Canine Fluid in the Lungs: Signs, Causes, and When It's an Emergency

Go to an emergency veterinary hospital now if your dog is struggling to breathe, breathing with an open mouth while resting, refusing to lie down, or has blue, gray, or very pale gums. Fluid in or around the lungs can be life-threatening, and a dog in breathing distress needs oxygen and treatment right away (Kuehn, 2024a; Linklater, 2024).

Direct answer

When people say a dog has "fluid in the lungs," they usually mean one of two different problems. Pulmonary edema is fluid inside the lung tissue and air sacs (Kuehn, 2024a). Pleural effusion is fluid in the chest cavity around the lungs (Kuehn, 2024b; Linklater, 2024). Both cause fast, labored breathing, and both need prompt veterinary care.

Fluid inside the lungs is most often caused by left-sided heart failure, usually from mitral valve disease or dilated cardiomyopathy. It can also follow head trauma, seizures, electrocution, choking or airway blockage, smoke inhalation, and severe illnesses such as sepsis or pancreatitis (Lang & Glaus, 2010). Fluid around the lungs has a longer list of causes, including heart failure, low blood protein, infection, bleeding, and cancer (Kladakis et al., n.d.; Linklater, 2024).

Cancer can be involved. Tumors in the chest can cause bleeding or fluid buildup around the lungs (Kuehn, 2024b; Linklater, 2024), and many cancers spread to the lungs (Fossum, 2013). The outlook depends on the cause, and for many disorders that cause fluid in the chest, it's guarded to poor (Kuehn, 2024b).

Key points

• Pulmonary edema is fluid in the lungs. Pleural effusion is fluid around them. They have different causes and different treatments (Kuehn, 2024a, 2024b).
• Signs include fast or labored breathing, open-mouth breathing, weakness, and refusing to lie down (Kuehn, 2024a, 2024b).
• Left-sided heart failure is the main cause of fluid inside the lungs (Lang & Glaus, 2010).
• Draining fluid from around the lungs can quickly improve breathing and helps find the cause (Linklater, 2024).
• Mesothelioma, a cancer of the chest lining, is a common cause of non-infected fluid around the lungs (Linklater, 2024).
• A clear chest X-ray doesn't rule out cancer spread, because lung nodules generally must be about 0.5 to 1 cm before they show up (Fossum, 2013).

Detailed explanation

Fluid in the lungs versus fluid around the lungs

These two problems sound alike but happen in different places.

Pulmonary edema is an abnormal buildup of fluid in the tissue, airways, or air sacs of the lungs (Kuehn, 2024a).

Pleural effusion is fluid in the space between the lungs and the chest wall. The fluid may be clear, bloody, milky (chylothorax), or infected (pyothorax) (Kuehn, 2024b; Linklater, 2024).

Your veterinarian usually tells them apart with chest X-rays or ultrasound, both of which readily show fluid around the lungs (Linklater, 2024). Our article on why dogs need X-rays for cancer explains what chest imaging shows.

Signs to watch for

Dogs with fluid in the lungs breathe faster and with more effort. Many breathe with their mouths open, and they often stand or sit rather than lie down, or lie only on their chest (Kuehn, 2024a). Dogs with fluid around the lungs may take rapid, shallow breaths, labor to breathe in, and seem weak (Kuehn, 2024b). Blue-tinged gums, tiredness, and a poor appetite can also occur, though dogs with only a little fluid may show few or no signs (Linklater, 2024).

Fast breathing during sleep is often the first sign you can measure at home. Healthy adult dogs rarely breathe more than 30 times a minute while asleep (Rishniw et al., 2012), and a rise above your dog's own baseline is an early warning of heart failure (Keene et al., 2019). Our article on early signs of CHF in dogs shows you how to count, and the Dog Cancer Tracker can store the numbers.

Breathing changes in a dog with cancer have many possible causes. Our articles on panting in dogs with cancer and coughing in dogs with cancer cover the overlap.

Causes of fluid inside the lungs

Veterinarians divide pulmonary edema into two groups (Lang & Glaus, 2010).

Heart-related (cardiogenic) edema happens when pressure backs up from a failing left side of the heart and forces fluid into the lungs. In dogs, the main causes are advanced mitral valve disease and dilated cardiomyopathy (Lang & Glaus, 2010). Dogs with left-sided heart failure most often cough, have trouble breathing, and can't exercise as they used to (Kittleson, 2024).

Non-heart-related (noncardiogenic) edema has several causes (Lang & Glaus, 2010):

• Blocked upper airways, from choking, laryngeal paralysis, tracheal collapse, or a flat-faced (brachycephalic) airway
• Brain injury, seizures, and electrocution, which can trigger what's called neurogenic edema
• Severe lung damage from smoke or toxic gas, inhaling vomit or food, pneumonia, sepsis, burns, or pancreatitis
• Rapid re-expansion of a lung after fluid or air is drained from around it

The outlook varies widely. Dogs with neurogenic edema have a good chance of full recovery with supportive care, while edema from severe, widespread lung damage carries a poor prognosis even with intensive care (Lang & Glaus, 2010).

Causes of fluid around the lungs

Pleural effusion can come from right-sided heart failure, disease of the sac around the heart, low blood protein, heartworm disease, a diaphragmatic hernia, bleeding disorders, trauma, and cancer, and sometimes the cause is never found (Kladakis et al., n.d.). An emergency specialist's list of the five most common types includes (Linklater, 2024):

• Blood (hemothorax): usually from trauma or clotting problems such as rat poison, and sometimes from tumors
• Lymph fluid (chylothorax): often with no known cause
• Infection (pyothorax): from a migrating foreign object, a chest wound, or severe pneumonia
• Inflammatory or cancer-related fluid: mesothelioma is a common cause
• Clear fluid (hydrothorax): from low blood protein, liver failure, heart disease, or too much IV fluid

Removing the fluid with a needle, often guided by ultrasound, can quickly ease breathing. Testing the fluid then helps identify the cause (Linklater, 2024). Dogs in distress may need oxygen and sedation first (Linklater, 2024).

