Research overview: effusive and non-effusive forms of FIP

FIP classically presents in two overlapping forms. The effusive (wet) form features protein-rich fluid accumulation in the abdomen or chest from vasculitis and leaky vessels, while the non-effusive (dry) form produces granulomatous lesions in organs, eyes, and nervous system without major effusion. The forms reflect differences in the host immune response, with stronger cell-mediated immunity tending toward the dry form. Many cats show mixed or shifting presentations. This overview synthesizes the clinical literature rather than a single study, describing how immune response shapes disease form, why the dry form is harder to diagnose, and how recognizing the spectrum of presentations helps clinicians pursue appropriate testing and, in the modern antiviral era, timely treatment.