The liver: versatile yet vulnerable: inflammation, poisoning, infection or cancer can present a life-threatening challenge to your dog.
Despite its size and capacity, however, the liver is by no means invulnerable. Indeed, says Sharon Center, DVM, a board-certified professor of internal medicine at Cornell University's College of Veterinary Medicine, the Cornell University Hospital for Animals (CUHA) sees many cases of liver disease. While she attributes this frequency of cases largely to the fact that CUHA is a referral hospital, she also notes that liver disease is very common in the general canine population.
Form and Function. Among all the organs of the body, the liver is exceeded in size only by the lungs and skin. Situated in close proximity to the stomach, spleen, pancreas and intestines, it is a heavy organ, its spongy tissue densely packed with vessels that receive blood through two major conduits. Most blood flows to the liver directly from the heart via the hepatic artery. It also receives blood from the spleen and gastrointestinal tract through the portal vein, returning it to the heart through a large vessel--the vena cava--for recirculation throughout the body.
The liver is divided into several sections (lobes), each of which contains thousands of tiny units called lobules. Every portion of the liver is able to carry out all of the organ's seemingly countless chores. These include, for example, the production of blood components that enable clotting; the metabolism of proteins, fats and carbohydrates as well as of drugs and hormones; the storage of vitamins and other nutrients and of blood that can be rushed into circulation if needed in an emergency; the conversion of ammonia--a potentially harmful byproduct of digestion--into a less toxic substance called urea; and the secretion of bile, a fluid that is discharged into the small intestine to aid in the digestive processes.
The liver also plays a vital role in detoxification--the mitigation of harmful substances that have entered the bloodstream. After blood has moved through the intestines, it passes through the portal vein to the liver and then to the rest of the body. In a healthy animal's body, these substances are either gotten rid of or are rendered harmless, after which the cleansed blood flows back to the heart and is then safely recirculated.
Lethal Threats. Given its proximity to other organs and its involvement in innumerable processes, the liver is subject to several disorders, all of which must be considered lethally dangerous. The most frequently observed liver disorders may be classified as inflammatory, toxic, infectious or neoplastic (cancerous). In addition, Dr. Center points out, the liver--sturdy as it is--can also suffer serious injury. "Whenever a dog is hit by a car," she says, "there's a chance for liver trauma."
Among all canine liver conditions, she notes, the most frequently observed at CUHA is chronic hepatitis, an inflammatory disorder that is most often diagnosed in middle-aged dogs (about six to eight years of age) but is also seen occasionally in young dogs. In the majority of cases, the initial source of tissue-damaging inflammation is unknown, although it is often presumed to be a consequence of an immune-mediated response to an infectious agent, such as a virus, or to a toxin or perhaps a drug.
Chronic hepatitis is particularly dangerous, Dr. Center points out, because affected dogs do not show clinical signs of illness, such as loss of appetite and weight loss, until the disease has reached an advanced stage. "Dogs tend to remain asymptomatic for long periods of time," she says, "before they become seriously ill." As a result, some animals with chronic hepatitis die within a short time following veterinary diagnosis.
Toxic insult to the liver results from the ingestion of harmful substances. These substances range widely and include, for example, poisonous plants, mushrooms, polluted water, spoiled food or garbage and human medications, such as Tylenol. "It's often difficult for us to identify the specific toxin that has caused the damage," says Dr. Center, "but these substances are probably at the source of more liver problems than we realize."
In some cases, the toxins are present in unlikely substances. In the recent past, for example, it was discovered that dogs can become deathly ill with liver disease by eating the leaves of a cycad, an exotic, palm-like plant often used as a bonsai. And only last year, owners and veterinarians were stunned to discover that canine liver failure was being caused by ingestion of dog food tainted with highly poisonous substances called aflatoxins.
Among infectious liver diseases, leptospirosis--a condition resulting from a bacterial infection--is a leading example. "Since the liver is in the middle of an animal's blood flow," says Dr. Center, "it is very often involved with and affected by a wide variety of infectious diseases. Fortunately, we have a very effective vaccine [originally developed at Cornell] for the major viral disease in dogs, which is canine infectious hepatitis."
Dogs are also vulnerable to the development of liver tumors, which can either arise in the organ or metastasize to the liver from elsewhere in the body. However, says Dr. Center, liver cancer is far less common than inflammatory problems that affect the organ.
Other notable liver problems in dogs include vacuolar hepatopathy (a degenerative disorder that develops secondary to high cortisone levels, stress or a systemic inflammatory condition) and portosystemic shunt, a congenital malformation that causes blood from the intestines to bypass the liver, thus sending uncleansed blood directly into systemic circulation. A liver disease called hepatic lipidosis, which is marked by the collection of fat within the organ, may occur in dogs but is far more common in cats.
Among all canine liver diseases, those with the poorest prognoses are end-stage cirrhosis, a condition in which functioning liver cells are replaced by scar tissue, and toxic insult caused by the ingestion of certain poisonous substances, such as the bonsai cycad plant or antifreeze.
Clinical Signs. To a great extent, says Dr. Center, the clinical signs are similar for all types of advanced liver disease. These include food avoidance, weight loss, vomiting, lethargy, a noticeable buildup of abdominal fluid and jaundice--a condition marked by yellowish discoloration of the skin, whites of the eyes and mucous membranes.
Early diagnosis of liver disease relies largely on a thorough physical examination, blood chemistry analysis, urinalysis, liver function tests, ultrasound imaging of the abdomen and, in some cases, chest X-rays. Treatment methods and their outcomes will depend on the type of liver disease that is diagnosed and the stage of its progression.
Although there may be no way to protect your dog from some types of liver disease, says Dr. Center, early diagnosis and treatment are likely to improve the prognosis should any disorder be present. Thus, she advises, owners should certainly have their animals undergo a thorough physical examination at least once a year that includes a blood chemistry panel and urinalysis. Furthermore, she urges, make sure that your dog does not have access to poisonous substances that it might ingest accidentally or out of curiosity.
Please note: Some tables or figures were omitted from this article.
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|Date:||Apr 1, 2012|
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