Early African Swine Fever Dynamics in Wild Boar After Reemergence in Catalonia, Spain: Surveillance-Based Estimates of Initial Spread.

African swine fever (ASF) reemerged in free-ranging wild boar in Catalonia, northeastern Spain, in November 2025, representing the first detection in Spain since 1994. We described the initial outbreak phase using surveillance data collected from 25 November 2025 to 23 February 2026. ASF virus (ASFV) positivity by real-time quantitative polymerase chain reaction (qPCR) was estimated overall and by surveillance stream; mortality timing was reconstructed probabilistically from estimated death-date intervals; and early front displacement was analyzed using leading-edge cumulative minimum convex polygons (MCPs), linear and generalized additive models (GAMs), segmented regression, and directional modeling. Among 1483 wild boars with definitive qPCR results, 201 were ASFV qPCR-positive, giving an overall qPCR positivity of 13.6% (95% confidence interval [CI] 11.9-15.4). Positivity was substantially higher in enhanced passive than in active surveillance (28.2% vs. 0.9%; positivity ratio [PR] 32.11, 95% CI 15.21-67.78; p < 0.001). A probabilistic cumulative mortality curve based on 198 positive carcasses indicated that, by the date of the first detected carcass, expected cumulative mortality had already reached 25.8 deaths (13% of reconstructed total mortality). Leading-edge analysis estimated an apparent mean radial front-displacement rate of 0.042 km/day (95% CI 0.029-0.055). The surveillance-derived distance-time relationship was nonlinear, indicating that the apparent front-displacement rate varied over the study period, with a segmented-regression breakpoint at 119 days and higher apparent front-displacement thereafter (0.106 vs. 0.021 km/day). The exploratory directional analysis indicated the fastest predicted advance toward the South-Southwest sector. These surveillance-derived estimates describe the observed leading edge under the surveillance and control conditions operating during the study period and should not be interpreted as direct estimates of ASFV transmission speed. These findings support an adaptive, risk-based response that prioritizes carcass search and removal and incorporates directional spread information to refine surveillance and zoning as the outbreak evolves. The analytical framework, combining reconstruction of estimated dates of death with simple descriptors of early epidemic spread, may support early epidemiological assessment of ASF incursions in other newly affected settings.