Understanding African swine fever outbreaks in domestic pigs in a sylvatic endemic area: The case of the South African controlled area between 1977–2017

Abstract

South Africa declared a controlled area for African swine fever (ASF) in 1935, consisting of the northern parts of Limpopo, Mpumalanga, North West and Kwa-Zulu Natal Provinces. The area was delineated based on the endemic presence of the sylvatic cycle of ASF, involving warthogs and argasid ticks. Occasionally, spillover occurs from the sylvatic cycle to domestic pigs, causing ASF outbreaks. In the period 1977 to 2017, 59 outbreaks of ASF were reported in domestic pigs within the ASF controlled area of South Africa. During these outbreaks, at least 4,031 domestic pigs either died or were culled. Season did not affect the number of reported ASF outbreaks, but the number of reported outbreaks in this area per year was thought to be slowly increasing, although not statistically significant. Outbreaks occurred predominantly in Limpopo province (93%) and were mostly due to contact (or suspected contact) with warthog or warthog carcasses. Clustering analysis of outbreaks found that the local municipalities of Ramotshere Moiloa, Lephalale and Thabazimbi had the highest relative risk for outbreaks. In 32 of the 59 outbreaks, the genotype of the ASF virus (ASFV) involved could be determined. Phylogenetic analysis of ASFVs detected in domestic pigs during the study period revealed that p72 genotypes I, III, IV, VII, VIII, XIX, XX, XXI and XXII had been involved in causing outbreaks within the ASF controlled area. No outbreaks were reported in the Kwa-Zulu Natal part of the controlled area during this period. South Africa is unlikely to eradicate all sources of ASFV as spillover from the sylvatic cycle in the controlled area continued to occur, but with the implementation of appropriate biosecurity measures pigs can be successfully farmed despite the presence of ASFV in African wild suids and soft ticks.

Extension of Sylvatic Circulation of African Swine Fever Virus in Extralimital Warthogs in South Africa

“…Ornithodoros ticks generally engorge blood meals in minutes to hours and detach while animals are at rest but can be conveyed passively on their hosts between burrows or to the environs of domestic pigs to transmit infection (53)(54)(55)(56). Transport of Ornithodoros ticks on live warthogs or carcasses to the environs of piggeries has been observed in the controlled area (16).…”

“…Thereafter, the regulatory measures proved effective until 2012 when a series of outbreaks on smallholdings in adjacent areas of GP and Mpumalanga Province (MP) were traced to initial illegal movement of infected pigs from LP followed by local spread of infection involving sale of animals at auctions ( 12 – 14 ). The virus genotype involved, XXII, had previously been detected in the district of origin of the infected pigs in LP ( 15 , 16 ). From 2016 onwards, there have been successive series of outbreaks of ASF in domestic pigs in GP, MP, North West Province (NWP), Northern Cape Province (NCP), and Free State Province (FSP) that cannot be linked to movement of infected pigs or products from the controlled area ( Figure 3 ) ( 15 , 18 – 20 ).…”

African Swine Fever Virus (ASFV): Biology, Genomics and Genotypes Circulating in Sub-Saharan Africa

“…For instance, most surveillance studies on the circulating genotypes of ASFV in Tanzania have been limited to outbreaks in pig farms [19,42,70]. Although other studies have tried to identify ASFV genotypes in African warthogs and ticks [23,44,66,68,74,81], no conclusive results have yet been obtained. For example, although seropositive ASFV samples was established in all sampled warthogs in the Serengeti ecosystem in Tanzania [81], the genetic makeup of the ASFV strains was not confirmed.…”

An outbreak of African swine fever in small-scale pigs, Gauteng, South Africa, July 2020

“…ASF virus evolved and is maintained in a sylvatic cycle between warthogs and soft ticks belonging to the Ornthidoros moubata complex (Janse van Rensburg et al, 2020b;Penrith et al, 2019). These ticks act as biological vectors and are essential for the transmission of ASF virus from warthogs to domestic pigs (Dixon et al, 2019;Janse van Rensburg et al, 2020a). ASF transmission between warthogs and domestic pigs through direct contact does not occur efficiently (Janse van Rensburg et al, 2020a;Penrith et al, 2019).…”

“…These ticks act as biological vectors and are essential for the transmission of ASF virus from warthogs to domestic pigs (Dixon et al, 2019;Janse van Rensburg et al, 2020a). ASF transmission between warthogs and domestic pigs through direct contact does not occur efficiently (Janse van Rensburg et al, 2020a;Penrith et al, 2019). ASF is endemic to many countries in Sub-Saharan Africa (World Organisation for Animal Health (OIE), 2020a) including parts of South Africa where it is a controlled animal disease in terms of the Animal Disease Act, Act No.…”