However, pleurisy and pleural effusion are different things. Pleurisy describes inflammation of the pleura, while fluid can also accumulate because of circulatory or lymphatic problems without primary inflammation. Fluid around the lungs differs from pulmonary oedema, where it accumulates within lung tissue. Both changes can also occur together.
When to see a vet urgently
Go to a vet immediately if your cat develops any of the following:
- Open-mouth breathing or marked use of abdominal muscles to breathe.
- Inability to rest normally, instead sitting to breathe.
- Bluish gums or other mucous membranes, or collapse.
- Clearly laboured breathing at rest with weakness.
Do not wait for blue discolouration before leaving. Find a vet near you.
Overview
Normally, the pleural space contains only a little fluid, allowing the lungs to slide during breathing. Disease disturbs the balance: accumulating fluid compresses the lungs and prevents normal expansion. Severe cases develop respiratory failure.
Pleurisy means inflammation of the pleura, often accompanied by inflammatory fluid. Its cause can be infectious or non-infectious.
The cause affects treatment and recurrence risk. Removing fluid may ease breathing quickly but does not, on its own, establish the underlying disease. Similar breathing patterns cannot distinguish blood, lymph and inflammatory fluid either.
Causes
- Heart failure, most often with hypertrophic cardiomyopathy.
- Feline infectious peritonitis, or FIP.
- Tumours, such as lymphoma, carcinoma or metastases.
- Pyothorax, a pus-filled infection of the pleural space.
- Chylothorax, accumulation of lymphatic fluid due to thoracic duct damage.
- Low blood protein from severe liver, kidney or intestinal disease.
- Injuries and bleeding, causing haemothorax.
Chylothorax does not always result from traumatic thoracic duct injury. Heart disease, a tumour or an obstruction of venous blood flow can hinder lymph drainage, for example. In many cases, the cause remains unknown despite investigation, termed idiopathic chylothorax.
As well as trauma, haemothorax may result from a clotting disorder or tumour. In pyothorax, bacteria may enter the pleural space by spreading from the lungs or through a penetrating chest injury, for example. Distinguishing these requires fluid testing and assessment of the whole cat.
Risk factors
- older cats with tumours;
- cats with chronic heart failure;
- young cats from catteries and multi-cat homes, with greater FIP risk;
- cats recovering from chest injuries;
- cats with chronic liver or kidney diseases.
The list guides suspicion but does not exclude other cats from investigation. FIP can develop at any age, for example, although young cats are an important risk group. Heart disease may also be unknown before respiratory signs appear, and not every affected cat has a murmur.
Typical symptoms
- rapid, shallow breathing;
- shortness of breath that worsens with exertion and also occurs at rest;
- sitting with the neck extended and elbows out, an enforced breathing posture called orthopnoea;
- cyanosis, meaning bluish mucous membranes;
- reduced appetite and lethargy;
- chest enlargement with a large fluid accumulation.
Earlier changes may include less movement, weight loss or a reduced appetite. A cat may repeatedly seek a different resting position. However, obvious effort when breathing in or open-mouth breathing indicates an urgent situation where home monitoring is insufficient.
Symptoms cannot reliably locate the respiratory problem. Asthma, pulmonary oedema and other diseases can cause similar difficulty. Squeezing the chest or forcing an examination position can increase breathing effort, so minimise handling during transport.
Complications and consequences
- severe respiratory failure;
- fluid accumulating again;
- sepsis with infected fluid;
- death without timely treatment.
Long-standing chylothorax can also irritate the pleural surface and cause changes that stiffen it. The lungs may then fail to expand fully even after fluid removal. Recurring fluid may require repeated drainage or other long-term treatment.
Diagnosis at the vet
- clinical examination and listening for quiet breathing sounds;
- chest X-rays showing opacity from fluid;
- pleural ultrasound to assess fluid amount and characteristics;
- thoracocentesis, puncturing the chest to collect a fluid sample;
- fluid cytology and biochemistry to distinguish transudate, exudate and chyle;
- a complete blood count and blood biochemistry to assess liver and kidney function and inflammation;
- further investigations for FIP, FeLV and tumours.
Supporting breathing before extensive investigations
At the practice, a cat may first receive oxygen and time to settle with minimal handling. If pleural fluid substantially impairs breathing, drainage may be needed before all X-rays or other extensive tests. The order is matched to condition so unnecessary stress does not worsen oxygen deprivation.
Ultrasound helps detect fluid and guide sampling, but cannot determine exact composition alone. Fluid can also obscure heart or tumour outlines on X-rays. Once breathing has improved and fluid is removed, imaging may be repeated to assess underlying changes.
What is tested in the fluid?
Cytology looks for inflammatory cells, bacteria and possible tumour cells, for example. Appearance, protein and cell count help classify fluid, but classification alone does not determine the underlying disease. Suspected bacterial infection also needs culture, potentially with samples suitable for both oxygen-using and anaerobic bacteria.
