Causes Of Anemia In Dogs And Cats

Why Is My Dog Or Cat Anemic?

Ron Hines DVM PhD

If your cat or dog is anemic, intent rapid breathing, weakness and pale gums are what you would probably notice first. Your veterinarian’s stethoscope might also detect a fast pulse and even a heart murmur if the anemia was severe.   Listen to what that might sound like here:

Anemia is the presence of too few red blood cells in your pet’s circulation. Your dog and cat’s blood is composed of a liquid portion, the plasma, and a cellular portion. That cellular portion is made up chiefly of red blood cells (=erythrocytes =RBCs) which carry oxygen throughout its body. The next most common elements are blood platelets. For every 20 red blood cells there is about one platelet (=thrombocyte). The other cellular types are the white blood cells (leukocytes). They are associated with the immune system. Compared to erythrocytes and blood platelets, there are far fewer white blood cells.

There are two basic kinds of anemia:

Regenerative Anemia

In the first kind, your pet’s body is loosing blood RBCs faster than they can regenerated by its bone marrow (through the process of erythropoiesis). But your dog or cat is still producing new red blood cells (RBCs) in its bone marrow at a normal or increased rate. This is called a regenerative anemia. The most common causes are trauma and internal parasites such as hookworms. A mark of a regenerative anemia is that your pet’s body, in a rush to correct the anemia, releases some of its RBCs a bit too early. Those young RBCs are called reticulocytes. An increase or decrease in the number of reticulocytes will help your veterinarian to decide which form of anemia is affecting your dog or cat. Regenerative anemias are sometimes called blood-loss anemias because substantial blood lost from the body in any way leads to it. Even heavy flea and tick infestations – particularly in immature dogs and cats can result in blood loss anemia.  Red blood cells can be prematurely lost in other ways. When your dog or cat is infected with one of the many red blood cell parasites carried by fleas and ticks, any red blood cells harboring the parasites are soon destroyed.  Cases of regenerative anemia tend to have better outcomes than non-regenerative anemias. 

Non-regenerative Anemia

In the second form of anemia, your dog or cat has lost its ability to manufacture sufficient new RBCs in its bone marrow to replace the ones that have lived out their lifetime. Red blood cells have a short lifespan. RBCs in healthy dogs and cats only function for a matter of months before they are filtered out of circulation by macrophages residing in your pet’s spleen and liver. Those lost red blood cells are replaced by fresh new ones generated in its bone marrow). Read about that here. The type of anemia that occurs when the dog or cat’s bone bone marrow can not proved replacement RBCs is  called a non-regenerative anemia (it was once called aplastic anemia).  

What Signs Might I See If My Pet Is Anemic?

In my mind, the terms “washed out”, “mopey” and “sluggish” comes to mind. Most anemic dogs and cats that are brought to me are described  by their owners as lacking normal vigor and alertness at home. They have low energy levels. They tend to sleep more and exhaust sooner on their daily walks than they once did. Anemic dogs and cats also tend to breath faster in an attempt to keep their bodies properly oxygenated. Their hearts beat faster for the same reason. Of course, a lot of illnesses could cause those same behaviors.  You might notice that your pet’s gums and the pink inner surfaces of its eyelids (palpebral conjunctiva) are abnormally pale. 

Because dogs and cats with anemia have fewer red blood cells, their blood is thinner (more watery). As a result, anemic animals commonly develop heart murmurs that your veterinarian might hear through his/her stethoscope. These audible heart murmur are due to the turbulence thinner blood creates as it flows through the valves of your pet’s heart. In female ferrets, the most common cause of aplastic/non-regenerative anemia is failure of a non-spayed female to complete ovulation. You can read about that here. In female dogs the most common cause is an estrogen injection given to abort a dog that was accidentally bred – the “mismating shot”, although testicular tumors are said to be capable of the same effect. Read about both here.  

The signs of anemia depend on its severity and how quickly the anemia occurs. With gradual onset of anemia, the pet’s body has time to adjust as best it can to its decreased red blood cell count. Animals that become anemic very quickly may die because their bodies just cannot handle the sudden loss in red blood cells and oxygen.

How Will My Veterinarian Determine If My Dog or Cat is Anemic?

When your veterinarian suspects that your pet is anemic, he/she will perform a simple office blood test  that determines the volume (percentage) of erythrocytes present in your pet’s bloodstream. This test for anemia is called a “packed cell volume” or “hematocrit” (= PCV, Hct). A drop of your dog or cat’s blood is introduced into a thin glass tube and spun in a centrifuge to separate the red blood cells from the blood serum or plasma. The shorter the column of red cells, the more anemic your pet is. Your veterinarian might also stain and examine a thin film of your pet’s blood on a slide to determine the characteristics of the red cells and blood platelets that are present. In this way, your veterinarian can often obtain clues that distinguish between regenerative and non-regenerative anemias. The search for clues as to the cause of anemia would be similar in ferrets and other pets as well. The shape (morphology) size and color of individual red blood cells in your pet’s blood are all important clues. You can read more about some of those clues here:  reticulocytosis,   anisocytosis and poikilocytosis.

Cats that are persistently anemic with no ready explanation need a PCR test run on their blood for evidence of hemotropic mycoplasma. The parasite is transmitted by fleas. Not all positive cats are anemic when they have these organisms.

The most common cause of regenerative anemia in young dogs and cats are intestinal parasites (generally hookworms). A fecal sample examination should detect them. Another common cause of anemia in young pets is heavy flea infestation. Although each flea only sucks a minute amount of blood the combined loss of blood in immature animals can be great. You can read about the health problems that fleas cause in pets through  this link . Hemoglobin, the red pigment of blood, contains iron. In both instances so much iron is lost from the body that the red blood cells subsequently produced are smaller than normal (microcytic anemia).

