| dc.description.abstract | African swine fever (ASF) is a rapidly lethal epidemic disease of domestic swine that represents a constraint, as a result of high morbidity and mortality, to the development of the smallholder pig industry in sub-Saharan Africa. The first case of the disease in Africa was officially reported in Kenya in the 1920s. Outbreaks have been reported in the country since 2001. The current study was conducted in western, central and north eastern regions of Kenya with the purpose of understanding the epidemiology of African swine fever virus in both domestic and sylvatic transmission cycles of the disease.
The objectives were to: 1) Characterize pig production systems in the major pig producing areas of Kenya; 2) Estimate the prevalence of ASF virus in contrasting pig production systems in Kenya; 3) Investigate reported outbreaks of ASF in the course of the study and assess possible causal relationships between current and recent outbreaks of ASF in the
country; 4) Isolate and characterize ASFV in Kenya. Western, Central and North Eastern regions of Kenya were selected for the study whereas Homabay, Kiambu, Thika, Kisumu, Kakamega, Busia, Machakos, Garissa and Mandera
districts were selected from these regions as study sites.
The study areas were selected because they represented contrasting pig production systems that were either free-ranging or stall-feeding or no pig keeping at all or were areas with frequent ASF outbreaks or were harboring wild and tick hosts. A systematic random sampling approach was used to factors for ASF from the 121 randomly selected pig-rearing households within the two contrasting pig farming regions; south-west Kenya was predominantly free-range while central region was stall-feeding. Northeastern region was selected as a control for wild pigs that had no interaction with domestic pigs. In addition, blood and tissue samples were collected from domestic pigs, bush pigs, and warthogs and whole ticks were also collected in the study areas for diagnosis and ASFV isolation.
Farms and wildlife sampling points were geo-referenced for spatial analysis. Data from the questionnaire were analyzed using descriptive statistics; K-means cluster analysis for farm characterization, and Generalized Linear Regression Models for assessment of potential risk factors for ASFV infections. Biological samples were analyzed using Enzyme Linked Immunosorbent Assays (ELISA), Immunoblotting, virus isolation, Polymerase Chain Reactions (PCR), virus genetic characterization and population genetic techniques.
The best characterization of the pig farms was by size of land in hectares into small (6 hectares) and large (18 hectares) farm sizes and the number of pigs per herd into small (9 pigs) and medium scale (150 pigs) herds. The domestic cycle of virus transmission was influenced by poor management practices such as inadequate biosecurity measures within farms. There appeared to be existence of inapparent infections in domestic pigs as shown by high virus prevalence by PCR technique estimated at 29.2% (95% CI, 21.6, 39.9) in south-west Kenya and 3% (95% CI, 0, 5.5) in central Kenya. Genotyping of both domestic and bush pig viruses using p72 and p54 genotyping and CVR amino acid sequencing revealed homology suggesting sylvatic cycle of ASFV transmission involving the two species in south-west Kenya. On the other hand, some ASFV isolates from ticks and warthogs from central Kenya were similarly closely related to ASFV isolated from domestic pigs from the same locality, suggesting viral transmission between the ticks, warthogs and domestic pigs.
The 5 outbreaks investigated occurred in 2010 and 2011 in $e study areas and were due to a single p72 genotype IX virus. These outbreaks were genetically and biologically associated with a closely related virus that has been present in areas around the Kenya-Uganda border region since 2006-2007. Characterization of viral isolates from domestic and bush pig in south-west Kenya by p72 and p54 genotyping showed homology to ASFV isolates from other parts of the East African region. However, on higher resolution by CVR amino acid sequencing of the same isolates complete homology was shown but was divergent from other isolates from Burundi and Tanzania from the same species. Overall, genetic characterization of the viral isolates from ticks, warthog, domestic and bush pigs revealed clustering with previous isolates from within the region isolated during the period 1959 to 2010, but were in a separate lineage from previous isolates from the southern Africa region.
Generally, epidemiology of ASFV in Kenya was influenced by pig production system, pig management practices, sylvatic reservoirs and virulent or avirulent virus strains in circulation. A further study is needed to assess possible phenotypic implications of ASF virus mutations, the extent to which outbreak patterns in Kenya and other countries are associated with the currently characterized group of viruses. The genetic relationships between bush pigs and domestic pigs and whether or not suspected hybridization between the two species that could have significant implications for disease epidemiology observed in south-west Kenya, does in fact occur should be looked at. Finally, assessment of impact of ASF on the pig value chain development in Kenya that was not covered in this study needs to be done. | en_US |
