Abstract
The persistence of African swine fever virus (ASFV) in endemic areas, with small-scale but regular outbreaks in domestic pigs, is not well understood. ASFV has not been detected using conventional diagnosis in these pigs or adjacent populations of resistant African wild pigs, that could act as potential carriers during the outbreaks. However, such data are crucial for the design of evidence-based control strategies. We conducted cross-sectional (1107 pigs) and longitudinal (100 pigs) monitoring of ASFV prevalence in local pigs in Kenya and Uganda. The horizontal survey revealed no evidence of ASFV in the serum or blood using either conventional or real-time PCR. One pig consistently tested positive using ELISA, but negative using PCR assays on blood. Interestingly, the isotype of the antibodies from this animal were strongly IgA biased relative to control domestic pigs and warthogs, suggesting a role for mucosal immunity. The tissues from this pig were positive by PCR following post-mortem. Internal organ tissues of 44 healthy pigs (28 sentinel pigs and 16 pigs from slaughter slabs) were tested with four different PCR assays; 15.9 % were positive for ASFV suggesting that healthy pigs carrying ASFV exist in the swine population in the study area. P72 and p54 genotyping of ASFV revealed very limited diversity: all were classified in genotype IX at both loci, as were virtually all viruses causing recent ASF outbreaks in the region. Our study suggests that carrier pigs may play a role in ASF disease outbreaks, although the triggers for outbreaks remain unclear and require further investigation. This study significantly increases scientific knowledge of the epidemiology of ASF in the field in Africa, which will contribute to the design of effective surveillance and control strategies.
Highlights
Pigs are increasingly contributing to improved nutrition and household incomes in regions of Africa where pork consumption and pig keeping are culturally acceptable
Five pigs were positive for African swine fever (ASF) virus (ASFV) by both conventional PCR [9] and Universal ProbeLibrary (UPL) real-time PCR [10] diagnostic assays using blood, with three of them being positive for ASFV in tissues, following post mortem (Table 1)
28 ear-tagged sentinel pigs that tested negative for virus in blood by PCR and serological assays, with the exception of two pigs (UG64/2013 which was seropositive during the entire study period and Ken13/ busia.3 which was positive in blood by conventional and UPL PCR from the first longitudinal time point at 3 months), were euthanized and tested for the presence of ASFV in the tissues
Summary
Pigs are increasingly contributing to improved nutrition and household incomes in regions of Africa where pork consumption and pig keeping are culturally acceptable. In Uganda, for example, pork is second only to beef in terms of meat production and accounts for at least a third of the current 10 kg yearÀ1 per capita meat consumption [1]. Current household monthly expenditure on meat, relative price of pork, preference for value-added pork products, prices of substitutes and response of households to improvements in pork quality have been shown to be associated with increased consumption of pork in some parts of Africa [2]. African swine fever (ASF), an infectious and lethal disease of domestic pigs, constrains the realization of economic benefits. It could potentially be transmitted or maintained in a pig population by Ornithodoros soft ticks, the presumptive ancestral arthropod reservoir and vector for transmission of the virus from warthogs, specialized African wild pigs which can carry the virus without signs of disease, to domestic pigs
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