu province (DRC)[8] in Sardina [22] and in Madagascar [23], where pens with mixed pigs of different species and ages were associated with ASFV positivity. Moreover, feeding pigs with kitchen leftovers is common in most pig-keeping households. This feed may be mixed with meat from ASFV-infected pigs. This increases the risk of consuming ASFV-contaminated products. These results corroborate with studies conducted in DRC [8], Uganda and Kenya [24], which reported swill feeding as a potential risk factor for ASF transmission. However, our finding contrasts with a study conducted in Malawi, where feeding kitchen leftovers was not a possible risk factor in ASFV transmission [25]. Additionally, free range system was a potential risk factor for both ASFV and PPV3 transmission. A free-range system, practised in several parts of North Kivu, allows pigs to move and scavenge around the farms. Hence, pigs in this system are more likely to be in contact with other infected domestic and wild pigs and possible pig products that may facilitate the spread of ASFV and PPV3.
Genomic comparison of North Kivu field isolates, together with other available partial sequences of ASFV genotype X and PPV3 available on GenBank, showed a high level of sequence conformity, and this is in agreement with previous study by Sliz et al.. [26]. Similarly, sequence analysis of PPV3 revealed overall high nucleotide homology between North Kivu PPV3 sequences and other PPV3 isolates previously reported in DRC, in particular South Kivu province [8], Cameroon, China and Germany, suggesting that the viruses might have also evolved from a common ancestor [9]. On the other hand, phylogenetic analysis revealed a high nucleotide identity (99–100%) with ASFV strains currently circulating in East Africa, indicating a common origin. The North Kivu ASFV isolates are closely related to isolates from South Kivu Eastern DRC, Kenya, Burundi, Tanzania and Uganda. This strongly suggests the historical and geographical connections between the ASF viruses and East African countries that mainly border DRC. However, genomic surveillance for ASF is poor and a lack of complete-genome sequencing from other regions likely biases this result. For instance, samples that were analysed in the current study were collected from local (indigenous) or cross-bred pigs, suggesting ASFV could probably spread from within Africa, specifically from the neighbouring province of South Kivu or from Uganda, which borders North Kivu province through regular pig trade.