Ascites is the buildup of fluid inside a dog’s abdominal cavity, enough to cause visible swelling of the belly. It is not a disease on its own but a sign that something else is going wrong, most commonly with the liver, heart, or intestines. The fluid accumulates when pressure inside blood vessels rises too high or when the blood loses proteins that normally keep fluid from leaking out of the bloodstream.
How Fluid Builds Up in the Abdomen
Under normal conditions, a small amount of fluid moves between blood vessels and surrounding tissues, and the lymphatic system drains any excess. Ascites develops when this balance breaks down. There are two main ways it happens.
The first is increased pressure inside the blood vessels that supply the liver and gut. When blood can’t flow through the liver efficiently, pressure backs up in the portal vein, the major vessel that carries blood from the intestines to the liver. This forces fluid out of the blood vessels faster than the lymphatic system can remove it. The liver itself begins weeping fluid from its surface when its internal lymphatic channels become overwhelmed.
The second is a drop in blood protein levels, particularly albumin. Albumin acts like a sponge inside blood vessels, pulling fluid inward and keeping it from leaking out. When albumin drops too low, fluid seeps into the abdomen, the space around the lungs, or the tissues under the skin. Many dogs with ascites have both high vascular pressure and low albumin at the same time, which accelerates the problem.
Common Causes in Dogs
Several diseases can trigger ascites. The underlying cause determines how the fluid looks, how the dog responds to treatment, and what the long-term outlook is.
Liver Disease
Chronic hepatitis and cirrhosis are among the most frequent causes. As the liver becomes scarred, its tiny blood channels narrow and stiffen, blocking normal blood flow. This raises pressure in the portal vein system, eventually pushing fluid into the abdomen. Advanced liver disease also reduces the liver’s ability to produce albumin, compounding the problem. Liver tumors, including hepatocellular carcinoma and metastatic cancers like hemangiosarcoma, can also obstruct blood flow through the organ and trigger fluid accumulation.
Right-Sided Heart Failure
When the right side of the heart can’t pump blood forward effectively, pressure builds in the veins that return blood to the heart. This elevated venous pressure has a double effect: it pushes more fluid out of blood vessels into surrounding tissues, and it simultaneously makes it harder for the lymphatic system to drain that fluid back into the bloodstream. The lymphatic valves can become dysfunctional under sustained high pressure, creating a cycle where fluid production outpaces drainage. Dogs with right-sided heart failure are particularly prone to ascites, while cats with heart failure tend to develop fluid around the lungs instead. Pericardial disease (fluid or scarring around the heart) and heartworm disease can produce the same effect.
Protein-Losing Conditions
Two conditions cause dogs to lose albumin faster than they can replace it. Protein-losing enteropathy (PLE) involves albumin leaking through a damaged intestinal lining, often due to severe inflammatory bowel disease. Protein-losing nephropathy (PLN) causes albumin to spill out through the kidneys into the urine. Both can drive albumin levels low enough to trigger ascites, though this typically represents advanced disease.
Cancer
Tumors throughout the abdomen can cause ascites through several routes: blocking blood or lymphatic flow, producing inflammatory fluid, or causing internal bleeding. Hemangiosarcoma, lymphoma, and cancers that spread widely through the abdomen are particularly common culprits. Cancer is the second most important cause of portal hypertension after liver disease itself.
Signs You Might Notice
The most obvious sign is a swollen, distended belly that develops over days to weeks. In some cases the swelling appears suddenly. Dogs may look like they’ve gained weight even though they’re actually losing muscle mass elsewhere on their body.
Because the fluid presses upward on the diaphragm, breathing changes are common. You may notice your dog breathing faster than usual, breathing with more effort, or preferring certain positions to get comfortable. Some dogs grunt when they exhale. Other signs that often accompany ascites include increased thirst and urination, decreased appetite, vomiting, diarrhea, and general lethargy. These reflect the underlying disease rather than the fluid itself.
How Ascites Is Diagnosed
A veterinarian can often detect abdominal fluid through a physical exam by feeling a wave-like motion when tapping the side of the belly. Ultrasound confirms the presence of fluid and helps identify the underlying cause by revealing changes in the liver, heart, or other organs.
The most informative step is drawing a sample of the fluid with a needle, a procedure called abdominocentesis. At least 5 to 6 milliliters of fluid per kilogram of body weight needs to be present in the abdomen for a successful tap. The fluid’s appearance and laboratory analysis reveal a great deal about what’s causing it.
Clear, watery fluid with low protein content (below 2.5 g/dL) points toward low albumin conditions like liver failure, PLE, or PLN. Fluid with moderate protein levels (2.5 to 3.5 g/dL) suggests heart failure or early liver disease. Cloudy or bloody fluid with high protein content (above 3.5 g/dL) indicates infection, cancer, or internal bleeding. Your vet will combine the fluid results with blood work, imaging, and the dog’s history to identify the specific cause.
Treatment and Management
Treating ascites means treating whatever disease is causing it. The fluid itself can be managed, but it will keep returning unless the root problem is addressed.
Diuretics help the kidneys excrete more sodium and water, reducing the amount of fluid the body retains. Sodium restriction plays a critical role alongside diuretics. Research in dogs with portal cirrhosis found that ascites did not re-form after drainage as long as dietary salt was withheld, but refeeding salt allowed the fluid to reaccumulate. This happens because the diseased body retains sodium first, and the excess fluid follows. Your vet may recommend a low-sodium prescription diet as part of ongoing management.
When fluid accumulates rapidly or severely enough to impair breathing, draining the abdomen with a catheter provides immediate relief. This is a temporary measure. Repeated large-volume drainage can itself worsen protein loss, since each time fluid is removed, albumin leaves the body with it.
Beyond fluid management, treatment depends entirely on the cause. Heart failure may respond to cardiac medications. Liver disease may require anti-inflammatory drugs, dietary changes, and long-term monitoring. Protein-losing conditions need specific therapy directed at the gut or kidneys. Cancer treatment varies widely depending on the tumor type and how far it has spread.
Prognosis Depends on the Cause
The outlook for a dog with ascites varies enormously based on what’s driving it. Some causes are manageable for months or years; others carry a very guarded prognosis.
When ascites stems from advanced liver disease with portal hypertension, the numbers are sobering. A study of dogs with portal hypertension from acquired liver disease found a median survival time of just 14 days, with nearly a third of dogs euthanized within two days of diagnosis. However, the same study showed that almost a quarter of dogs survived longer than two months, and over 10% were still alive more than a year later. Dogs with ascites and cirrhosis across multiple studies had median survival times around 22 to 23 days, though some individuals lived considerably longer.
Dogs whose ascites is caused by right-sided heart failure may do better if the cardiac condition responds to medication. Those with treatable infections or certain forms of protein-losing enteropathy can also have reasonable outcomes. Cancer-related ascites generally carries a poor prognosis, particularly with aggressive tumors like hemangiosarcoma.
The presence of jaundice alongside ascites is a warning sign. In one analysis, jaundice was the only factor independently associated with a higher risk of death after controlling for age, roughly doubling the hazard compared to dogs without it. Age alone, interestingly, did not significantly affect survival in that study.