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American Maltese Association

A National Breed and Member club of the American Kennel Club

Liver Shunt/Microvascular Dysplasia

Written by: 
Vicki Fierheller

Portosystemic Vascular Anomalies (PSVA) and Microvascular Dysplasia (MVD)

It is now recognized that Maltese and other terrier type breeds (Yorkshire Terriers, Cairn Terriers, Tibetan Spaniels, etc.) all share a congenital inherited disorder of the liver.


One form is PSVA or better known as liver shunt. Liver shunt is a blood vessel that carries blood around the liver instead of through it. There are different kinds of shunts, but in Maltese the most common form seen is the extrahepatic (outside the liver) shunt. As one can imagine, having a shunt or any liver dysfunction is not a good thing as the liver plays an important role in the body. The liver detoxifies many chemicals, converts glucose into glycogen for food storage and aids in digestion. Clinical signs are most often seen at a young age. Puppies with shunts tend to be poor doers. They are small in size (runt of the litter), often have digestive problems (diarrhea, constipation, vomiting, and anorexia) and may exhibit neurological abnormalities from toxin buildup (circling, apparent blindness, head pressing, dementia, aggression, seizures and lethargy). The neurological signs are generally most noticeable after a meal. There may be increased drinking and urination. Some animals may not show any signs until older and occasionally (up to 20%), some dogs may appear asymptomatic. Dogs with liver shunts are slow to recover from some drugs such as barbiturates or sedatives. Others will develop bladder infections and stones from ammonium biurate crystals. Upon palpation, the kidneys may feel enlarged. When a shunt is suspected, the first course of action is bloodwork. In a general panel, the following may appear: low blood urea nitrogen (BUN), creatinine, cholesterol and glucose; higher liver enzymes (ALT, AST), low protein (albumin) and smaller than normal red blood cells (microcytosis). The most important blood test is the Serum Bile Acids Test, which will be discussed further on. Treatment for PSVA is extremely variable as is the prognosis. It may include surgical ligation (closing off the affected vessel so the blood will go through the liver) and/or dietary and drug management.


Rather than one or more large vessels that bypass the liver as in PSVA, MVD occurs at the microscopic cellular level. The blood will circulate through the liver, but at a compromised level varying from dog to dog. Of the two liver disorders, MVD is by far the most prevalent in Maltese. In fact, a large majority of the Maltese population has MVD. MVD dogs are divided into two groups: symptomatic and asymptomatic. ****Fortunately virtually all Maltese with MVD are asymptomatic and live full long healthy lives with no medical intervention There is no surgical treatment for MVD. The very few MVD dogs that are symptomatic (similar to the PVSA symptoms) may require dietary management (a low protein diet such as L/D) and drug treatment (lactulose, antibiotics) usually with good success. Again, the Total Serum Bile Acids Test is utilized as a test for MVD.

Testing And Differentiating between PSVA and MVD:

As mentioned above for both PSVA and MVD, the Total Serum Bile Acids Test (TSBA) is done as a screening test to determine liver function. It is not a genetic test, but it is the best tool we currently have. TSBA is the collection of a pre-meal blood sample and a 2-hour post-meal blood sample. Random 'fasting' bile acid tests are not reliable; paired samples around a meal must be done. Approximately 20% of dogs tested will have higher pre-meal than post-meal TSBA values. Because TSBA values cannot diagnose a specific liver disease, testing dogs greater than 4 years old that have a history of illness may disclose high bile acids due to another disease process rather than the PSVA/MVD trait. Research has shown that any TBSA lab value over 25 uMol/L is considered abnormal. ****In other words, in Maltese, anything over 25 uMol/L means there is PVSA or MVD present. TSBA testing in Maltese can be very confusing to a veterinarian not familiar with MVD in Maltese.  Many veterinarians automatically assume anything over 25 uMol/L is a liver shunt when in actuality, it is likely MVD.  In Maltese, there is a huge gray area between MVD and a possible shunt.  Some experts suggest that results up to 70 uMol/L (with a normal ALT) is likely MVD.  The vast majority of dogs with a liver shunt will have TBSA results in the hundreds and an elevated ALT.  It is possible for a Maltese with MVD to have very high TBSA results, but that is the exception, not the rule.  If the TBSA is over 25 uMol/L, it is important to differentiate between PVSA and MVD.  Fortunately, there is now a further blood test that helps to differentiate PSVA from MVD called the Protein C test. Low Protein C activity is uncommon in MVD dogs, but occurs in greater than 90% of dogs with PSVA as well as in dogs with severe forms of acquired liver disease. Therefore a low Protein C result (<70%) indicates PVSA while a high Protein C result (>70%) indicates MVD. The Protein C test is a non-invasive and non-expensive method compared to costly imaging such as ultrasound, colorectal scintigraphy, CT scanning or invasive surgical procedures such as exploration and liver biopsy.


Current research substantiates that PSVA and MVD are related genetic disorders. In fact, it is possible to have both in the same dog. The mode of inheritance is most consistent with autosomal dominance with incomplete penetrance, meaning the trait is not associated with the sex chromosomes, has varying severity and is determined by a gene that one parent can pass to offspring. For breeding purposes all breeding stock should be bile acid tested. PSVA dogs should not be used for breeding. Because so many Maltese have asymptomatic MVD, it would be very hard to find stock that has normal bile acid values. If Maltese breeders only keep dogs with values less than 25 nMol/L, the gene pool would become so small that the breed would end up a disaster from other genetic faults being concentrated. Also, it has been shown that breeding dogs with normal bile acid values still have produced affected dogs with both PVSA and MVD. Until a genetic marker is found, common sense breeding is the only way to go. It is also recommended that all puppies have a TSBA test done (at 4 months of age or older) before or soon after going to their new homes to establish a bile acid baseline. Proactive assessment of the bile acids will limit the awkward circumstances imposed when a puppy is suddenly recognized to have abnormal bile acids by a pet owner's veterinarian. This can lead to unnecessary diagnostic confusion and unwarranted invasive tests.

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