Research overview: feline coronavirus biotypes and the internal mutation hypothesis

Feline coronavirus circulates widely as a generally mild enteric virus (FECV), but in a minority of infected cats it mutates within the host into a virulent biotype (FIPV) capable of replicating in macrophages and causing systemic FIP. The internal-mutation hypothesis holds that FIP arises de novo in each cat from its own enteric infection rather than spreading directly as an FIP-causing virus. Researchers have sought the specific genetic changes, particularly in the spike gene, that drive this shift. This overview synthesizes the biotype literature rather than one citation, describing the evidence for in-host mutation, why most coronavirus-positive cats never develop FIP, and how understanding this transition guides diagnosis and risk assessment.