Ascites in dogs, the buildup of fluid in the abdomen, is almost always a sign of an underlying disease rather than a condition on its own. The most common causes are right-sided heart failure, liver disease, and conditions that cause severe protein loss. Identifying which one is responsible matters because the fluid itself is just a symptom, and treating it effectively depends on addressing the root problem.
How Fluid Builds Up in the Abdomen
Under normal conditions, a small amount of fluid moves in and out of the spaces between your dog’s organs. This movement is governed by a balance of pressures: the force of blood pushing fluid out through vessel walls and the pulling force of proteins (mainly albumin) that draw fluid back in. When something disrupts this balance, fluid leaks faster than the body can reabsorb or drain it, and it pools in the abdomen.
Three things can tip that balance. First, pressure inside the blood vessels can rise too high, forcing more fluid out than normal. Second, the protein levels in the blood can drop too low, weakening the body’s ability to pull fluid back. Third, blood vessel walls can become leaky from inflammation or infection, allowing fluid and proteins to escape freely. Most diseases that cause ascites involve one or more of these mechanisms. The lymphatic system normally acts as a safety valve, draining excess fluid back into circulation, but when the rate of fluid leakage overwhelms lymphatic capacity, the abdomen begins to fill.
Heart Failure
Right-sided heart failure is one of the most common causes of ascites in dogs. When the right side of the heart can’t pump blood forward efficiently, blood backs up into the veins returning from the body. This raises pressure throughout the venous system, and fluid is pushed out of the blood vessels into the abdominal cavity, the chest, and the tissue around organs like the liver and spleen.
Several heart conditions can lead to this. Dilated cardiomyopathy, where the heart muscle weakens and stretches, is a major one, particularly in large breeds. Pericardial disease, where fluid or thickening around the heart physically restricts it from filling properly, produces the same venous backup. Dogs with long-standing mitral valve disease (a left-sided problem) can eventually develop right-sided failure too, because chronic pressure buildup in the lungs forces the right side of the heart to work harder until it fails. Heartworm disease can also obstruct blood flow through the right heart and the large veins entering it.
A rare congenital defect called cor triatriatum dexter, where a membrane divides the right atrium, can cause ascites even in young dogs by blocking normal blood flow.
Liver Disease
The liver plays a central role in ascites for two reasons: it produces albumin, the main protein that holds fluid in the bloodstream, and it receives a huge volume of blood through the portal vein. When liver disease damages either of these functions, ascites often follows.
Chronic hepatitis and cirrhosis are the most common liver-related causes. As inflammation and scarring progress, the tiny blood channels inside the liver (sinusoids) become stiff and narrow. This raises pressure in the portal vein, the vessel that carries blood from the intestines to the liver. The increased portal pressure triggers a chain of events: blood vessels in the gut dilate, which paradoxically increases blood flow into the already congested portal system, worsening the pressure further. The body responds by retaining sodium and water, adding even more fluid volume.
At the same time, the diseased liver produces less albumin. When blood albumin drops below about 2 g/dL, the pulling force that normally keeps fluid inside blood vessels becomes too weak, and fluid escapes into the abdomen. In advanced liver disease, fluid essentially weeps from the surface of the liver itself as overwhelmed lymphatic channels beneath its capsule can no longer keep up.
Congenital vascular problems can also cause portal hypertension in younger dogs. Portal vein atresia, where the portal vein fails to develop properly, and arteriovenous malformations, where high-pressure arterial blood flows backward into the portal system, are less common but well-documented causes. Blood clots in the portal vein and tumors that compress it can produce the same effect in older dogs.
Protein-Losing Conditions
Any disease that drains protein from the body can cause ascites by lowering oncotic pressure, the force that keeps fluid in the bloodstream. Two conditions are particularly important in dogs.
Protein-losing enteropathy (PLE) involves the loss of protein through a damaged intestinal lining. Inflammatory bowel disease, intestinal lymphoma, and lymphangiectasia (where the lymphatic vessels in the gut wall become dilated and leaky) can all cause it. PLE typically lowers both albumin and globulin levels in the blood, a pattern called panhypoproteinemia. When your vet sees both protein types dropping simultaneously, PLE moves high on the list of suspects.
Protein-losing nephropathy (PLN) works differently. Here, the kidneys’ filtering units become damaged and allow albumin to spill into the urine. Unlike PLE, PLN tends to lower albumin selectively while globulin levels stay closer to normal. A urine protein-to-creatinine ratio helps distinguish kidney-related protein loss from gut-related loss. Breeds like Soft-Coated Wheaten Terriers and Bernese Mountain Dogs are genetically predisposed to PLN.
