The normal abdomen houses organs that are bathed in a certain amount of intra-abdominal fluid. This fluid helps to maintain sterile environment for the organs and helps to fight infection, keep and cells and tissues moist and healthy, and promote cellular renewal and tissue repair. When free fluid in the abdomen accumulates that is excessive and even distends the patient, this is referred to as ascites.
Ascites is really not a disease in and of itself, but a syndrome or clinical sign that arises secondary to some other primary disease process. There are certain diseases that can commonly lead to ascites that I will outline here:
1.) Diseases of the liver may lead to an increase in the pressure of its major venous drainage, a blood vessel known as the hepatic portal vein. This subsequent increased pressure leads the leakage of the serum portion of the blood through fenestrations in blood small blood vessels normally utilized for nutrient exchange. Another consequence of liver disease that can contribute to ascites is the liver not having the ability to synthesize adequate amounts of the serum protein albumin. In addition to being a major carrier protein in the blood stream, albumin also carries a charge that keeps the aqueous (water) portion of the blood stream in the blood vessels at the level of the capillaries, small blood vessels utilized for nutrient exchange at the tissue level. Insufficient amounts of albumin therefore will lead to extravasation of the aqueous portion of the blood at the capillary level and collect in body cavities such as the abdomen. Liver diseases that can lead to ascites include: portosystemic shunt, chronic active hepatitis, cholangiohepatitis, cirrhosis, and canine infectious peritonitis.
2.) Congestive heart disease – Left sided heart failure commonly leads to fluid in the lungs, while right sided heart failure more commonly tends to lead to fluid accumulation in the abdomen.
3.) Kidney failure – The large serum protein, albumin, is normally unable to escape the blood stream via the kidneys’ individual filtration apparatuses within the kidneys known as nephrons. In cases of kidney failure where disease extends to nephrons, albumin can be lost in the urine. Excessive loss of albumin leads to albumin deficiency and a syndrome that result in line with albumin deficiency that occurs with liver disease: ascites.
4.) A number of infectious diseases can cause abdominal effusions, the most common of which in veterinary medicine is feline infectious peritonitis (FIP).
Free fluid in the abdomen can be treated by tapping the abdomen utilizing a a drainage technique called abdominocentesis. Some cases will respond to treatment with a diuretic depending on the cause of ascites, however, severe ascites usually requires abdominocentesis to manage initially. Further management of ascites involves treating the primary disease that is causing it.
Roger Welton, DVM
President Maybeck Animal Hospital
CEO, Founder, Web-DVM