Understanding Microvascular Dysplasia in Dogs
Microvascular dysplasia (MVD), sometimes called portal vein hypoplasia, is a condition that affects the liver's microscopic blood vessels. In this disorder, the small portal veins inside a dog's liver are either underdeveloped or completely missing. This leads to decreased blood flow through the liver tissue, which can have significant effects on how well the organ works.
How Does MVD Affect the Liver?
The liver relies on a steady supply of blood to perform its many vital functions: processing toxins, synthesizing proteins for growth and repair, and maintaining normal metabolism. When these tiny vessels are missing or too small, the liver gets less blood than it needs. As a result, it may struggle to remove toxins from the bloodstream or produce enough proteins for healthy body function.
Who Gets MVD?
This condition is most commonly found in small breed dogs. Breeds with a higher risk include:
- Yorkshire Terriers
- Cairn Terriers
- Maltese
- Miniature Poodles
- Shih Tzus
- Lhasa Apsos
- Cocker Spaniels
- West Highland White Terriers
- Dachshunds
- Bichon Frises
MVD is rare in large breed dogs and cats, though some feline cases have been described.
Causes and Inheritance
MVD is considered inherited or congenital. Genetic studies suggest it's passed down through families in a polygenic fashion—meaning several genes contribute to risk. Both male and female dogs can be affected. Even parents with normal test results can produce puppies with MVD if they carry certain risk alleles.
Symptoms: What Do Owners Notice?
The severity of symptoms varies widely. Many dogs show no signs at all, while others display subtle or more serious effects:
- Poor weight gain or muscle development; smaller size than littermates
- Lethargy or seeming less active/intelligent than expected
- Gastrointestinal issues: vomiting, decreased appetite, diarrhea, pica (eating non-food items)
- Increased thirst and urination; straining or blood during urination due to urinary tract infections or stones
- Rarely, neurologic signs like head-pressing, ataxia (wobbly walking), seizures, or behavior changes if hepatic encephalopathy develops
The clinical signs of MVD are often milder than those seen with larger portosystemic shunts (PSS), which are bigger vascular anomalies sometimes present alongside MVD.
Diagnosing Microvascular Dysplasia
The diagnostic process usually starts with laboratory tests:
- Bloodwork: May be normal or show mild anemia; decreased proteins (albumin), BUN (blood urea nitrogen), glucose; increased liver enzymes.
- Urinalysis: Sometimes reveals dilute urine or ammonium biurate crystals.
- Bile acid testing: The serum bile acid test (before and after eating) is key—dogs with MVD often have mildly to moderately elevated postprandial bile acids.
- Protein C activity: Usually normal (>70%) in MVD but decreased in larger shunts.
Imaging studies, such as abdominal ultrasound, may show a small liver or reduced portal vein perfusion but can't confirm MVD alone. To definitively diagnose MVD, veterinarians must rule out larger shunts using advanced imaging (ultrasound, CT scan, MRI, nuclear scintigraphy) and confirm characteristic microscopic changes on a liver biopsy. Biopsies should come from multiple lobes since severity can vary within the same dog. Needle biopsies aren't recommended because samples are too small for accurate diagnosis.
Treatment Options for Dogs with MVD
MVD generally doesn't progress significantly over time for most dogs. Many live normal lifespans without ever developing illness related to their condition. Treatment depends on how sick the dog is:
- If there are only mild lab changes and no symptoms: No specific treatment or diet change needed.
- If there are clinical signs (especially neurologic): Management focuses on controlling metabolic effects.
- Diets: Restrict protein using highly digestible sources like dairy/soy; prescription hepatic diets preferred.
- Lactulose: Used to reduce ammonia absorption from intestines.
- Antibiotics: Metronidazole may be prescribed short-term to alter gut bacteria.
Liver supplements, such as S-adenosylmethionine (SAMe), vitamin E, milk thistle (silymarin), and ursodeoxycholic acid might be considered but haven't shown clear benefit for symptom-free dogs. Probiotics and yogurt could support gut health but evidence of effectiveness is limited here too.
Lifestyle & Monitoring Recommendations
Affected dogs should have regular monitoring of their liver enzymes, protein levels, and ammonia rather than frequent bile acid retesting. Owners need to watch for new symptoms—some dogs may develop secondary problems like urinary tract infections or stones as they age. If a macroscopic shunt is also present, surgery may help correct that issue—but not MVD itself.
MVD & Breeding Decisions
Avoid breeding affected animals! Because MVD has a strong hereditary component and high prevalence in certain breeds, breeding carriers risks passing it on even if both parents seem healthy by current tests. Selective breeding without genetic testing can't eliminate this disorder from lines.
The Takeaway on Microvascular Dysplasia in Dogs
MVD is an inherited defect of the tiny vessels within the liver that leads to reduced portal perfusion and impaired hepatic function. Most often seen in small-breed dogs worldwide, it ranges from silent to mildly symptomatic—and rarely causes severe disease. Diagnosis combines breed predisposition with lab findings (especially bile acids), imaging studies to rule out larger shunts, and confirmation by histopathology of the liver tissue itself. Most affected pets do well long-term but need ongoing monitoring—and responsible breeding practices are essential to reduce future cases.