Another cause of regenerative blood loss anemia in dogs is the administration of non-steroidal anti-inflammatory medications including aspirin, ibuprofen, phenylbutazone and naproxen and pyroxicam. These medications commonly cause bleeding ulcers of the stomach and small intestine leading to anemia. They are even more toxic to cats. These type of drugs sometimes given to dogs suffering from arthritis. Similar NSAID medications that veterinarians dispense long-term for problems like arthritis are less likely to cause stomach and intestinal bleeding. However, occasionally they can produce similar effects.  

Some pets become anemic because their blood does not clot normally. Read about many of the health issues responsible for that here.  

How Will My  Veterinarian Treat My Pet’s Anemia?

Mild to moderate cases of anemia are treated with blood-building vitamins and minerals called hematinics and treatment of the underlying cause. Severe cases of anemia often require transfusions. Luckily, severe transfusion reactions seem to be less common in pets than in human beings. Anemia never occurs without an underlying deficiency, disease or accident. So correcting anemia without correcting its underlying cause is never a permanent cure for your pets problem. Anemias that occur in animals less than half-way through their natural lifespan often respond well to treatment when the underlying cause is identified and corrected. Often, anemias that occur in older pets do not have as favorable an outcome. Mild to moderate anemia, in itself, is not a painful or distressing condition – its chief symptom is a lack of energy. So in older anemic pets, lifestyle changes, nutritional support and keeping underlying or coexisting health problems at bay is often enough to keep the pets happy.

What is Red Blood Cell Hemolysis?

Hemolysis is the sudden destruction of red blood cells within the veins and arteries of the body. This can be caused by the ingestion of toxic materials, bacterial and viral infections, defectively produced red blood cells, autoimmune disease and parasites of the blood (Haemobartonella [the old name for hemotrophic mycoplasma] and babesia). These are sometimes sudden crisis events. Transfusions alone are not effective in treating hemolysis because the new blood is destroyed as quickly as it is added. Many cases of hemolytic anemia in pets are treated with antibiotics and drugs that slow the destruction of red blood cells (corticosteroids). The gums and white portions of the eyes of animals with hemolytic anemia are often yellowish (icteric) due to the presence of an excess of destroyed hemoglobin products within the body (bilirubin).

Both Tylenol® (acetaminophen/paracetamol) and onions can cause anemia in cats.

Accidental eating of zinc-based coinage (US pennies) will cause anemia in all species of animals. An auto-immune disease (autoimmune hemolytic anemia) is a common cause of hemolysis in older adult pets, particularly dogs.

Non-regenerative Anemias

A common cause of non regenerative anemia in cats is infection with the feline immunodeficiency or feline leukemia virus . In non-regenerative anemias it may be necessary to examine samples of the blood-forming marrow of the bones to determine the cause. This procedure is called a bone marrow biopsy.  There are forms of non-regenerative anemia that are a sign of autoimmune disease. A suspicion that autoimmune red blood cell destruction underlies anemia is a positive is the organism  coombs or ANA test.

The Feline Leukemia Virus

Often, the first signs of Feline leukemia (FLV) in cats are unexplained chronic low-grade fevers and anemia. These anemias are due to the effects of the FLV on the blood forming elements within your cat’s bone marrow. You can read more about feline leukemia through this link.

The Feline Immunodeficiency Virus

As with FLV, feline immunodeficiency virus (FIV) often causes anemia in cats through its effect on bone marrow. Complicating matters, cats with FIV often have poor appetites and will not eat a balanced diet. Diets low in iron, and vitamin B-12 can exacerbate these anemias. Cats with FIV or FLV may also have decreased resistance to the blood parasites that often cause anemia in themselves. You can read more about feline immunodeficiency virus through this link

Fight Abscesses In Cats

Indoor/outdoor cats that fight, as well as unneutered stray tomcats are very susceptible to septicemias (bacteria in the blood stream) and subcutaneous abscesses. These often result in toxic, non-regenerative anemias early in the condition. Once the infections coalesce into an abscess and drains, the anemia almost always subsides unless the pets are already positive for FLV, FIV or both those virus.


Many forms of cancer liberate toxic products into the bloodstream that suppress the formation of red blood cells in the pet’s bone marrow. Generally, the cancers that cause anemia are the more life-threatening forms of cancer. Small, benign tumors do not cause anemia. In certain types of cancer, erythropoietin production by the kidneys, as well as its activity on the bone marrow, is inhibited by cancer-produced cytokines (the substances that mediate inflammation). Chemotherapeutic drugs – drugs used by veterinarians to combat cancer , autoimmune or allergic disease are also capable of causing non-regenerative anemia. You can read more about cancer in pets through this this link.

Anemia Due To Kidney Failure

Kidney failure in all animals leads to a buildup of toxic waste products in the blood stream that suppresses red blood cell formation (uremia). In chronic renal failure, your pet’s kidneys cease to produce sufficient amounts of a hormone, erythropoietin, necessary for blood cell formation in the bone marrow. Human erythropoietin has been available to veterinarians since 1989. However, it has not worked as well as we had hoped in dogs and cats because each animal species’ erythropoietin is slightly different from the other. Most cats eventually build up immunity to human erythropoietin. Perhaps, the lives of cats suffering from non-regenerative anemia might be improved by administering synthetic (recombinant) feline erythropoietin (rfEPO) developed at Cornell Veterinary School. (read here) I do not believe that it is readily available. There have also been a few studies on treating this problem in cats with gene therapy (read here). Studies on the use of canine erythropoietin were also conducted. (read here) Since nothing more was published since then, I assume results were not as  promising as initially hoped.

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