When investigating chyle, fluid fat measurements may be compared with blood. A milky appearance alone does not confirm chylothorax. If FIP is possible, fluid is assessed for compatible cell and protein changes and, where needed, coronavirus genetic material or antigen. An ordinary coronavirus antibody finding alone does not confirm FIP.
Heart ultrasound helps establish the contribution of failure or other heart disease. Suspected tumours sometimes need a separate cell or tissue sample despite fluid examination. Blood tests may assess anaemia, protein levels and other organs.
Treatment
- First aid: thoracocentesis to remove fluid and restore breathing.
- Treatment of the underlying disease:
- for heart failure, diuretics and management of hypertrophic cardiomyopathy;
- for FIP, antivirals and supportive care;
- for tumours, surgery or chemotherapy;
- for chylothorax, surgery and a low-fat diet;
- for pyothorax, antibiotics and drainage;
- supportive care with oxygen, fluids and nutrition.
Thoracocentesis is performed by a vet; it is not home first aid. If large amounts form or fluid rapidly returns, a chest drain may be placed. This permits repeated drainage without a new needle puncture each time. A cat with a drain needs appropriate hospital monitoring.
The components are not applied identically to every cause. Diuretics are needed in heart failure, for example, but do not replace drainage of pus and treatment of bacterial infection. Intravenous fluids are also assessed individually: breathing difficulty or heart failure is not automatically treated with large fluid volumes.
Diet and possible surgery for chylothorax are planned individually. Low-fat food alone is generally insufficient to prevent lymph accumulation. Surgery may aim to redirect lymph flow but does not succeed in every cat. Long-standing pleural changes and the underlying disease affect outcome.
Antivirals have changed the prognosis for FIP, and fluid accumulation does not automatically mean a hopeless situation. Diagnostic certainty, general condition and response are assessed together. For cancer, options depend on tumour type and spread, rather than fluid volume alone.
Home monitoring and recurrence
Discharge instructions cover medicines, nutrition, activity and follow-up. Easier breathing after drainage does not mean the cause is cured. Contact a vet immediately if breathing becomes laboured again or your cat cannot find a resting position. Do not increase diuretics yourself.
Once your cat is stable, the vet may advise monitoring resting breathing at home. Observe a sleeping or calm cat that is not purring. One chest rise and fall is one breath. Record findings and compare them with your cat's baseline and the vet's contact instructions. Counting must not delay care when breathing is difficult.
Monitor appetite, weight trends and alertness too. Persistent poor intake and weight loss need nutritional reassessment. With long-term disease, follow-up monitors both the condition and possible fluid recurrence. Prognosis is refined according to findings and response.
Prevention
- Take at-risk cats, including those with heart disease and older cats, for regular assessment.
- Vaccinate against infectious diseases.
- Monitor and treat chronic liver, kidney and heart diseases.
- Prevent injuries.
Vaccines do not prevent every cause of effusion, such as heart disease or idiopathic chylothorax. Vaccination is planned according to infection risk. A breathing change in an at-risk cat should not wait until the next routine check.
Important for cat owners
Pleural effusion is life-threatening and needs urgent help. Remember that laboured breathing, sitting with the neck extended, open-mouth breathing or sudden severe lethargy warrants an immediate veterinary visit. Timely thoracocentesis and treatment of the cause can save your cat's life.
This article provides general information and does not replace veterinary care. If you notice signs of illness in your cat, contact a vet.
Frequently asked questions
Is pleural fluid the same as pulmonary oedema?
No. Pleural fluid is around the lungs, while pulmonary oedema involves fluid in lung tissue. Both can impair breathing and occur in the same cat.
Can ultrasound identify the cause of the fluid?
Generally not alone. Ultrasound helps find fluid and other changes, but cytology, composition and other investigations are needed to establish the cause.
Does draining the fluid cure the cat?
It can ease breathing quickly, but the underlying disease needs its own treatment. Fluid may return, so agreed monitoring matters.
Is low-fat food enough to treat chylothorax?
Generally not alone. Diet may be part of care, but drainage and sometimes surgery are also needed. Selection depends on the cat's situation and findings.
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
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- Pleural Effusion in the Cat: A Practical Approach to Determining Aetiology, Journal of Feline Medicine and Surgery, accessed 2026-11-03.
- Guideline for Feline Infectious Peritonitis, ABCD, accessed 2026-11-03.
- ACVIM Consensus Statement Guidelines for the Classification, Diagnosis, and Management of Cardiomyopathies in Cats, ACVIM / Journal of Veterinary Internal Medicine, accessed 2026-11-03.
- Initial Triage and Resuscitation of Small Animal Emergency Patients, Merck Veterinary Manual, accessed 2026-11-03.
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