Interrelationships of warthogs (Phacochoerus africanus), Ornithodoros ticks and African swine fever virus in South Africa

Abstract

Sylvatic circulation of African swine fever virus (ASFV) in warthogs and eyeless Ornithodoros ticks that live in warthog burrows historically occurred in northern South Africa. Outbreaks of the disease in domestic pigs originated in this region. A controlled area was declared in the north in 1935 and regulations were implemented to prevent transfer of potentially infected suids or products to the rest of the country. However, over the past six decades, warthogs were widely translocated to the south where the extralimital animals flourished to become an invasive species. Initial transfers were made from a source considered to be free of ticks and virus, but multiple unrecorded translocations accompanied the subsequent burgeoning of the wildlife industry. Since 2016, there have been outbreaks of African swine fever (ASF) in pigs outside the controlled area that cannot be linked to transfer of infected animals or products from the north. An investigation in 2008–2012 revealed that the presence of Ornithodoros ticks and ASFV in warthog burrows extended marginally across the boundary of the controlled area. Furthermore, a recent revision of the taxonomy of Afrotropical Ornithodoros ticks included description of new species and indicated that some species were present outside the controlled area. The present project was undertaken to confirm whether there is sylvatic circulation of ASFV beyond the controlled area through testing warthog sera for presence of antibody and Ornithodoros ticks for presence of virus. Although warthogs are widely hunted in South Africa, logistic problems precluded the recruitment of adequate numbers of samples from hunters, and hence opportunistic use was made of samples collected during warthog culling operations in large nature reserves to augment samples obtained from hunters or collected in association with provincial veterinary authorities on farms in the vicinity of past outbreaks of the disease. The Greater Kruger National Park (GKNP) inside the controlled area was sampled as representative of an environment where circulation of ASFV in ticks and warthogs is endemic, while Addo Elephant National Park (Addo ENP) was included in the study for contrast since the presence of ASFV had never been recorded in the Eastern Cape Province at the time of sampling in 2019, although there were outbreaks of the disease about 250 km east of the national park in the following year. Specific evidence of sylvatic circulation of ASFV was sought in Mokala National Park (Mokala NP) because contact with warthogs had been reported in outbreaks of the disease in domestic pigs in the Northern Cape Province and adjacent western Free State Province. Stored serum samples were also obtained opportunistically from 44 warthogs culled in 2017 in the provincial Rolfontein Nature Reserve (Rolfontein NR), 100km south of Mokala NP. Besides serum for antibody tests and whole blood samples taken with EDTA anticoagulant, opportunity was taken to collect samples of selected visceral organs and lymph nodes from culled warthogs for determining ASFV presence in warthog tissues to be reported separately from the present project. Antibody to ASFV was detected by blocking ELISA in 201/207 (97.1%) warthog sera from GKNP, 0/66 sera from Addo NP, 0/44 from Rolfontein NR, 76/77 (98.7%) from Mokala NP, 26/29 (89.6%) dried blood samples from hunted warthogs from properties inside the controlled area and 8/12 (66.7%) warthogs from properties outside the controlled area. A total of 5078 ticks was collected at 45 locations in 7/9 provinces during 2019–2021 and assayed as 711 pools for virus content by qPCR, while 221 pools were also analysed for tick phylogenetics. Viral nucleic acid was detected in 50 tick pools representing all four members of the Ornithodoros (Ornithodoros) moubata complex known to occur in South Africa: O. (O.) waterbergensis and O. (O.) phacochoerus species yielded ASFV genotypes XX, XXI, XXII at 4 locations and O. (O.) moubata yielded ASFV genotype I at two locations inside the controlled area. Outside the controlled area, O. (O.) moubata and O. (O.) compactus ticks yielded ASFV genotype I at 7 locations, while genotype III ASFV was identified in O. (O.) compactus ticks at a single location. Two of the three species of the eyed O. (O.) savignyi complex ticks known to be present in the country, O. (O.) kalahariensis and O. (O.) noorsveldensis, were collected at single locations and found negative for virus. The only member of the Pavlovskyella subgenus of Ornithodoros ticks known to occur in South Africa, O. (P.) zumpti, was collected from warthog burrows for the first time, in Addo National Park in the Eastern Cape Province where ASFV had never been recorded, and it tested negative for the ASF viral nucleic acid. Since ASF subsequently occurred in Eastern Cape Province, a short communication on the finding of O. (P.) zumpti in warthog burrows was published separately to stimulate investigation of the possible involvement of the tick in ongoing outbreaks there. The current observations extend the known distribution ranges of some tick species and provided presumptive evidence of their role as vectors of ASFV. While it was confirmed that there is sylvatic circulation of ASFV outside the controlled area in South Africa, there is a need for more extensive surveillance and for vector competence studies with various species of Ornithodoros ticks, including O.(P.) zumpti, and O. (O.) compactus that had previously only been known as a parasite of tortoises.