Urbanisation as an Ecological Filter: Decoupling Lassa Virus Hazard from Spillover Risk in West Africa

Lassa fever is classically defined as a rural zoonosis constrained by the agricultural niche of its reservoir, Mastomys natalensis. However, current risk models rely on historical sampling heavily biased toward rural settings (>67%). Here, I reconstruct the realised niche of M. natalensis using an Integrated Multi-Species Occupancy Model (IMSOM) accounting for biotic interactions with invasive rodents. Contrary to climatic predictions of urban exclusion, I identify a cryptic reservoir niche in West African peri-urban fringes. While the reservoir and pathogen are urban-tolerant, analysis reveals a non-linear Socio-economic Shield proxied by infrastructure quality, dampening spillover in city cores. This creates an urban paradox displacing transmission from the shielded centre to hyper-endemic peri-urban fringes. Accounting for this shield and antibody waning ({lambda}=0.03), I estimate 2.6 million annual infections (Range: 0.9-4.4 million). This challenges conservative estimates based on lifelong immunity and aligns with the high force of infection required to sustain observed seroprevalence. Spatially, the model identifies widespread silent districts, where ecological conditions are primed for transmission but clinical reporting is absent. These findings suggest Lassa fever has the potential to emerge as a peri-urban disease, necessitating a paradigm shift in surveillance to address the blind spots of rapid urbanisation.