Epidemiology of African swine fever in Nigeria

Abstract

African swine fever (ASF) is a highly contagious transboundary viral infection of domestic pigs that has serious socio-economic implications on people’s livelihood, international trade and food security. It is still a major limitation to profitable pig production and presently, it is threatening the pig industry internationally with current outbreaks in the Russian federation and the Caucasus. Since 1996, the disease has made major incursions into the West African sub-region. In this study, a combination of classical epidemiologic (statistical), economic, laboratory (serological, virological and molecular) and evidence-based tools were used to determine the prevalence of ASF in Nigeria, map the temporal situation of the virus, estimate the economic implications of infection with justification of alternative control (biosecurity), compare and contrast the virulence genes (Thymidine kinase, TK) and finally evaluate the effectiveness of ethnoveterinary preparations used in the management of ASF. Nine percent (9%) of serum samples and 48% of tissue samples tested were positive for ASF virus antibody and genome, respectively. Areas with high pig-related activities (marketing, consumption and farming) have higher prevalences compared with areas with less pig activities. Farm-gate buyers, marketing systems and transport of untested pigs within the country appeared to assist with the circulation of the virus. Using the financial model of partial budgeting and benefit-cost analysis, ASF outbreak in a 122-sow unit may lead to a loss of up to ZAR7,475,867.27 (US$910,836.70) in a single year while the implementation of biosecurity and its effective monitoring can prevent these losses with certain other social benefits and give a benefit-cost ratio of approximately 29 in return, but the cost of its implementation may result in a 9.70% less annual profit. Since the identification of factors that supports infection on pig farms in the sub-region remains the key component in the development of a risk-based approach to control the disease, most plausible risk factors and biosecurity measures previously identified were analysed in this study with a univariable/multivariable conditional logistic regression analytic models. Presence of an abattoir in a pig farming community (OR = 8.20; P < 0.001) and the presence of an infected pig farm in the neighbourhood (OR = 3.26; P = 0.02) were significant risk factors. There was a marginally significant negative association (protective) between risk of ASF infection and sharing farm tools and equipment (OR = 0.35; P = 0.05). For the biosecurity measures evaluated, food and water control (OR = 0.14; P < 0.001), separation/isolation of sick pigs (OR = 0.14; P = 0.004) and washing and disinfection of farm equipment and tools (OR = 0.27; P = 0.02) were negatively associated (protective) with ASF infection. Consultation and visits by veterinarian/paraveterinarians when animals were sick (OR = 8.11; P = 0.002), and pest and rodent control were positively associated with ASF infection of Nigerian farms (OR = 4.94; P = 0.002). The leaf, root and stem portions of Ancistrocladus korupensis possessed some chemical compounds with antiviral potentials and extracts and fractions from the plant showed very good antiviral (virucidal) activities in-vitro against ASF virus (NIG/99). It also has certain cytotoxic principles and narrow therapeutic index. Further studies on the maximization of the ethnoveterinary potentials of the plant invivo and in-vitro while reducing its cytotoxic potentials will be needed. Using molecular characterization, similar unresolved topologies were observed within the European, South America, Caribbean and West African (ESAC-WA) genotype and the mean character distances on the coded data set revealed least possible loss of information that would have otherwise been ignored in pairwise- or complete- deletion distance analysis. The size of the coding ORF for the TK protein varies between isolates but the majority of isolates code for a protein of 196 amino acids. These isolates comprise of viruses from Europe, West, Central and Southern Africa. A smaller TK gene product of 185 and 194 amino acids, caused due to a frameshift mutation at nucleotide position 561 in many of the East African isolates resulting in stop codons immediately thereafter or further downstream (nucleotide position 571 in Malawi 3). Despite the smaller TK protein product size, certain nonsense insertions of differing length were responsible for some considerably larger TK-PCR products. This TK protein heterogeneity is unexpected in an enzyme with such an important function and these size differences may have an effect on virulence. It is concluded that strains from southern Africa may have a shared evolutionary history with strains of the ESAC-WA genotype but may differ from the evolutionary lineage from East Africa. It is also suggested that a link exist between the sylvatic cycle, domestic tick cycle and the truncated TK products. Finally, putting in place a comprehensive routine surveillance and testing system to rapidly eliminate all pigs in infected farms, reorganization of the market and transportation systems for pigs, implementation of carefully planned on-farm biosecurity protocols, and giving consideration to the option of compensation to encourage reporting of outbreaks will possibly achieve a significant reduction in high ASF prevalence in Nigeria. It will be desirable to eliminate certain risky farm-related practices and behaviours (e.g the removal of all pig abattoir from within the pig communities, isolation of infected neighbourhood farm) while entrenching farm-level biosecurity as these appear to be the key to controlling ASF within the subregion. In conclusion, the outcomes of this research can be used to plan long-term strategies for countries like Nigeria, and assist the ASF unaffected countries that are at risk of infection to organize and secure their animal (pig) resources, so that Africa can be free from the significant effects of ASF and explore options of international markets.