Why Does My Cat Have a Swollen Belly?

A distended abdomen in a cat has very different implications depending on age and how quickly it developed. In kittens, the most common cause is intestinal parasites. In adults, it's more concerning and needs veterinary assessment.

Parasites (roundworms, hookworms) are the #1 cause of pot-bellied kittens. Kittens can acquire worms from their mother before birth or through nursing. A heavy worm burden causes a distinctly rounded belly even as the rest of the kitten appears thin. This is easily treated with deworming medication.

Obesity creates a gradually enlarging belly, typically with fat also deposited over the ribs and along the spine. You can distinguish obesity from fluid by feel: fat feels soft and squishy, while fluid feels taut and firm.

Fluid accumulation (ascites) is the most concerning cause. It indicates serious underlying disease: congestive heart failure, liver disease/failure, FIP (feline infectious peritonitis — a usually fatal viral disease), or cancer. The abdomen feels firm and "fluid-wave" positive (tapping one side creates a wave felt on the other). The cat is typically unwell — lethargic, not eating, possibly with labored breathing.

FIP deserves special mention: the "wet" form causes massive abdominal (and sometimes chest) fluid accumulation. It most commonly affects cats under 2 years old. Until recently it was universally fatal, but new antiviral treatments (GS-441524) are showing remarkable success.

Other causes: pregnancy (intact female cats), bladder obstruction (especially male cats — this is an emergency), and organ enlargement from tumors, polycystic kidney disease, or liver disease.