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Cat Spleen: What it is, & How it Works

Last Updated on November 30, 2023 by Catster Editorial Team

Ask most people what their cat’s heart does, and they’ll tell you it pumps blood. How about the lungs? They breathe in oxygen and exhale carbon dioxide. The kidneys? They filter toxins from the bloodstream and put them in the urine. The spleen? That’s easy. It … it …

Admit it. You have no idea what the heck your cat’s spleen does. Don’t be embarrassed. You’re in good company. The average person is clueless about all things splenic. I think it’s time we shed a little light on this most puzzling of organs.

Diagram by Shutterstock

Here are spleen basics

The spleen is a dark red organ that looks like a giant tongue. It is longer than it is wide and is covered by a thick fibrous capsule. Located near the stomach, it’s on the left side of the body. However, the exact location of the spleen isn’t fixed. Depending on its size and shape and the size of the surrounding organs, the spleen can migrate around the abdomen and visit the right side of the body.

The spleen performs a surprisingly large number of functions. Here are the four most important:

  • Production of red blood cells: The bone marrow is the primary site of red blood cell production, and the spleen is the next major site.
  • Storage of red blood cells: The spleen holds a fair amount of blood. If the body was suddenly in need of extra red blood cells, the spleen can contract, releasing red blood cells into the bloodstream.
  • Filtration: Think of the spleen as a giant filter that traps and removes old or abnormal blood cells from the circulation.
  • Immunity: The spleen traps bacteria, foreign proteins, and other microbes and presents them to cells in the immune system, so an immune response can be initiated.

With all of these important roles, you’d think the spleen would be essential for life. Surprisingly, it can be surgically removed if necessary, and most animals will be fine. However, it’s better to have one than to not.

Photo by Andrew Marttila

When things go amiss

Disorders of the spleen are much more common in dogs than in cats. Splenic disorders can be generally categorized as either primary or secondary. A primary splenic disorder is one in which the spleen itself is the site of the illness. The spleen can also be affected secondarily by a systemic disease that is occurring somewhere else in the body.

When things go awry, spleenwise, the spleen usually grows bigger. Enlargement of the spleen is called splenomegaly. This is not something a cat owner would be able to detect. Splenomegaly is usually found during the physical examination, at the part of the exam where the veterinarian carefully presses on the abdomen to feel the internal organs.

Once splenomegaly is discovered on examination, your veterinarian will recommend diagnostic tests to help determine the cause. Blood tests and X-rays may provide important information. Abdominal ultrasound, however, is an excellent, non-invasive procedure to distinguish localized versus generalized splenomegaly and to further define the condition.

In most cases, however, a definitive diagnosis can only be achieved by obtaining a sample of cells from the spleen for analysis. The sample is usually acquired via fine needle aspiration, a procedure in which a needle, attached to a syringe, is gently inserted into the spleen. Material is then aspirated into the hub of the needle, and the contents sprayed onto a microscope slide. The slide is evaluated by a clinical pathologist. If this does not yield a diagnosis, abdominal exploratory surgery may be warranted.

Sadly, infiltration of the spleen with cancer cells is the most common cause of splenomegaly in cats. The most common cancer affecting the feline spleen is mast cell tumor. Hemangiosarcoma (a very bad tumor; my cat Crispy died from this) is the next most common, followed by lymphosarcoma.

Fortunately, disorders of the spleen are much less common in cats, compared to dogs. When they do occur, the prognosis will vary, depending on the cause.

Splenomegaly breakdown

Splenomegaly occurs in two forms: localized and generalized.

Localized splenomegaly: Where one focal area of the spleen is enlarged, called a “splenic mass.” More common in dogs.

Generalized splenomegaly: A diffuse enlargement of the entire spleen. More common in cats.

About the Author

Dr. Arnold Plotnick
Dr. Arnold Plotnick
Dr. Arnold Plotnick is the founder of Manhattan Cat Specialists, a feline-exclusive veterinary practice on Manhattan’s upper west side. He is also an author of The Original Cat Fancy Cat Bible. Dr. Plotnick is a frequent contributor to feline publications and websites, including his own blog, Leisure Commando. He lives in New York City with his cat Glitter.

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