PATHOGENESIS:
FCoV replicates in the gut but it is thought that in FIP a mutant spreads to major organs in macrophages and monocytes. Immune complexes form within the walls of small blood vessels leading to an inflammatory response which damages vessels and permits the escape of fibrin-rich serum constituents into the intercellular spaces, giving the characteristic FIP fluid.

The presence of circulating anti-coronavirus antibodies potentiates the disease probably accelerating the uptake of virus into mononuclear cells as well as contributing to damaging immune complex formation. Disease depends on multiple factors which are not fully understood. Probably a genetic element in susceptibility.