What Causes Ascites in Dogs and How Is It Treated?

Ascites is the accumulation of free fluid in a dog’s abdominal cavity, and it is almost always a sign of a serious underlying disease rather than a standalone condition. Heart failure, liver disease, cancer, and severe protein loss are the most common drivers, and the specific cause shapes both the treatment and the outlook. Because fluid can build up gradually or appear suddenly after an emergency like internal bleeding, recognizing it early and getting a proper diagnosis matters more than simply draining the belly.

What Causes Fluid to Build Up in a Dog’s Abdomen

The abdomen normally contains a tiny film of fluid that lubricates the organs as they move. Ascites develops when something tips the balance so that fluid enters the abdominal space faster than it can be reabsorbed. Several broad mechanisms are at work, and most dogs with ascites have one or more of the following going on:

Increased blood pressure in the portal system: The portal vein carries blood from the intestines and spleen through the liver. When the liver is scarred or obstructed, pressure backs up and forces fluid out of blood vessels into the abdomen.
Low blood protein: Albumin in the bloodstream acts like a sponge that holds fluid inside vessels. When albumin drops, fluid leaks out into tissues and body cavities.
Heart failure: A failing right side of the heart cannot accept returning blood efficiently, so pressure rises in the veins draining the abdomen and fluid weeps out.
Inflammation or infection: Peritonitis and other inflammatory conditions make blood vessel walls leaky, allowing protein-rich fluid and white blood cells to flood the abdomen.
Lymphatic obstruction: When lymph channels in the abdomen are blocked or ruptured, lymphatic fluid (sometimes chyle, a milky fluid rich in fat) accumulates.
Bleeding: A ruptured tumor or traumatic organ injury can fill the abdomen with blood.

In practice, many dogs have more than one mechanism contributing at the same time. A dog with advanced liver disease, for instance, often has both portal hypertension and low albumin levels working together to push fluid into the belly.

Liver Disease and Portal Hypertension

Liver-related causes are among the most frequently diagnosed reasons for ascites in dogs. Chronic hepatitis, cirrhosis, and idiopathic hepatic fibrosis can all raise pressure inside the portal venous system. A retrospective study of 17 dogs that developed ascites from pre-sinusoidal portal hypertension found that idiopathic hepatic fibrosis and chronic hepatitis accounted for the majority of cases.¹ In these dogs, the fluid was a pure transudate, meaning it had very low protein and cell counts, which is a pattern that points toward high venous pressure rather than inflammation.

When bile flow is chronically obstructed, the liver can develop sinusoidal portal hypertension along with dramatic changes in blood circulation. Experimental models have shown that portal venous pressure roughly doubles in affected dogs, while blood flow through the portal vein drops to less than half its normal volume. The body compensates by opening shunt vessels that bypass the liver, but those shunts come with their own problems, including poor toxin clearance and a tendency toward further liver deterioration.²

For owners, the takeaway is that ascites tied to liver disease often signals advanced damage. The liver has a remarkable ability to compensate for injury, so by the time fluid appears in the belly, a large proportion of liver function may already be compromised.

Heart Failure

Right-sided congestive heart failure is the classic cardiac cause of ascites in dogs. When the right ventricle cannot pump blood forward effectively, pressure builds in the veins returning blood from the body. The hepatic veins and the veins draining the intestines are especially affected, and the result is fluid leaking from congested capillary beds into the abdominal space. Dogs with pericardial effusion, where fluid surrounds the heart and restricts its filling, can present in a similar way because the effect on venous return is the same.

Breeds prone to dilated cardiomyopathy, such as Doberman Pinschers and Great Danes, and breeds susceptible to tricuspid valve disease can develop ascites as a prominent feature of their heart failure. In these dogs the fluid tends to be a modified transudate with moderate protein levels, reflecting the chronically elevated venous pressure. Treatment focuses on the cardiac disease itself, with diuretics used to reduce the fluid load while the underlying heart condition is managed.

Bleeding Into the Abdomen

Hemoperitoneum, free blood in the belly, can look like ascites at first glance. In dogs, the most common cause is a ruptured splenic mass, and the tumor behind it is often hemangiosarcoma, a highly malignant cancer of blood vessel walls. A study comparing small and large dogs with spontaneous hemoperitoneum found that splenic hemorrhage was diagnosed in about 61% of large dogs but only about 43% of small dogs. Small dogs were more likely to bleed from the liver or from other sites like the kidney or adrenal gland.³ Hemangiosarcoma was more closely tied to splenic hemorrhage and occurred more often in large breeds.

Dogs with sudden hemoperitoneum often collapse without warning. Pale gums, a rapid heart rate, and a distended belly that appeared over hours rather than days are hallmarks of internal bleeding. This is a genuine emergency, and whether the cause is a tumor, a traumatic injury, or a bleeding disorder, the dog typically needs stabilization with intravenous fluids or a blood transfusion followed by surgery to control the source.

