Why Is My Cat or Dog’s Reticulocyte Count Low Or High?

Ron Hines DVM PhD

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Causes Of Most Abnormal Blood & Urine Tests

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Your Dog And Cat’s Blood Reticulocyte Count

Reticulocytes are your pet’s youngest red blood cells (=RBCs = erythrocytes). They are slightly larger than your cat or dog’s older, more mature, red blood cells (erythrocytes) that should make up most of the red cells in their blood.

Reticulocytes retain a bit of lacy material that was once part of the protein-making factories (ribosomes) present in the bone marrow cell (hemopoietic stem cells) from which they originated. After a few days in your dog or cat’s circulation, that material disappears and reticulocyte becomes an ordinary adult red blood cell. Although not exactly describing the same thing, increased reticulocyte numbers and polychromasia generally go hand in hand (occur together).

If your dog or cat becomes anemic and its bone marrow is healthy, the number of reticulocytes in its blood stream will rise as your pet’s bone marrow attempts to increase RBC numbers. When reticulocyte counts do not rise in an anemic pet, veterinarians call the situation an unresponsive (aplastic) anemia. The cause is usually located in your dog or cat’s bone marrow where these cells form.

When your veterinarian considers the importance of your pet’s reticulocyte count, he/she must do so knowing your pet’s packed cell volume (PCV,   Hct). That shouldn’t be a problem because a reticulocyte count is usually a standard part of a dog or cat’s CBC / WBC test panel that your veterinarian ordered to be run on the blood sample.

Because the blood of anemic pets is more dilute, there will be fewer reticulocytes per volume of blood than in non-anemic animals. That would falsely lower your pet’s reticulocyte count on the lab report. To get around that, laboratories apply a reticulocyte index (RI) value to adjust for any anemia that might be present.

Reasons Why Your Dog Or Cat’s Reticulocyte Count Might Be High (= Reticulocytosis) :

The usual cause is lack of enough working red blood cells = anemia. That could be due to a recent traumatic blood loss. It could be high blood-sucking parasite loads. 

When your cat or dog becomes anemic for any reason, its entire body – including its kidneys are starved for oxygen. That low oxygen situation causes healthy kidneys to produce a hormone called erythropoietin. Erythropoetin encourages your pet’s bone marrow to increase its production of new red blood cells. In the process, there will be more of these younger reticulocyte-stage RBCs released into your pet’s blood stream than would normally be found there.

So, anemia from any cause should cause an increase in your pet’s reticulocyte count. Rather than list all of those causes of anemia again, find them listed for the reason your pet’s PCV or hemoglobin might be low. In puppies and kittens, heavy flea infestation and hookworms are the most common cause.

Intravascular (within the blood vessels) red blood cell destruction (hemolysis) can also increase your pet’s reticulocyte count. When an event like that occurs, it is usually due to an autoimmune disease in which the pet’s own antibodies are mistakenly directed against its own red blood cells. Onion ingestion can cause this as well (as can an accidentally mismatched blood transfusion).

There are mixed feelings as to whether cats that eat bentonite-containing cat litter are predisposed (more susceptible) to anemia with an associated elevation in their reticulocyte counts. Only a few poorly verified instances where that might have occurred have been reported.

What If My Cat Or Dog Has A High Reticulocyte Count But Is Not Anemic?

An elevated (high) reticulocyte count can persist for a while after a pet has overcome a period of anemia – say after having bled severely due to a car accident. Higher than normal reticulocyte counts in the absence of anemia have also been found to occur more frequently in dogs taking NSAID medications like carprofen (Rimadyl®) or meloxicam (Metacam®) to treat their arthritis or other chronic pain conditions. Veterinarians suspect that chronic stomach and/or intestinal bleeding caused by these NSAID medications is the underlying cause. Any cause of chronic bleeding – seen or unseen – or extreme exertion can have a similar effect. (read here & here)

Reasons Why Your Dog Or Cat’s Reticulocyte Count Might Be Low:

Anemias due to sudden blood loss, caught before your pet’s bone marrow has not had time to increase its RBC production, can have low blood reticulocyte numbers. It takes 2-4 days from the time of the blood loss before your pet’s reticulocyte numbers begin to climb.

Blood loss over extended periods of time, such as intestinal bleeding from parasites or a tumor, can deplete your pet’s iron reserves. That eventually results in a high reticulocyte count when it is a “responsive anemia” (an anemia trying to repair itself). If that does not occur, it is called an unresponsive anemia. That is usually because your pet’s bone marrow has run out of the iron needed for new red blood cell formation. That is why giving an appropriate amount of an iron supplement (a hematinic) is a wise decision after severe blood loss has occurred. The same problem can occur in chronic intestinal inflammations such as IBD in cats or IBD in dogs where folate and vitamin B12 as well as iron are depleted.

Non-regenerative (aplastic) anemias can also occur in cats and dogs when tumors or pre-tumorous cells replace or crowd out normal bone marrow stem cells. Similar non-regenerative anemias with low reticulocyte counts occur due to cytokines released in the advanced stages of numerous chronic inflammations and cancers. Pets chronically ill or at the end of life from many of these causes are often moderately anemic. Many of these cases have nothing directly to do with your pet’s red blood cell generating process. 

I have found over the years that pets in this situation seem to know that their life is ending and accept that. They are calm, loving and not fearful. I believe that the loving thing to do for them at that point is to spoil them with love and affection. Offer them whatever they might be willing to consume in the way of fluids and nutrition – not some foreign-tasting, prescription product they are unaccustomed to eating. I believe that “heroic” veterinary interventions at that point in life is not in your dog or cat’s best interest. I suggest that you not get talked into treating your pet “heroically” by aggressive or corporate veterinary practices.

Low reticulocyte count anemias are a common problem in advancing feline leukemia (FLV). It can also occur in panleukopenia virus infections, and it has been associated with ehrlichia infections in dogs.

A non-regenerative, low reticulocyte anemia, can also occur when pets have consumed lead or zinc-containing objects. Methimazole, used to treat hyperthyroid cats, has also caused aplastic, low reticulocyte count anemias in humans. It is unclear if that is a possible side effect in hyperthyroid cats. Various medications given to dogs and cats for the treatment of cancer have cell-destroying properties that can damage the pet’s blood forming bone marrow cells as well. That is why the CBC/  WBC counts of pets receiving those chemotherapy medications need to be frequently monitored.

Complementary Tests:

CBCWBC and blood chemistry panel,   folate level,   blood iron level and vitamin B12 level.   Bone marrow biopsy when initial tests and examinations fail to identify the cause. When reticulocyte counts are high or low, other tests chosen by your veterinarian to determine why and where blood loss is occurring or what bone marrow issue is causing your dog or cat to have difficulty producing reticulocytes.


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