Sometimes, our pets can find themselves in breathing difficulty. There can be various causes of this but fluid in or around the lung tissues is a common cause in both cats and dogs. When the fluid forms within the air-filled sacs of the lung tissue, it is known as pulmonary oedema. The lungs are surrounded by a membrane called visceral pleura which is connected to the membrane that lines the inside of the chest cavity, called parietal pleura. When fluid forms between these two pleural membranes, the so-called pleural space, it is known as a pleural effusion. 

Symptoms of Pleural Effusion 

As a pleural effusion forms, the lungs ‘float’ within the fluid-containing pleural sac. This prevents the lungs from fully inflating during breathing causing difficulty breathing. Symptoms will be related to the need to put in more effort with breathing, but the speed of onset and severity of symptoms will depend upon the cause, how much fluid has accumulated, and whether the problem has developed suddenly or slowly. 

Early or mild cases may have a subtle change in the rate or depth (or both) of their breathing. This may be initially hard to see when they are at rest, but it may become more pronounced with exercise or play. Therefore, an affected pet can show some exercise intolerance or may seem lethargic or less interactive. 

With progressive, severe, or sudden cases, breathing difficulties will be much more pronounced. The rate of breathing will be increased in an effort to inhale enough oxygen for the body to function. As the lungs fail to adequately inflate, the depth of breathing can seem quite shallow or appear laboured with the abdomen sucking in and out. While healthy cats and dogs will usually breathe through their noses at rest, switching to open-mouth breathing can occur in the presence of a pleural effusion. 

Unlike dogs who pant with exercise and excitement, cats rarely breathe through their mouths, so if they do so, it can indicate severe compromise. If inhaled oxygen levels drop, the affected animal’s gums may develop a blue tinge. If you are ever concerned about your pet’s breathing, it is important to seek veterinary attention as soon as possible. Breathing difficulties can progress rapidly, and the sooner the problem is detected, the less suffering your pet will endure. 

Diagnosing Pleural Effusion 

One of the first things that your vet will do is listen to your pet’s lungs with a stethoscope. Healthy airways will sound very quiet, with even breath sounds across both lungs. When fluid forms, it blocks the transmission of sound, so areas of the lungs may sound muffled. Your vet may tap or ping the outer chest with a finger or utensil to listen for a difference in the percussion of sound between areas or sides of the chest. 

An oxygen saturation monitor may be attached to an area of thin skin, such as an ear flap, to assess how well the body is distributing oxygen to the tissues around the body. If this number is too low, your pet is in distress, or their gums are blue, your vet will probably need to administer oxygen through a mask, nose prongs or inside a tent before any further tests can occur. 

Because many things can cause breathing problems, imaging such as an x-ray or ultrasound is needed to diagnose a pleural effusion. For both of these, patients need to sit still and tolerate being put in positions that allow a good image to be taken. This may involve the use of some form of sedation. Your vet should discuss with you the risks associated with the use of these drugs. 

As gravity causes the fluid to pool in the lower portion of the cavity, there is sometimes a fluid line evident on x-rays. Multiple x-rays at different angles are usually necessary to determine the side, and severity of a pleural effusion. When severe, the structure of the heart and lungs may be completely hidden from view. 

Once identified, your vet will want to take a sample of this fluid to identify the type. This helps narrow down the possible causes and assists your vet in recommending additional tests or treatment plans. Different causes create different characteristics within the fluid which can be analysed by your vet clinic or an external laboratory. 

The process of collecting a sample is called thoracocentesis. Where tolerated, as much fluid as possible will be drained to allow the lungs to expand again. There are some risks with the procedure, but removal of even small amounts of fluid can produce immediate improvements in breathing. 

Your vet will likely recommend taking additional x-rays after draining, because the heart and lungs should be more visible once the fluid has been reduced. This helps to pick up structural changes which may have been hidden on the initial images.  

Types of Pleural Effusion 

The pleural effusion can be a watery consistency, a thicker fluid, or bloody, depending on the cause. Colour, consistency and components give clues as to the trigger. An examination under a microscope can assess the number and type of cells present. A handheld device called a refractometer is used to check protein levels. Most clinics can check these immediately. If indicated, your vet may want to send a sample to a laboratory for bacterial culture. 

Watery fluids with low protein and few cells are called a transudate. Watery fluids with moderate levels of protein and cells are called modified transudates. More viscous fluids are either exudates or chyle. Exudates refer to fluids with higher protein and high numbers of infection-fighting cells called neutrophils. Chyle is a milky fluid containing triglycerides, a type of fat. Blood may appear watered down or will have a similar appearance to blood drawn out of a vein. 

Causes of Pleural Effusion 

Heart Disease 

Heart failure arises when the heart can no longer compensate for an underlying disease or dysfunction. An ultrasound will likely be necessary to confirm this. Heart failure can affect the left side, the right side, or both. Due to reduced blood flow out of the heart, fluid accumulates within areas, causing a pressure change. This pressure change forces fluid out of the vessels and into neighbouring tissues. 

Although this effusion is usually watery, heart failure can occasionally cause an effusion due to disruption of lymphatic flow. The lymph system is part of the body’s immune defence. Chyle is a milky substance comprising lymph and fats which have been absorbed from the digestive system. An effusion of chyle into the chest is called chylothorax. 

Cancer 

Cancerous growths in the chest can be primary or secondary. Primary growths start in this location, whereas secondary growths represent a cancer that has spread (metastasised) from elsewhere. The lungs are a common place for many cancers to metastasise to. These growths can cause watery effusions. 

