Estimation of the Farm-Level Basic Reproduction Number for African Swine Fever Outbreaks in the Philippines

Abstract

African swine fever (ASF) genotype II has severely impacted the Philippine swine industry since 2019, affecting backyard and commercial farms as well as native wild boar populations, with significant economic and food security implications. Understanding ASF transmission dynamics is crucial for effective control and management strategies. However, farm-level transmission patterns remain unclear, and a comprehensive analysis incorporating both national and island-specific estimates has yet to be conducted. Using ASF outbreak data from 2019 to 2022, we estimated the herd-level basic reproduction number (Rh) for the Philippines and its three major islands—Luzon, Visayas, and Mindanao—using deterministic and stochastic models. Epidemic descriptive analysis and Spearman’s correlation results indicate that Luzon’s data most accurately reflect national epidemic trends. Our estimation model for Rh shows that while the long-term trend of ASF approaches a threshold near 1, indicating a shift toward endemicity, the Rh value does not consistently remain below 1 during the study period, suggesting ongoing outbreaks. Luzon, the outbreak’s origin with the highest swine farm density, mirrors national trends, whereas Mindanao and Visayas exhibit distinct transmission patterns influenced by local production systems and swine demographics. These results highlight the need for region-specific ASF management strategies, including enhanced biosecurity in Luzon and community-based surveillance in Visayas and Mindanao. Adapting control measures to regional transmission patterns can improve ASF management and support the long-term recovery of the Philippine swine industry.