Epidemiology of African swine fever in wild and domestic swine: Factors for its persistence in Uganda

Abstract

The pig industry in Uganda has more than doubled in the last decade with national pig population put at approximately 3.6 million pigs by 2012 with most of this being kept by smallholder farmers. African swine fever (ASF), a fatal, haemorrhagic disease of domestic pigs, poses a threat to pig farming and is currently endemic in most of sub-Saharan Africa. To get an insight into the factors that relate to ASF outbreaks at farm-level and the possible role of the domestic pigs to the maintenance of the disease in the domestic pig population, a longitudinal study was performed in one of the major pig producing areas in central Uganda. Potential risk factors associated with outbreaks of ASF were investigated using a questionnaire-based survey administered to farmers. The possible presence of apparently healthy ASF-virus (ASFV) infected pigs, which could act as long-term carriers of the virus was also investigated. Blood was taken from 715 pigs (241 farms) and 649 pigs (233 farms) to investigate presence of ASFV and antibodies, during the periods of June-October 2010 and March-June 2011, respectively. The association between ASF outbreaks during the study period and the risk factors was assessed. Fifty-one (21%) and 13 (5.6%) farms reported ASF outbreak in the previous one to two years and during the study period, respectively. The incidence rate for ASF prior to the study period was estimated at 14.1 per 100 pig farm-years and 5.6 per 100 pig farm-years during the study. There were no seropositive pigs. Three pigs tested positive for ASFV using real-time PCR. There was no evidence for existence of long-term carrier pigs from the analysis of blood and serum. Potential ASF risk factors were present on both small and large-scale pig farms, however, no risk factors were significantly associated with ASF reports during the study. Different practices in the pig value chain and their association with the transmission of ASF within and between farms and pork traders were investigated on 101 pig farms and 30 traders (slaughter slab, pork butchery and roasted pork restaurant operators) in the districts of Kabarole, Mityana, Moyo, Mukono, Soroti, and Tororo in Uganda. Key informant interviews with the District Veterinary Officers (DVOs) were conducted as well, between May-November 2011. Twenty six percent of the farmers reported having had the disease on the farm in the last one year. None of the risk factors was statistically significant for ASF outbreaks but potential risk factors for ASF existed on the farms and among pork traders. To understand the role of bushpigs in epidemiology of ASF, 4 bushpigs were captured from Lake Mburo Conservation Area and Masaka at the wildlife-livestock interface, blood and sera samples taken and analysed for antibodies against ASFV. The bushpigs were also fitted with harnesses mounted with GPS/GSM tracking collar for monitoring movement. Two of these had antibodies against ASFV. Possible interactions between the bushpigs and the domestic pigs was noted. Analysis for anti-tick antibodies using the recombinant rtTSGP1 showed that 18 (3.6%) individual sera out of the 500 sera from both domestic and wild pigs (4 bushpigs and 1 warthog) had antibodies, an indication that there could be exposure of the domestic pigs from the different regions to the tick vector, Ornithodoros moubata. Analysis of the p72 sequences from ASFV sequences from this study revealed that they all belonged to genotype IX, with few differences. The samples were from Masaka, Rakai, Mityana, Gulu, and Moyo. The sequences grouped with other viruses from previous studies. There is a need for increased sensitisation of farmers and traders involved in the pig value chain on the potential risk factors for effective disease prevention and control. The absence of carrier pigs requires further investigation in order to better inform disease control strategies. The presence of anti-tick antibodies requires further investigation into the actual presence of soft ticks on pig farms if control and prevention are to be appropriately addressed. The study also recommends a non-conventional approach to disease control to bridge disparity between policy on quarantine for disease control and farmer copying mechanisms.