X-rays of the abdomen, ultrasound or an advanced imaging system such as a CT or MRI can help identify other cancerous growths. A biopsy will be necessary to identify the type of cancer present. Lymphoma is a common type of cancer in this area. 

Infections 

Infections that get into the pleural space can result in the production of pus (a type of exudate) as with an abscess. This is known as a pyothorax. This can occur as a result of severe pneumonia but is more likely to occur with deep bite wounds or similar penetrating wound injuries. It can also occur as a result of a foreign body getting into the chest cavity, such as a burrowing grass seed that has entered through the skin, or a swallowed object that pokes through from the oesophagus. 

Feline Infectious Peritonitis (FIP) is a possible complication from the Feline Coronavirus. FIP can trigger inflammation of blood vessels (vasculitis) which can allow fluid to leak out. If the affected vessels are in the chest, a watery pleural effusion can occur. Some affected cats may also have fluid loss into the abdomen as well (ascites). FIP is more common in young cats. 

Trauma 

Sometimes our pets unfortunately get injured through falls, fights, and vehicle strike. Both blunt force and penetrative trauma can cause pleural effusions. The type of effusion will depend upon what damage has occurred. 

Bleeding will result from ruptured vessels. Twisting of a lung lobe (lobe torsion) or a tear in the diaphragm (diaphragmatic hernia) can lead to watery effusions. If the lymphatic system within the chest is damaged, a chylothorax could occur. A pyothorax may develop if foreign debris enters the chest during the traumatic event. Sometimes, an effusion can be a delayed complication after a traumatic injury. 

Hypoalbuminaemia 

A low level of the blood protein albumin is known as hypoalbuminaemia. This occurs if albumin is under produced for demand or is lost out of circulation. Reduced production occurs with some liver diseases. Excessive losses occur with some forms of intestinal disease or kidney disease. It can also occur with severe inflammation. A blood test will be recommended to check protein levels and assess organ function. An ultrasound of the intestinal tract and kidneys will also be recommended. 

Other Causes 

Chylothorax can occasionally occur spontaneously. When no cause is found, it is referred to as idiopathic. Torsion of a lung lobe can be idiopathic also. Although a diaphragmatic hernia is usually traumatic, it can be a birth defect. These hernias allow organs from the abdomen to move into the chest space. This physically compromises breathing but also causes effusions. 

Exposure to rat baits (rodenticides) can result in poisoning. These products affect the blood’s ability to clot and can lead to bleeding into tissues and body cavities. A clotting blood test can be used to diagnose rodenticide toxicity. Also, any disease that triggers blood vessel inflammation can lead to a pleural effusion. 

Treatment of Pleural Effusions 

As effusions result from some injury or underlying disease process, treatment is focused on managing the underlying condition (where possible) and keeping fluid out of the pleural space. After the initial drainage, some animals may require repetitive drainage to combat ongoing fluid formation. This may necessitate the placement of a fixed chest drain. 

Where a foreign body or focal abscess is identified, surgery will likely be required to extract these. If a pyothorax is present, the pleural space may need to be flushed to help reduce the infection debris. These procedures have risks and may involve referral to a specialist veterinary facility. Antibiotics will be started upon confirmation of a pyothorax but may need added to or changed once culture results are obtained from a laboratory. 

With idiopathic chylothorax, medical management can be utilised. As chyle contains absorbed fats, a low-fat diet and appropriate nutritional supplements can help control this condition. If medical management is not successful however, surgical intervention may be necessary. 

Where heart failure, cancer, or FIP are the cause, treatment may be palliative or symptomatic. Although all causes of pleural effusion can be serious and life threatening, these causes carry a much poorer prognosis because the effusion often results from advanced or ‘end-stage’ disease. A relatively new drug for FIP may be appropriate for some cats. 

Regardless of cause or type, treatment is often intensive, at least in the initial days. A hospital stay of several days will often be necessary, or referral to a specialist hospital could be required. Medical therapy such as antibiotics are usually necessary for multiple weeks. Some individuals will tolerate this level of intervention better than others. You and your vet will need to weigh up the balance of likely treatment success against the costs and intensity of that care. 

Conclusion 

A pleural effusion is one of several conditions that can cause breathing distress in pets. Swift veterinary intervention is important whenever you notice your pet struggling to breathe. A diagnosis requires imaging and a sample of fluid. The severity at presentation and underlying cause will impact the risks of treatment and the likely outcome of any interventions. Your vet can discuss with you the risks and prognosis of your pet’s specific condition. 

Further Reading: 

Ayoob-Wagner, A.L. (2024) ‘Stabilization of patients with pleural effusion prior to referral’ Clinician’s Brief.  

Beatty, J., Barrs, V. (2010) ‘Pleural effusion in the cat: a practical approach to determining aetiology’ Journal of Feline Medicine & Surgery, 12(9), pg. 693-707.  

Berry, C.R., Huguet, E.E., and Cole, R.C. (2022) ‘Radiographic diagnosis of pleural effusion and pulmonary edema in dogs and cats’ Today’s Veterinary Practice, 1.  

Murphy, K. and Papasouiotis, K. (2011) ‘Pleural effusions in dogs and cats 2. Placement of tubes and treatment’ In Practice, 33(10), pg. 526-530.  

Waddell, L. (2018) ‘Pleural Effusion’ in Textbook of Small Animal Medicine & Surgery I&II