How cancer can cause fluid in or around the lungs

Tumors in the chest are one cause of bleeding around the lungs (Kuehn, 2024b), and cancer can also cause inflammatory fluid and milky lymph fluid there (Linklater, 2024). Because harmless reactive cells in the fluid can look like cancer cells, a pathologist should review the sample (Linklater, 2024).

Cancer in the lungs is more often spread from somewhere else than a tumor that started there. Cancers with a high likelihood of spreading to the lungs include mammary carcinoma, thyroid carcinoma, hemangiosarcoma, osteosarcoma, transitional cell carcinoma, squamous cell carcinoma, and oral and toe melanoma (Fossum, 2013). Chest X-rays in dogs with lung tumors are checked for enlarged lymph nodes and fluid around the lungs (Fossum, 2013). See our lung cancer in dogs hub and lung cancer symptoms article.

A normal chest X-ray is reassuring, but it isn't the final word. Lung nodules generally need to be about 0.5 to 1 cm before X-rays can show them, and contrast CT is the most sensitive way to find them (Fossum, 2013). Our articles on what lung cancer looks like on X-rays and whether your dog's cancer has spread explain more.

Treatment and outlook

Treatment depends on the cause. For fluid in the lungs, oxygen gives short-term relief, the underlying cause must be treated, and diuretics are typically used to remove excess fluid (Kuehn, 2024a). For fluid around the lungs, drainage can relieve breathing trouble and help with diagnosis (Kuehn, 2024b). Other options include surgery when indicated, antibiotics for infection, a chest tube when fluid keeps returning, and a procedure called pleurodesis for chronic, relapsing cases (Kladakis et al., n.d.). For lymph fluid with no known cause, surgery succeeds in 88% to 94% of dogs (Linklater, 2024).

For many disorders that cause fluid in the chest, the outlook is guarded to poor (Kuehn, 2024b). If cancer is the cause, ask your veterinarian or oncologist which options can keep your dog comfortable. Our article on when palliative care is the best choice may help.

When to contact a veterinarian

Go to an emergency hospital now if your dog:

• Is struggling to breathe or breathing with an open mouth while resting
• Won't lie down, or will only lie on the chest
• Has blue, gray, or very pale gums
• Collapses, even briefly
• Had head trauma, a seizure, an electric shock, smoke exposure, or a choking episode and is now breathing hard

Call your veterinarian the same day if:

• Your dog's sleeping breathing rate is repeatedly above 30 per minute or clearly above its usual rate
• A cough is new or getting worse
• Your dog has cancer and its breathing has changed

Questions to ask your veterinarian

• Is the fluid in my dog's lungs or around them?
• What do you think is causing it: the heart, cancer, infection, or something else?
• Should the fluid be drained and tested?
• Would a CT scan show anything the X-rays can't?
• What sleeping breathing rate should prompt me to call you?
• If cancer is the cause, what can we do to keep my dog comfortable?
• What signs mean the fluid is coming back?

References

Fossum, T. W. (2013). Pulmonary neoplasia [Conference proceedings]. World Small Animal Veterinary Association World Congress. VIN. https://vin.com/doc/?id=5709865

Keene, B. W., Atkins, C. E., Bonagura, J. D., Fox, P. R., Häggström, J., Fuentes, V. L., Oyama, M. A., Rush, J. E., Stepien, R., & Uechi, M. (2019). ACVIM consensus guidelines for the diagnosis and treatment of myxomatous mitral valve disease in dogs. Journal of Veterinary Internal Medicine, 33(3), 1127-1140. 

Kittleson, M. D. (2024). Heart failure in dogs. Merck Veterinary Manual, Dog Owners Version. Retrieved November 10, 2026, from https://www.merckvetmanual.com/dog-owners/heart-and-blood-vessel-disorders-of-dogs/heart-failure-in-dogs

Kladakis, S. E., Adamama-Moraitou, K. K., & Rallis, T. S. (n.d.). Pleural effusions in the dog: Etiology, pathogenesis, diagnosis and treatment. Journal of the Hellenic Veterinary Medical Society. https://ejournals.epublishing.ekt.gr/index.php/jhvms/article/view/15684 

Kuehn, N. F. (2024a). Pulmonary edema in dogs. Merck Veterinary Manual, Dog Owners Version. Retrieved November 10, 2026, from https://www.merckvetmanual.com/dog-owners/lung-and-airway-disorders-of-dogs/pulmonary-edema-in-dogs

Kuehn, N. F. (2024b). Accumulation of fluid or air in the chest cavity in dogs. Merck Veterinary Manual, Dog Owners Version. Retrieved November 10, 2026, from https://www.merckvetmanual.com/dog-owners/lung-and-airway-disorders-of-dogs/accumulation-of-fluid-or-air-in-the-chest-cavity-in-dogs

Lang, J., & Glaus, T. M. (2010). Cardiogenic and non-cardiogenic pulmonary edema: Pathomechanisms and causes [Conference proceedings]. World Small Animal Veterinary Association World Congress. VIN. https://www.vin.com/doc/?id=4516320

Linklater, A. (2024, May 30). Top 5 causes of pleural effusion in dogs. Clinician's Brief. https://www.cliniciansbrief.com/article/pleural-effusions-fluid-lungs-chest

Rishniw, M., Ljungvall, I., Porciello, F., Häggström, J., & Ohad, D. G. (2012). Sleeping respiratory rates in apparently healthy adult dogs. Research in Veterinary Science, 93(2), 965-969.