The Typical Patterns of Clinicopathological Changes in Dogs With Liver Disease
In a dog with liver disease, the typical patterns seen will differ with where the disease lies, there will be differences depending on if the liver, bile duct, or both is affected. Once liver disease is suspected further tests may be performed to help confirm liver dysfunction and get a diagnosis to aid treatment.
It is important to note that there can be no clinicopathological changes in some dogs with liver disease due to the large reserve capacity that livers have, this means even with the disease the liver can continue to function normally and it may require more severe, widespread disease before it can be detected with testing.
Some common tests and their findings are listed below:
Biochemistry
Complete blood count
Urinalysis
Imaging
Other clinicopathologic tests include bile acid stimulation tests, cytology, histology, and checking clotting times.
The Four Liver Enzymes in Dogs
There are four liver enzymes that are tested in dogs, these enzymes differ in where they come from and can give us valuable information about certain diseases occurring in the liver, however they can also be elevated from other causes and elevations sometimes don’t give the whole answer.
These four enzymes are broadly divided into two groups, those that become elevated when there is damage to the liver cells (hepatocellular enzymes) and those that become elevated when there is a problem with the bile duct (cholestatic enzymes)
AST or aspartate aminotransferase can be elevated when there is damage to the liver cells (hepatocytes) but also when there is damage to other cells in the body such as muscle cells. This enzyme is less commonly tested because it can be elevated due to non-liver causes and typically parallels ALT which is more specific meaning veterinarians usually rely on the information ALT provides instead.
Non-liver elevations of AST can be pancreatitis and muscle disease.
ALT or alanine aminotransferase is a useful enzyme because it is found primarily in hepatocytes meaning an elevation of ALT on blood can more clearly indicate liver disease. ALT is also produced by the kidneys and intestines. When there is damage to the hepatocytes, ALT is released into the blood and can be measured on blood tests (biochemistry).
Elevation in ALT shows there has been damage to the hepatocytes but that doesn’t mean there is definitely liver disease as diseases outside the liver can cause elevated ALT also (pre-liver). Examples include heart disease, intestinal inflammation, and severe dental disease. In general, these pre-liver problems will only cause minor elevations in ALT whereas true liver disease will cause a larger elevation.
Because ALT is more specific to liver disease at high levels a large elevation of ALT is a sign to investigate possible liver disease further. Severe increases of ALT don’t automatically mean irreversible damage, it is possible for liver function to return to normal with appropriate treatment, thanks to the regenerative capacity of the liver.
Examples of diseases that cause hepatocellular disease include certain toxins, chronic inflammation (hepatitis), and genetic disorders
Cholestatic enzymes are those found in the bile duct that extends from the liver.
ALP or alkaline phosphate is contained in cells that line the bile ducts, blockage of the bile ducts (cholestasis) causes increased production of ALP leading to an elevation seen in blood work. Elevated levels of ALP don’t always indicate a problem with bile flow as there are many forms of ALP produced in the body including from bones (B-ALP), as well as steroid-induced ALP (C-ALP). Some breeds can naturally have elevated ALP, such as Miniature Schnauzers, Huskies, and Scottish Terriers.
Other non-liver-related diseases can also cause elevations in ALP such as pancreatitis, gallbladder disease (where bile is produced), and certain endocrine (hormonal) diseases.
ALP is the least liver-specific enzyme but also the most commonly elevated liver enzyme meaning it can cause concern of liver disease without being true. An elevation in only ALP may not signify a liver problem and may not require further investigation.
GGT or γ-glutamyl transpeptidase is another enzyme contained in the cells that line the bile ducts and can also become elevated when there is cholestasis. GGT levels typically parallel ALP levels and having elevations in both suggests cholestatic. GGT is a very sensitive indicator of liver or bile disease and can be useful to confirm that a problem is not liver-related, if GGT is not elevated then liver disease may not be the problem.
Examples of diseases that cause cholestatic disease include nodular hyperplasia, toxins, neoplasia, and gallbladder problems (e.g. gallbladder mucocele).
Why is age and breed important?
Puppies will normally have an elevated ALP because ALP is also produced from growing bones, not just liver cells. Some older dogs will have a benign accumulation of water, carbohydrates, or fat in the liver cells (so called vacuolar hepatopathy)… Others can develop nodules as an aging change. These diseases typically causes primarily an elevated ALP and do not really affect liver function or require treatment.
Elevations in ALT and ALP do not necessarily mean the liver is in failure, or not working properly. However, your veterinarian may recommend another blood test, called bile acids (before feeding and after feeding), to assess liver function. Another test that can be used to assess how the liver is working is bilirubin.
This is the yellow pigment that builds up in the body when an animal is jaundiced. If the bilirubin is increased, this already indicates decreased liver function (unless the patient is also severely anemic) – so bile acids testing in this situation is not indicated as it will not provide additional information.
Certain breeds are prone to particular diseases. For instance, an older Dachshund with elevated liver enzymes may have Cushing’s syndrome Labradors, Bedlington terriers, Westies, and Dobermans are predisposed to chronic hepatitis (inflammation of the liver). Shelties and Cocker Spaniels are prone to gallbladder problems.
The fact that we see certain diseases in specific breeds suggests that there is a genetic component to some of these processes.