When to see a vet urgently
Go to an emergency vet immediately if an enlarged abdomen is accompanied by any of the following signs:
- difficulty breathing or open-mouth breathing;
- collapse or extreme weakness;
- severe abdominal pain, repeated vomiting and a rapidly deteriorating condition;
- sudden abdominal enlargement after an injury, or pale mucous membranes at the same time.
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Overview
Normally, the abdominal cavity contains only a small amount of fluid that helps internal organs slide against one another. With ascites, this balance is disrupted and too much fluid accumulates. It may be a clear transudate or, with inflammation, a cloudy exudate. The fluid presses on abdominal organs and the diaphragm, making breathing difficult and worsening general condition.
Not every enlarged abdomen is caused by free fluid. Increased fatty tissue, intestinal gas, an enlarged organ or an abdominal mass can also change its shape. Investigations first establish what is causing the enlargement. The appearance of the abdomen alone does not distinguish ascites from these possibilities.
Free abdominal fluid can also be blood or urine leaking from damaged urinary structures. These situations have more specific names because their causes and urgency differ from an ordinary accumulation of tissue fluid. A change in abdominal size or your cat's condition after an injury therefore needs investigation even without a visible wound.
Causes
Ascites can develop for several reasons, including:
- heart diseases, such as heart failure and cardiomyopathies;
- liver diseases, such as cirrhosis, chronic hepatitis and tumours;
- kidney failure and low blood protein levels;
- tumours in the abdominal cavity;
- infections, such as feline infectious peritonitis (FIP);
- injuries and bleeding within the abdominal cavity.
Albumin is a blood protein, and a very low level may be associated with fluid accumulation. Albumin can be lost in urine with kidney disease or through the intestine with severe intestinal disease. Liver disease, meanwhile, can disrupt protein production. Blood and urine tests help distinguish these possibilities.
In right-sided heart failure, pressure in the body's veins can rise and fluid can collect in the abdomen. This form of heart failure is uncommon in cats. A cardiac cause is still possible, however, and cannot be inferred from abdominal size alone. An ultrasound of the heart and other cardiac investigations may therefore be part of the assessment.
Risk factors
- older cats with chronic diseases;
- viral diseases, including FIP and FeLV;
- cats that are overweight or have metabolic disorders;
- a history of heart, liver or kidney disease.
The risk of ascites depends above all on the disease causing it. Being overweight or having general metabolic risk factors does not alone establish that there is fluid in the abdomen; information on shared risk factors in cats is limited. A young cat can also develop FIP, for example, so age does not rule out an important cause.
Symptoms
Symptoms of ascites develop gradually as fluid accumulates:
- an enlarged abdomen that does not become smaller after eating or passing stools;
- difficulty breathing because of pressure on the diaphragm;
- tiredness and reduced activity;
- loss of appetite and weight loss despite an enlarging abdomen;
- a sensation of fluid movement when the abdomen is felt.
The rate of fluid accumulation varies. With peritonitis or internal bleeding, a cat's condition may change quickly, while some other causes progress more slowly. General condition, breathing and pain are therefore at least as important to observe as abdominal size.
A fluid-like movement noticed at home does not confirm the diagnosis, and its absence does not rule out fluid. The vet combines feeling the abdomen with imaging and other investigations. Also explain whether your cat has lost weight elsewhere while the abdomen has grown, and how long appetite has been poor.
Possible complications
Ascites always indicates a serious underlying disease. Without investigation and treatment of the cause, the condition progresses. Fluid can press on internal organs, disrupt heart and lung function and cause chronic respiratory failure. Sometimes an infection or tumour is responsible, making the prognosis worse still.
The word ascites alone does not determine the prognosis. Peritonitis, heart failure, protein loss and tumours require different treatments. Fluid accumulation associated with FIP is no longer automatically regarded as untreatable either: antiviral medicines have produced good results in treatment planned by a vet.
Diagnosis at the vet
Investigating ascites requires veterinary diagnostic procedures such as:
- a clinical examination and palpation;
- abdominal ultrasound;
- X-rays;
- blood and urine tests to identify the underlying cause;
- examination of the abdominal fluid, including cytology, bacterial culture and biochemistry.
The composition of the fluid helps establish whether it is associated with infection, a tumour or a metabolic disorder.
The history establishes how quickly the abdomen enlarged, previous illnesses, possible injury and changes in eating, urination and passing stools. The vet also listens to the heart and lungs. This looks for evidence of a problem affecting the whole body rather than treating the abdomen as an isolated area.
A blood count may reveal anaemia, changes consistent with inflammation or platelet abnormalities. Biochemistry examines albumin and values reflecting liver and kidney function, among other things. A urine sample helps establish whether kidney findings are accompanied by protein loss. An abnormal protein value therefore does not identify the affected organ on its own.
Why take a fluid sample?
Abdominocentesis means collecting a sample of abdominal fluid with a thin needle. The vet selects a safe sampling site and uses ultrasound to help if needed. The sample can be examined under a microscope and used for other targeted tests. Testing the fluid differs from removing a large volume to make your cat more comfortable.
