Structure and Function of the Liver in Cats
Dr. Bari Spielman
Congenital portosystemic shunts are defects in the portal vein that leads to the liver. In animals with these defects, the portal vein bypasses the liver and materials that are normally carried from the intestines do not reach the liver. Certain materials (e.g. ammonia) continue to circulate in the blood until they reach toxic levels.
What Are Common Diseases of the Liver?
Since the liver is involved in many biochemical processes, many different diseases can affect it. A variety of clinical signs may be seen with liver disorders; however, in many cases, one of the earliest signs of disease is jaundice. Jaundice occurs when the blood contains an excessive amount of bilirubin, causing a yellow color to the skin, gums, and sclera (whites of the eyes). Ascites (fluid in the abdominal cavity), another clinical sign seen with liver disease, occurs when there is impairment of blood flow through the portal vein. This leads to hypertension (high blood pressure) in the portal vein, which causes fluid to leak out of the portal vein into the abdomen. Low albumin protein levels in the blood from liver disease may also cause ascites.
The liver has a remarkable ability to produce new cells to replace its own diseased or damaged cells. This regenerative capability allows the liver to return to normal function in some cases.
Some examples of liver diseases include:
Viral, parasitic, protozoal and bacterial infections may affect the liver and gall bladder. Bacterial infections may result in abscessation within the liver. An important cause of liver disease in the cat is feline infectious peritonitis virus.
Hepatic lipidosis is a syndrome seen in cats with sugar diabetes. Abnormalities in the metabolism of glucose and fat in diabetic cats cause an accumulation of fat in the liver that may eventually result in liver dysfunction. Idiopathic hepatic lipidosis also occurs in the cat, and its cause is unknown. Cats that are overweight and have a decreased intake of calories (often through a loss of appetite for a prolonged period) are prone to idiopathic hepatic lipidosis that leads to liver failure.
Hepatotoxins (agents or drugs harmful to the liver) can cause severe, sometimes irreversible liver disease. Examples include heavy metals (e.g. lead, arsenic, thallium, copper), anti-inflammatory drugs, certain antibiotics and anesthetics, anticonvulsant medications, and certain anti-parasite drugs and dips.
Cirrhosis occurs when scar tissue replaces healthy liver cells. Cirrhosis may develop from any chronic, long-standing liver disease.
Cholangiohepatitis is an inflammation of the bile carrying structures and the surrounding liver tissue. Two forms are seen in the cat. One form develops when infections from the nearby intestines invade the bile ducts. The other form is not related to infection and may be an immune disease.
Both benign and malignant tumors may develop within the liver. These tumors may develop only in the liver, or may spread to the liver from other organs. A common liver tumor in the cat is lymphosarcoma, and this cancer is often associated with infections with the feline leukemia and feline immunodeficiency viruses.
What Types of Diagnostic Tests Are Used to Evaluate the Liver?
There are many diagnostic tests that are helpful when evaluating the liver.
Initial tests usually include a complete blood count, biochemistry profile and urinalysis. A blood count may reveal evidence of infection, anemia or low protein. A biochemical profile may reveal elevations in liver enzymes and bilirubin (cause of jaundice), and/or decreases in glucose, protein, blood urea nitrogen and cholesterol. It may also show electrolyte abnormalities. A urinalysis may show increased excretion of protein by-products.
Other laboratory tests may be considered, depending upon results of the initial blood tests. Bile acids are blood tests that assess the function of the liver and the amount of liver that is diseased. Blood ammonia, blood steroid and amino acid levels may be measured. Serology tests for certain viruses, protozoa and fungal diseases may be considered.
Abdominal radiographs (x-rays) may show changes in liver size and shape. They may also reveal the present of ascites (abdominal fluid), gallstones, and abnormalities in other abdominal organs.
Chest x-rays may be performed to look for signs of metastatic tumors, fluid in the chest, and problems with the diaphragm.
Abdominal ultrasonography is very helpful in evaluating the internal structures of the liver. It provides valuable information about the consistency of the liver and can often identify blood vessel shunts, cysts, abscesses and tumors. It is a noninvasive procedure that often necessitates the expertise of a specialist and/or referral hospital. Your veterinarian may refer your cat to a veterinary internal medicine specialist to perform an ultrasound.
A clotting profile is often performed when there is evidence of chronic or severe liver disease. If the liver cannot manufacture normal amounts of clotting factors, then the animal will be very prone to bleeding disorders. Evaluation of clotting function is particularly important before any attempt is made to biopsy the liver.
Advanced imaging tests that may be helpful in diagnosing liver disease or shunts include radioisotope studies, CT scans, MRIs and dye contrast studies of the portal vein (portogram).
A liver biopsy is often necessary to determine the specific type of liver disease present. A liver biopsy may be performed under the guidance of an ultrasound, through laparoscopy (insertion of a small rigid scope into the abdomen), or by surgical opening of the abdomen (exploratory laparotomy). Material retrieved for biopsy can be submitted for culture and for microscopic examination.