Infection and Septic Peritonitis

When bacteria invade the abdominal cavity, the body responds with a flood of inflammatory fluid. Septic peritonitis is one of the most dangerous forms of abdominal effusion, and it most commonly follows gastrointestinal perforation from a foreign body, a ruptured intestinal tumor, or a surgical dehiscence. A large retrospective review evaluated 161 dogs diagnosed with septic peritonitis based on the presence of intracellular bacteria or positive culture from the abdominal fluid.⁴

One of the most useful bedside tests for distinguishing septic from non-septic effusions is comparing the glucose level in the abdominal fluid against the glucose level in the blood. Research in dogs showed that when the blood glucose was more than 20 mg/dL higher than the peritoneal fluid glucose, the test was perfectly accurate at identifying a septic effusion. Measuring the difference between blood and fluid lactate worked similarly well.⁵ These quick comparisons can guide the decision to take a dog to surgery without waiting for culture results, which typically take days.

Less Common Causes

Chylous Ascites

Chylous ascites occurs when chyle, the milky lymphatic fluid that carries dietary fat from the intestines, leaks into the abdominal cavity. It is rare in dogs. A report of three cases found varying causes: one dog had a ruptured mesenteric lymphatic vessel identified during surgery, another had evidence of lymphatic obstruction, and the third developed chylous ascites as a complication of a diagnostic procedure performed for a different condition.⁶ The milky appearance of the fluid, along with a high triglyceride concentration, helps distinguish it from other types of effusion.

Uroperitoneum

A rupture anywhere along the urinary tract, whether the bladder, a ureter, or the urethra, can spill urine into the abdomen. The resulting uroperitoneum causes a distinctive pattern on blood work: rising kidney values (azotemia) and dangerously high potassium (hyperkalemia), paired with abdominal effusion. The potassium elevation is the most immediately life-threatening part because it can trigger fatal heart rhythm abnormalities.⁷ Trauma is a common cause, but bladder tumors and urinary stones can also erode through the urinary tract wall.

How Veterinarians Diagnose the Cause

Recognizing that a dog has fluid in its abdomen is usually straightforward. The bigger challenge is figuring out why. Diagnosis typically involves imaging, fluid analysis, and blood work, each contributing a different piece of the puzzle.

Ultrasound

Abdominal ultrasound is the quickest way to confirm free fluid and start looking for a cause. The AFAST (Abdominal Focused Assessment with Sonography for Trauma) protocol uses five standard views of the abdomen and includes a scoring system that gives a rough estimate of how much fluid is present.⁸ A study comparing AFAST findings against CT scans in recently traumatized dogs found moderate to excellent agreement for detecting free peritoneal fluid.⁹

One thing worth knowing: a small amount of free peritoneal fluid can be normal, especially in young female dogs. A study enrolling 92 healthy dogs found that 93% of juveniles and 52% of adults had some detectable fluid on AFAST.¹⁰ So a tiny amount of fluid on ultrasound is not automatically a disease finding. The context matters: how much fluid, what the dog’s clinical picture looks like, and what the fluid analysis shows.

Fluid Analysis

Once fluid is identified, the next step is to collect a sample with a needle (abdominocentesis) and send it for analysis. The lab measures the total protein concentration, the cell count, and examines the cells under a microscope. Veterinary guidelines recommend cutoffs of 3,000 cells per microliter and 2.5 g/dL of protein to help classify the fluid.¹¹ Fluid that falls below both cutoffs is a transudate, pointing toward portal hypertension or low albumin. Fluid above both is an exudate, suggesting infection or inflammation. Fluid between the two is called a modified transudate, which can be seen with heart failure, early cancer, or a mix of causes.

The cell count and protein level are just the starting point. The microscopic exam can reveal bacteria inside white blood cells (pointing to septic peritonitis), malignant cells (pointing to cancer), or red blood cells making up most of the sample (pointing to hemorrhage). Additional tests like triglyceride levels (to identify chyle), creatinine levels (to identify urine), and the glucose and lactate comparisons described earlier further narrow the diagnosis.

Treatment Approaches

There is no single treatment for ascites because there is no single cause. The fluid itself is a symptom, and draining it without addressing the underlying problem only provides temporary relief at best. That said, when the fluid buildup is severe enough to compromise breathing or cause significant discomfort, therapeutic drainage (paracentesis) is done as a short-term measure while longer-term treatment is set up.