The sample's appearance provides clues, but a clear or cloudy appearance does not confirm the diagnosis alone. Tests look for inflammatory cells, bacteria or cells suggesting a tumour, for example. Results are compared with blood values and imaging. If the sample does not provide an answer, a cell or tissue sample from a suspicious organ may be needed.
Investigating suspected FIP
In FIP, the inflammatory response affects areas including those around blood vessels, and fluid may accumulate in the abdomen or chest cavity. Not every cat exposed to coronavirus develops FIP. A routine antibody finding indicates exposure and does not establish the cause of ascites on its own.
The diagnosis is assessed using a combination of symptoms, blood tests, fluid characteristics and, when needed, tests directed at the virus. PCR can identify viral genetic material in tissue or fluid, but it too has limitations. A single result is not interpreted separately from the overall situation, because both false-negative and false-positive results can occur.
Treatment and follow-up
The first aim of treatment is to secure breathing and circulation and correct any severe disturbances. Peritonitis may require hospital care, fluid therapy, antimicrobials and sometimes a blood transfusion. If intestinal contents, urine or bile, for example, are leaking into the abdomen, surgery may be needed to correct the source of the leak.
Fluid accumulation associated with heart failure may be treated with diuretics and other medication suited to the heart condition. The choice depends on the cause and severity of the failure. Ascites itself does not mean that every cat should receive the same diuretic. The vet also assesses kidney function and fluid balance.
If the volume of fluid interferes with breathing or causes other significant problems, the vet may consider removing it. This may relieve symptoms, but the fluid can return unless the underlying cause is controlled. The need for drainage and its extent are assessed individually. Sampling and drainage for relief may be part of the same care plan, but they have different aims.
A cat with liver disease or a prolonged poor appetite may need nutritional support. The diet is tailored to the diagnosed disease; the same liver or kidney diet does not automatically suit every cause of ascites. A feeding tube can provide nutrition if needed. Eating enough is part of treatment after a procedure too.
If FIP is suspected, antiviral treatment is decided according to the diagnosis and your cat's overall situation. Care may also include support for nutrition, fluid balance and other symptoms. New treatment options do not remove the need for prompt investigation, because similar-looking fluid accumulation may be caused by an entirely different disease.
What should you monitor?
At home, record appetite, activity, breathing effort and changes in abdominal size. A care plan for a cat with heart disease may include monitoring breathing rate during sleep or rest. Repeated observations help the vet assess the response to treatment. Visible difficulty breathing still requires treatment without a prolonged period of measurements at home.
Contact the treating vet if the abdomen starts growing again, your cat vomits or eating worsens. Follow-up may involve blood values, urine tests and, if needed, fluid examination or imaging. The schedule depends on the disease identified and the treatment response. Medication is adjusted according to follow-up findings, rather than abdominal appearance alone.
Prevention
Because ascites results from other diseases, prevention is based on managing them:
- regular veterinary check-ups;
- vaccination and protection against infections;
- appropriate nutrition to support the liver and kidneys;
- weight management and prevention of excess weight;
- monitoring and early treatment of chronic diseases.
Vaccinations do not prevent every cause of ascites. Managing FIP risk emphasises measures such as hygiene and avoiding crowded, stressful conditions, but preventing all infections can be difficult. Chronic disease care is planned according to its own findings. New abdominal enlargement needs investigation despite previous preventive care.
This text provides general information and does not replace veterinary care. If your cat develops symptoms, contact a vet.
Frequently asked questions
Does a swollen abdomen always mean ascites?
No. Fat, gas, enlarged organs and various masses can also enlarge the abdomen. Ultrasound and other investigations help establish whether free fluid is present in the abdominal cavity. Appearance alone is not enough for a diagnosis.
Is ascites cured by removing the fluid?
Removing fluid can improve comfort or breathing, but the underlying disease needs its own treatment. Otherwise, fluid may build up again. The vet decides whether drainage is needed and how other treatment should continue.
Is ascites in a cat always FIP?
No. Possible causes also include heart and liver diseases, protein loss, tumours and various forms of leakage into the abdominal cavity. Diagnosing FIP also requires combining several findings. The cause of fluid accumulation is not inferred from your cat's age alone.
This article is for general information only and is not a guide to treatment. If you notice the first signs of illness, contact your vet.
Paw4care editorial team
Sources
- Testing for Abdominal Enlargement in Cats, VCA Animal Hospitals, accessed 2026-11-02.
- Feline Infectious Peritonitis, Cornell Feline Health Centre, accessed 2026-11-02.
- Peritonitis in Cats, Merck Veterinary Manual, accessed 2026-11-02.
- Heart Failure in Cats, Merck Veterinary Manual, accessed 2026-11-02.
- Disorders of the Liver and Gallbladder in Cats, MSD Veterinary Manual, accessed 2026-11-02.