A fractional-order analysis of recurrent Lassa fever transmission in Nigeria

The recurrence of Lassa fever in Nigeria is sustained by human, hospital, rodent, and environmental routes. However, many existing models do not simultaneously capture asymptomatic infection, nosocomial spread, imperfect isolation, and epidemiological memory. This study develops a Caputo fractional-order model that integrates these mechanisms within a single system. The model is shown to admit a unique, non-negative, bounded solution in a positively invariant feasible region. The human-environment reproduction threshold is derived using the next-generation matrix method, while the rodent subsystem yields a separate reservoir threshold. The Laplace-Adomian decomposition method (LADM) is used to obtain an approximate analytical solution, and the model is calibrated directly to confirmed weekly incidence data reported by the Nigeria Center for Disease Control and Prevention for epidemiological weeks 1-14 of 2026. The fitted fractional order is α̂=0.92, which indicates data-supported memory during the recurrence window. The fractional model gives the sum of square error (SSE=26.20), the root mean square error (RMSE=1.37) cases/week and the mean absolute percentage error (MAPE=2.26%), while the independently fitted classical model gives SSE=764.24, RMSE=7.39 cases/week, and MAPE=13.49%. Under the fitted fractional baseline, R₀=1.7653. Pathway decomposition attributes 64.34% of R₀ to asymptomatic community spread, followed by nosocomial transmission (17.79%), symptomatic community spread (12.45%), environmental exposure (4.27%), and imperfect-isolation leakage (1.14%). Sensitivity, p-profile and environmental-feedback analyses show that hidden community spread is the leading contributor to Lassa fever spread, although its share is strongly dependent on the fraction of asymptomatic progression. Environmental contamination and persistence of rodents mainly support recurrence rather than dominate early growth. These results suggest that sanitation and rodent control alone are insufficient; effective intervention should combine rapid community screening, strict infection prevention in health facilities, prompt isolation that accounts for leakage, environmental sanitation, and rodent reduction.