Sero-prevalence and risk factors associated with African swine fever on pig farms in southwest Nigeria

Background: African swine fever (ASF) is one of the major setbacks to development of the pig industry in Nigeria. It is enzootic in southwest Nigeria. We determined the sero-prevalence and factors associated with ASF among-herd seropositivity in 144 pig farms in six States from southwest Nigeria during the dry and rainy seasons using indirect Enzyme Linked Immunosorbent Assay (ELISA) for ASF IgG antibodies. An interviewer-administered questionnaire was used to collect information on demography, environmental and management factors. We performed descriptive statistics, and univariate and multivariable analyses to determine the among-herd sero-prevalence of ASF and its associated factors. Results: The overall herd sero-prevalence of ASF was 28 % (95 % Confidence interval (95 % CI) 21 - 36); it was significantly higher (P < 0.05) in the dry season (54 %; 95 % CI 37 - 70) than the rainy season (18 %; 95 % CI 11 - 27). In the univariate analysis, having a quarantine/isolation unit within 100 m radius of a regular pig pen (OR = 3.3; 95 % CI 1.3 - 8.9), external source of replacement stock (OR = 3.2; 95 % CI 1.3 - 8.3) and dry season (OR = 5.3; 95 % CI 2.2 - 12.7) were risk factors for ASF among-herd seropositivity. In the multivariable logistic regression, there was interaction between season and herd size. Our final model included season, source of replacement stock, herd size and interaction between herd size and season. Herds with an external source of replacement always had higher ASF sero-prevalence compared with herds with an internal source. The herd size effect varied between seasons. Conclusions: The ASF herd level sero-prevalence in southwest Nigeria was higher in pig herds with an external source of replacement stock and in the dry season. The effect of season of the year the samples were taken on ASF seropositivity was modified by herd size. We encourage strict compliance with biosecurity measures, especially using an internal source of replacement stock and measures that minimize movement on pig farms in southwest Nigeria, in order to enhance ASF free farms.