Cat Bloated Abdomen: Fluid, Mass, or Gas?

A cat with a swollen belly is one of 4 things: fat, gas, free fluid, or a solid mass and uterus. The first 2 are cosmetic, and the last 2 are not, which is why the tape measure and a pair of hands matter more than the look of the cat. Fluid that reaches 100 ml or more in a 4 kg cat starts to show as a pear-shaped outline with a tight, shiny midline, and the same outline can be a pregnant uterus at 60 days.

Fluid or solid: what your hands tell you

Put the cat standing on a firm surface and place one flat hand on each side of the belly. Free fluid behaves like water in a bag: press one side and the other side bulges, tap one flank and feel a wave arrive at the other hand 1 second later. Fluid also sinks, so a cat held up by the front legs shows a heavy, pendulous lower belly. A solid mass feels different: a discrete firm lump that keeps its shape when you push, or a tubular structure running forward from the pelvis that is the uterus.

Fat is soft, symmetrical, and does not slosh. Gas is drum-like and high-pitched under the fingers, and it moves when the cat rolls. Pregnancy at day 60 to 65 gives you firm, separate fetal bumps the size of a walnut, and a mammary chain that has doubled in width 2 weeks before birth. A closed pyometra in a cat aged 3 to 10 feels like a full, tender sausage and comes with drinking, vomiting, and a temperature over 39.5°C.

The weekly tape measure

Measure at the same landmark every time: the point 1 cm behind the last rib, at the level of the umbilicus, with the tape snug but not denting the skin, and the cat standing. Write down the number in centimeters with the date, and mark the tape line with a dot of livestock marker so the next measurement lands in the same place. Weigh the cat on the same day, because fluid adds 1 kg per liter.

Normal fluctuation across a week is 0 to 1 cm. A gain of 2 cm or more in 7 days is a call to the vet, and 3 cm in 7 days with any breathing change means go now. In a cat already diagnosed with FIP or heart failure, weekly circumference plus sleeping respiratory rate is the cheapest monitoring pair there is: a circumference up 2 cm and a sleeping rate up from 24 to 34 is a relapse, not a bad day.

What the swelling usually is

Cause	Typical age	Fluid volume	Untreated course
FIP, wet form	under 2 years	100-500 ml, refills in 5-10 days	days to 8 weeks
Right heart failure	1-7 years	50-300 ml	days to weeks
Liver disease	8-14 years	100-1000 ml	weeks to months
Pregnancy	1-8 years	20-80 ml amniotic	63 days, normal
Pyometra	3-10 years	50-500 ml pus	1-5 days, emergency

FIP: the effusion that refills

Feline infectious peritonitis produces a yellow, sticky, protein-rich fluid with more than 3.5 g/dL of protein, and it tests positive on the Rivalta drop test. The pattern is a young cat, often under 2 years, from a multi-cat household, with a fever that does not answer antibiotics, a belly that grows 1 to 2 cm a week, and often fluid around the lungs at the same time. Antiviral treatment runs 12 weeks and is expensive, but it changes the outlook completely, so the fluid sample is worth taking on day 1 rather than after 3 weeks of guessing.

Heart failure and liver disease produce the same shape with different chemistry. Right-sided heart failure from hypertrophic cardiomyopathy gives a low-protein fluid under 2.5 g/dL, jugular pulses, and cool ears. Liver disease brings albumin under 2.0 g/dL, so the fluid is a pure transudate and the cat looks thin everywhere else. Chylous fluid, milky and high in triglycerides above 100 mg/dL, points to lymphoma or trauma.

Clotting tests before the needle

Any tap goes through the abdominal wall and can hit a vessel, so 4 things are checked first: platelet count above 50,000 per µL, prothrombin time and partial thromboplastin time within 1.5 times normal, no history of rodenticide exposure, and a packed cell volume that can afford to drop 2%. Cats with liver disease or FIP can have deranged clotting and still look stable, which is exactly why the tests run before the needle rather than after. Ultrasound guidance is standard, a 22 gauge needle is enough, and most taps pull 3 to 5 ml for cytology, culture, and protein measurement.

Removing fluid is treatment as well as diagnosis: 10 to 20 ml per kg comes off a stable cat and can drop the respiratory rate within 20 minutes. Take more than that in one sitting and you risk a drop in blood pressure, so large volumes get drained over 2 or 3 sessions. Never tap a cat that is fighting for breath; stabilize with oxygen first.

Swollen belly plus fast breathing is the emergency line

Fluid or gas pushing up on the diaphragm steals lung volume, and a cat cannot compensate the way a dog does. The line is 40 breaths a minute at rest, or any open-mouth breathing, or gums that have gone pale or gray. Gastric distension belongs on the same list: a cat with a hard, drum-tight belly, unproductive retching, and drooling has a surgical emergency, described in gastric dilatation in cats, and the clock is 6 hours, not 24.

The other look-alike is a colon packed with hard stool, where the belly is firm along the left side and the cat has produced nothing for 48 hours; that pathway is covered in constipation and megacolon. If the cat has chewed a plant or licked a product, add poisoning to the list and check the toxin reference before assuming fluid, because some toxins cause gas and ileus within 12 hours.

Five steps for the next 7 days

1. Photograph the cat from the side and from above, in daylight, on the same floor spot.
2. Measure the circumference 1 cm behind the last rib and log it with the date.
3. Count the sleeping breathing rate and log it beside the circumference.
4. Book a visit if the belly grows 2 cm in a week, or if either number moves with appetite loss.
5. Bring the log, the photos, and the weights; the trend decides whether this is a tap or a wait.

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