Medical Management

Diuretics are the mainstay of medical management for ascites caused by heart failure or liver disease. Furosemide, a loop diuretic that drives sodium and water excretion through the kidneys, is usually combined with spironolactone, which blocks the hormone aldosterone that the body produces in excess when it senses low blood volume. A case study documented clinical recovery of a dog with ascites managed on this combination along with supportive care, with the dog returning to normal feeding and the fluid resolving within about two weeks.¹²

Dietary changes play a supporting role. Salt restriction helps reduce fluid retention, and high-quality protein in the diet supports albumin production by the liver.¹³ Dogs on diuretics need regular monitoring of their kidney values and electrolytes, because aggressive fluid removal can tip a dog from too wet to dangerously dehydrated in a short window.

Surgery and Interventional Procedures

When the cause is surgically correctable, such as a bleeding splenic mass, a ruptured bladder, or a perforated intestine, surgery is the definitive treatment. For septic peritonitis in particular, surgical exploration to find and fix the source of contamination is standard care, often paired with abdominal lavage and drain placement.

For dogs with refractory ascites that keeps recurring despite medical therapy, some experimental procedures have been tried. One report described a peritoneo-vesical shunt system, essentially a small pump implanted in the abdomen that automatically moves ascitic fluid into the bladder so the dog urinates it out. The device eliminated the need for repeated drainage over ten weeks, and by the end of that period the ascites had resolved and the dog’s blood protein levels had normalized.¹⁴ Another group evaluated saphenoperitoneal shunting, which redirects ascitic fluid into the venous system through the saphenous vein. The success rate was about 50%, with complications including infection and herniation in the remaining cases.¹⁵ These are niche procedures available primarily at referral centers and reserved for situations where conventional options have been exhausted.

What the Prognosis Looks Like

Prognosis varies enormously depending on the underlying cause, but ascites in general is a marker of advanced disease, and the numbers reflect that. In a study of dogs with chronic hepatitis, those that had ascites at the time of diagnosis survived a median of only about two weeks from diagnosis, compared to over two years for dogs with the same liver disease but no ascites.¹⁶ That stark difference underscores how much the fluid’s presence signals the degree of organ failure rather than being the main problem itself.

A broader look at dogs with intra-hepatic portal hypertension found a median survival of just 14 days, with nearly a third euthanized within two days of diagnosis. However, about a quarter survived longer than two months, and occasional dogs lived for years. The presence of jaundice, a sign that the liver can no longer process bilirubin, was the strongest predictor of a poor outcome.¹⁷

The outlook is quite different for surgically correctable causes. A dog with a ruptured bladder from trauma, for example, can recover fully once the rupture is repaired and the metabolic derangements are corrected. Similarly, dogs with hemoperitoneum from a benign splenic hematoma often do well after splenectomy. The prognosis darkens considerably when the bleeding is caused by hemangiosarcoma, which carries a very short expected survival even with surgery and chemotherapy.

What Owners Should Watch For

A gradually enlarging belly is the most obvious sign, but early ascites can be surprisingly easy to miss, especially in overweight dogs or breeds with deep chests. Other clues include a dog that seems reluctant to lie down flat, breathes with more effort than usual (because fluid pressing on the diaphragm limits lung expansion), or loses interest in food while gaining weight. Some owners notice a sloshing feeling when they place their hands on opposite sides of the belly and tap gently.

If you notice any of these signs, getting to a veterinarian promptly changes the math on outcomes. Many of the treatable causes of ascites, like a ruptured bladder or a benign bleeding mass, have dramatically better results when addressed quickly. And even for causes that carry a poorer prognosis, early diagnosis gives you and your vet more room to try medical management before the disease has progressed to its most advanced stage.

One practical point that surprises some owners: repeated large-volume paracentesis, while it relieves discomfort, can itself cause problems. Each time fluid is drained, protein is removed along with it. In dogs that are already low on albumin, this creates a cycle where drainage temporarily helps but accelerates protein loss, making the fluid return faster and more aggressively. This is why veterinarians generally prefer to manage ascites medically when they can, reserving drainage for cases where breathing is compromised or the dog is visibly miserable.

Breed and Size Patterns

Certain breeds appear more often in the ascites caseload, largely because of their predisposition to the diseases that cause it. Doberman Pinschers are overrepresented in chronic hepatitis, which can progress to cirrhosis and portal hypertension. Golden Retrievers and German Shepherds carry a higher risk of hemangiosarcoma, the splenic cancer most commonly responsible for sudden hemoperitoneum. Cavalier King Charles Spaniels and other breeds prone to valvular heart disease can develop ascites as part of their heart failure progression.

Body size also matters diagnostically. As noted, large dogs with sudden abdominal fluid are more likely to be bleeding from the spleen, while small dogs with the same presentation are more likely to have hepatic or non-splenic bleeding sources.¹⁸ That difference can steer the diagnostic workup in different directions and affects which imaging or surgical approach the veterinarian prioritizes.