The epidemiology of the 2016-2017 African swine fever outbreaks in the Northwest and Free State provinces of South Africa

Abstract

African Swine Fever (ASF) is a viral disease, which is endemic in most sub-Saharan African countries with warthog and tick vector (Ornithodoros) acting as biological reservoirs of the virus. A high mortality rate is evident among domestic pigs, and international trade becomes negatively affected including swine products. The African Swine Fever outbreak was confirmed by laboratory results issued on the 6th of June 2016. It occurred in the North West Province where smallholder farmers reported deaths in the free-roaming pigs. During the same period, another suspected outbreak of ASF was reported on a farm near Koffiefontein in the Free State. This project aimed at establishing the source of the ASF outbreak in the Free State and North West provinces by conducting an epidemiological assessment, investigating possible risk factors, and assessing the role played by the sylvatic cycle. Face-to-Face interviews and direct observations were used to collect primary data. Affected pigs (n=2 539) were recorded in both provinces. Pigs that succumbed to the ASF virus were (n=880) in Free State and (n=664) in North West province whereas pigs that were culled, in both provinces, were respectively (n=571) and (n=424). Blood samples from live pigs comprised sera (n=174), blood on ethylene diamine tetra-acetic acid (EDTA) (n=67), and from dead pigs, tissue samples (n=44) were submitted to the laboratory for analysis. Samples from warthogs namely, EDTA blood (n=2), sera (n=9,) and tissue (n=10) were also submitted to the laboratory. Methods used included virus isolation, enzyme linked immunosorbent assay (ELISA), and PCR. Antibody ELISA was used to assess exposure to the African Swine Fever virus (ASFV). The polymerase chain reaction test was used to quantify the prevalence of pig exposure to the ASFV using tissue and EDTA samples. Blood samples on EDTA from domestic pigs (n=67) yielded 23 out of 67 samples (34%) positive for ASFV which suggests that healthy pigs carrying ASFV existed in the affected area. Twenty-six out of 67 (39%) tested negative with the rest, 18 out of 67 (27%) not tested. The tissue samples (n=44) revealed that 23 out of 44 (52%) tested positive and 21 out of 44 (48%) were negative for ASFV. Of the serum samples (n=174), 18 out of 174 (10%) were positive for antibody detection and 138 out of 174 (79%) were negative. From assessments, EDTA samples (n=2) revealed that both samples 2 out of 2 (100%) were positive for ASFV, and tissue samples from warthog’s carcasses (n = 10) revealed that 9 out of 10 (90%) were positive while 1 out of 10 (10%) was negative for ASFV detection. Lastly, serum samples (n = 9) revealed that 7 out of 9 sera (78 %) tested positive while 2 out of 9 sera (22%) tested negative for antibodies against ASFV. To establish the involvement of the sylvatic cycle, sampling of warthog burrows with Ornithodoros moubata was carried out. A total of 88 ticks were recovered from the burrows and the laboratory results demonstrated that 10 out of 88 tick samples (11.4%) collected from warthog burrows in Koffiefontein in the Free State tested positive for ASFV deoxyribonucleic acid (DNA), 68 out 88 (77.3%) were negative, and 10 out of 88 (10.4%) yielded inconclusive results. The positive results indicated a possibility that the sylvatic cycle has contributed to the dissemination of the ASFV. In addition, the questionnaire and farm observations revealed a lack of biosecurity as a major concern.