There are many possible causes of respiratory issues in dogs, and although some may be relatively superficial, others can be life-threatening. Learn more about this possible cause of fluid in your dog’s lungs.

One reason a dog could be struggling to breathe is pleural effusion. While not the most common cause, pleural effusion can indicate a serious underlying condition. Understanding more about pleural effusion in dogs could help you spot the concerning signs and get the prompt veterinary treatment needed.

What Is Pleural Effusion in Dogs?

Pleural effusion is a condition in dogs where fluids build up in the space between the heart and lung and chest wall. The fluid isn’t in a dog’s lungs, but rather surrounding the lungs.

Pleural effusion is sometimes confused with pulmonary edema. However, pulmonary edema is the accumulation of fluid in dogs’ lungs, rather than around them.

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Dr. Adesola Odunayo, DVM, is a Michael Schaer Endowed Distinguished Professor of Emergency and Critical Care Medicine at the University of Florida. She explains that pleural effusion, also known as water on the lung, is the accumulation of fluid in the pleural cavity. Also called the pleural space, the pleural cavity is the space in the chest that surrounds the lungs. In this space, thin tissue membranes, called pleura, cover the outer side of the lungs and the inside of the chest cavity.

It’s normal for the body to produce a small amount of pleural fluid for lubrication. However, too much of any type of fluid within the pleural cavity is abnormal. It ends up leading to respiratory problems and the potential for the lungs to collapse.

What Can Cause Pleural Effusion in Dogs?

Causes of pleural effusion in dogs are wide-ranging. Some possible causes include:

• Trauma

• Heart failure

• Cancer

• Ingestion of rodenticides like rat poison

• Infection

• Low protein levels

• Disease affecting the thoracic duct (which carries immune system cells back to the heart)

• Heartworm

• Liver or kidney disease

• Diaphragmatic hernia (a hole in the diaphragm)

A veterinarian might identify hemothorax as the cause of the fluid in a dog’s lungs. Hemothorax relates to bleeding in the pleural cavity (typically caused by things like trauma or ingestion of rat poison). A vet may find that pyothorax (pus in the pleural cavity) could cause the pleural effusion.

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Symptoms of Pleural Effusion in Dogs

Dr. Odunayo explains that clinical signs of pleural effusion primarily relate to struggling to breathe. Respiratory distress is common in cases of pleural effusion because the condition leads to reduced lung expansion, ultimately reducing the amount of oxygen a dog can get. Many dogs suffer from partial or full lung collapse.

“An affected dog usually has a fast breathing rate, also called respiratory rate. Dog owners may also notice labored breathing, purple or gray-colored gums or tongue, or breathing with the mouth open,” she says. A dog may also have a fever, stop eating, start coughing, or show signs of exercise intolerance and lethargy. The severity of the symptoms usually corresponds to the volume of fluid in the pleural cavity or how quickly the pleural cavity is filling. Depending on the cause of pleural effusion, there may also be other symptoms.

“It’s important to note that these signs generally indicate something is wrong with the respiratory system, and may not always be specific for pleural effusion,” Dr. Odunayo says. If your dog is having any breathing difficulties, seek veterinary support right away.

Diagnosing Pleural Effusion in Dogs

It’s important that a vet examines a dog to diagnose them in a systematic way. A vet will usually identify pleural effusion with various diagnostic imaging tests. Dr. Odunayo says a chest ultrasound is an easy, quick, and minimally stressful method.

However, she explains that this equipment isn’t available in all veterinary practices. Vets often use X-rays for diagnosis. “However, it can sometimes be a stressful test for a dog with respiratory distress, due to the need to have them lay still on the X-ray table,” she says.

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Fluid on a dog’s lungs is typically a sign of trauma or an underlying serious disease. Depending on a dog’s symptoms, other tests a veterinarian may need to arrange include:

• Thoracocentesis, followed by analysis of fluid
• Thoracoscopy to look at the space inside of the chest but outside of the lungs
• Blood, urine, or fecal tests to identify things like internal parasites, cancer, infectious diseases, or organ disease
• CT scan, MRI, biopsy, or fluid-analyzing flow cytometry test (which allows the vet to look closely at cells) to identify diseases such as cancer

Typically, a veterinarian will perform these tests once a dog is stable (and once immediate risk from the fluid around the lungs is over). X-rays, ultrasounds, and analysis of the removed fluid can also help uncover the cause of any underlying conditions.

Treatment of Pleural Effusion in Dogs

Prompt treatment for pleural effusion is very important. The first goal of treatment is to remove the fluid putting pressure on the lungs. Dr. Odunayo explains the veterinarian will perform this via a procedure called thoracentesis (or thoracocentesis). “The veterinarian inserts a needle or a catheter into the chest to drain the fluid,” she says.

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It’s rare for there to be complications with this procedure, and removing the fluid allows the lung to expand again. Dr. Odunayo says this immediately makes the dog more comfortable and restores oxygen to the pleural tissue.

Sometimes, a chest tube will remain in a dog for a period of time if the fluid is excessive or continues to develop rapidly. A chest tube allows for repeated drainage and cleaning of the pleural cavity. If the fluid is infectious, a vet may prescribe antibiotics for a dog.

But treating the pleural effusion isn’t enough on its own. A veterinarian will need to identify and treat the underlying cause to prevent the fluid from gathering again. This will provide a dog with the best chance of having a high quality of life.

Outlook and recovery times vary, depending on the severity of the pleural effusion, a dog’s overall health, and the seriousness and stage of the underlying condition. A dog is more likely to have a favorable prognosis if their owner seeks prompt veterinary treatment once they notice symptoms.

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