Global stability analysis of within-host Zika virus–chikungunya virus co-infection with CTL immune responses

We formulate and analyse a mathematical model for the within-host co-infection dynamics of Zika virus (ZIKV) and chikungunya virus (CHIKV), incorporating latently infected cells and cytotoxic T lymphocyte (CTL) immune responses. We establish the nonnegativity and boundedness of solutions. Using the next-generation matrix method, we derive the basic reproduction numbers (RZL) and (RCL) for the ZIKV and CHIKV submodels, respectively, and the overall reproduction number (R0L=max{RZL,RCL}). Invasion reproduction numbers, (RZL,inv) and (RCL,inv), are derived to characterise the ability of either virus to invade an established infection. Global stability of the equilibria is established using Lyapunov functions. We investigate three therapeutic protocols: antiviral treatment blocking viral entry, treatment reducing viral production, and IL-2 therapy enhancing CTL responses. Sensitivity analysis of (RZL) and (RCL) is performed, and numerical simulations support the stability results. Simulations indicate that combined antiviral and immune-based interventions suppress co-infection by reducing viral replication and enhancing immune-mediated clearance.