Cancer and Abdominal Masses
Tumors in the abdomen can cause ascites through several routes. A tumor on the liver or spleen may bleed, producing a hemorrhagic effusion. Cancers like lymphoma or carcinomatosis can seed the lining of the abdomen, triggering inflammation and fluid production. Masses that compress the portal vein or vena cava obstruct blood flow and raise venous pressure, much like heart failure does from a different direction. Hemangiosarcoma, a common and aggressive cancer in dogs, frequently causes bloody abdominal fluid when tumors on the spleen or liver rupture.
Infections and Inflammation
Infectious peritonitis, though less common in dogs than in cats, causes a protein-rich, cell-heavy fluid to accumulate. Bacterial infections from a ruptured intestine, a perforated ulcer, or a migrating foreign body can spill bacteria directly into the abdomen, producing septic peritonitis. Pancreatitis, ruptured abscesses, and bile leakage from a damaged gallbladder also trigger inflammatory fluid buildup. These effusions tend to be exudates, meaning they contain high levels of protein (often above 4 g/dL) and large numbers of inflammatory cells.
Chylous Ascites
When the lymphatic system in the abdomen is damaged or obstructed, lymph fluid rich in fat leaks into the abdominal cavity. This is called chylous ascites, and it has a characteristic milky appearance. The fluid contains triglyceride levels higher than those in the blood, usually more than twice the serum concentration, and often above 100 mg/dL. Causes include lymphatic tumors, trauma to the lymphatic duct, and sometimes heart failure that raises pressure in the veins where lymph normally drains.
What Ascites Looks Like
The most visible sign is a swollen, round, or “pot-bellied” abdomen. In early stages, the swelling can be subtle and easy to mistake for weight gain. As fluid accumulates, you may notice your dog becoming restless, reluctant to lie down, or breathing faster than usual. The fluid presses upward on the diaphragm, making it harder for the lungs to expand fully. Some dogs lose their appetite or seem uncomfortable after eating.
If your dog has had ascites before, Tufts University veterinary cardiologists suggest monitoring their waist size with a soft tape measure or tracking daily body weight. A sudden increase in either can signal fluid returning before the belly looks obviously distended.
How Vets Identify the Cause
Diagnosing ascites is straightforward. A physical exam, often combined with ultrasound, confirms fluid is present. The harder question is why. Your vet will likely recommend abdominocentesis, drawing a sample of the fluid with a needle, and analyzing it.
The fluid’s protein content and cell count help narrow the possibilities. A low-protein fluid with very few cells (a pure transudate, with protein below 2.5 g/dL and fewer than 1,500 cells per microliter) points toward low albumin from liver failure, PLE, or PLN. A higher-protein fluid with moderate cell counts (a modified transudate) suggests heart failure, portal hypertension, or a mass compressing veins. A fluid packed with cells and protein (an exudate) indicates infection, inflammation, or cancer spreading along the abdominal lining.
Blood work measuring albumin, liver enzymes, kidney values, and a complete blood count provides the next layer of information. Chest X-rays check for heart enlargement or fluid around the lungs. Echocardiography (heart ultrasound) can confirm or rule out heart failure. Abdominal ultrasound looks for liver changes, masses, or enlarged lymph nodes.
How Ascites Is Managed
Treatment targets the underlying cause. Draining the fluid with a needle (therapeutic abdominocentesis) provides immediate relief when the volume is large enough to cause breathing difficulty or significant discomfort, but it’s a temporary measure. The fluid returns unless the driving disease is controlled.
For heart failure, the standard approach combines diuretics to reduce fluid retention, medications that help the heart pump more effectively, and drugs that block the hormonal systems driving sodium and water retention. Spironolactone is commonly used alongside a loop diuretic like furosemide. If furosemide at higher doses stops working well, a related but more potent diuretic called torsemide may be substituted.
For liver disease, managing portal hypertension and supporting liver function are the priorities. A low-sodium diet helps limit fluid retention. Diuretics can reduce the volume of ascites, but they must be used cautiously because aggressive fluid removal can worsen kidney function in dogs with advanced liver disease.
For protein-losing conditions, treatment focuses on controlling the intestinal or kidney disease responsible for the protein loss. Dogs with PLE may need dietary changes, immunosuppressive therapy, or both, depending on the underlying intestinal disease. PLN is managed with medications that reduce protein leakage through the kidneys, along with dietary modification.
Infectious causes like septic peritonitis are surgical emergencies. The source of contamination needs to be found and repaired, the abdomen flushed, and antibiotics started promptly. Cancer-related ascites depends entirely on the tumor type and how far it has spread.