WBCs and platelets in greyhounds

We discussed the differences in greyhound RBCs and non-greyhound RBCs.  There are two other types of blood cells that are different in greyhounds vs. non-greyhound dogs, the White Blood Cell (WBC) and the platelet. 

RBCs, WBCs, and platelets are made in the bone marrow.  After they are mature they are sent into the blood stream.  As discussed previously, the RBC carries oxygen to the tissues.  The WBC’s job is more complex; lets look at it first. 

WBCs are immune cells.  They fight off viral, bacterial, parasitic, and fungal infections.  This is an important job, and there are multiple cells types and they each specialize on a certain intruder. 

I am going to break down WBCs cells lines as simply as possible.  I want this to be easy to understand for everyone!  If there are any questions please comment or email and I will be happy to address this further. 

There are five main types of WBCs.  Some of them are granulated and have a nucleus or control center (RBCs do not have this).  These granulations are particularly important.  The granules are enzymes within the cell that improve cellular function—i.e. they are the marinade on the foreign evader the WBC is going to destroy.  These are the types of WBCs found in the body:

1.     Neutrophils: most numerous WBC.  Nucleated and have granules.  Responsible for ridding the body of bacterial and fungal infections.

2.     Eosinophils: nucleated with granules.  Responsible for ridding the body of parasites.  Assist in allergic reactions.  Help destroy cancer cells. 

3.     Basophils: nucleated with granules.  Basophils control the histamine response in inflammation.

4.     Lymphocytes: Multiple subsets (T and B cells) of the lymphocyte—complex cell with very important functions.  Some subsets have granules others do not.  All have nuclei.  Help the body rid itself of viral and bacterial invaders and also create memory cells so that when the same infection enters the body again, it will be prepared. 

5.     Monocytes: nucleated without granules.  They have longer lives than the other WBCs.  When mature they migrate to the tissues and organs and hang out there to take care of any potential bacterial evaders. 

With all of this complexity it is hard to believe the immune system does not go haywire more often!

Now lets look at the differences in greyhound labs values vs. non-greyhound dogs. Lower WBC counts are often seen in greyhounds.  This has been well defined and is currently not a point of concern, as it is considered a breed trait.  The eosinophil of the greyhound has been widely studied.  Beginning in the 1960s the greyhound was discovered to have a vacuolated eosinophil, meaning that the granules within the cell would not stain for microscopic examination (FYI staining is a method of putting a dye into cells allowing them to be better evaluated by microscope).  This was odd, as non-greyhound dogs’ eosinophils will stain orange with a quick stain method.  The lack of the staining was concerning, as it appeared to be a toxic cell.  The concern for abnormalities prompted large work-ups looking for a source of the inflammation.  Fortunately, these changes were not pathologic.  This prompted more studies.  In 2005, there was a study published in Veterinarian Clinical Pathology.  This study looked a 49 greyhound blood smears and compared them to 200 non-greyhound dog blood smears.  A more advanced staining method was used than in the 1960’s study.  This study showed that structurally and chemically, the greyhound’s eosinophil was the same as the non-greyhound dogs.  This suggests that there were different staining properties in the greyhound eosinophil but overall it was the same cell seen in non-greyhound dogs.  The cell was deemed not pathologic.

In addition to the changes in the WBC there are also changes in the platelet counts of greyhounds.  First things first, what is a platelet? Platelets are tiny blood cells that are made in the bone marrow with RBCs and WBCs.  They are responsible for blood clotting.  

Greyhounds are known to have low platelet levels.  This is considered to be a breed trait as long as the count is not below 100,000/μL.  Again, I ask why is the greyhound’s platelet different from non-greyhound dogs?  Well there are multiple theories about this.  The first came about in 1994 by PS Sullivan.  This theory has been supported multiple times since and focuses on stem cell competition.  Do you remember the RBCs we talked about a few days ago?  Well, Sullivan posited that when the bone marrow begins creating the building blocks of blood, there is a competition between the cells destined to be platelets and the cells destined to be RBCs.  Due to the need for more RBCs in the greyhound, the poor platelets lose this fight—leading to lower platelet counts.  Another theory focuses on the mature RBCs—due to the amount of oxygen molecules on the RBC, there is a mild loss of oxygen overall when the RBCs release the oxygen into the tissues, and this mild hypoxemia causes an increase in the production of RBCs, thus causing a decreased production of platelets—remember these cells are in competition.  Both of these studies show that the competition between the RBC and the platelet either in the blood or the bone marrow is most likely why greyhounds have lower platelet counts. 

Naturally one would think that greyhounds would be “easy bleeders” due to their low platelet counts.  Well, they are “easy bleeders” but it is not due to the lack of platelets.  Actually, the time it takes for a greyhound to create a platelet plug or the “closing time” is very similar to non-greyhound dogs.  This suggests that greyhounds’ platelets are more active than non-greyhound dogs.  These researchers also looked at the risk for bleeding disorders in greyhounds, but this was very rare and not related to their “easy bleeding” tendencies.  However, the clot strength in the greyhound was weaker than non-greyhound dogs—this most likely is the cause of the “easy bleeding” in greyhounds.  

So why would the greyhound be able to clot quickly with a low platelet count but then create a substandard clot? Aging that points back to the high level of RBCs in the blood.  Due to the high RBC mass in the blood, the plasma (fluid in the bloodstream after removal of RBCs, WBCs, and Platelets) has less Fibrinogen (a protein that aids in clotting).  This is the confusing part; there is actually no difference in the fibrinogen levels in greyhounds vs. non-greyhounds.  The mass of the RBCs within the blood is to blame for the pseudo hypofibrinogenemia.  Additionally, it is thought that greyhounds have enhanced fibrinolysis (the ability to break down clots).  This would keep the greyhound from clotting, as they are at risk due to the increased viscosity of blood during exercise. 

Photo Credit: Cindy Frezon

 To sum up the bleeding tendency of the greyhound goes something like this: greyhounds have low platelet numbers but active platelets and these platelets create clots quickly but the clots lack stability due to low levels of fibrinogen due to elevated RBCs in the blood.  Furthermore, is has been theorized that greyhounds have active fibrinolysis and can break up clots quicker than non-greyhound dogs. 

Whew! Now we have looked at the RBCs, WBCs, Platelets, and hemostasis in the greyhound.  It has been great for be to learn more about this.  I now have an understanding of why these levels are different from non-greyhound dogs and I think this adds to the undeniable coolness of the greyhound! The last series on labs will be next week and it will focus on liver and thyroid functions.  Hope you check it out!  

Iazbik MC, Couto CG. Morphologic characterization of specific granules in Greyhound eosinophils. Vet Clin Pathol. 2005; 34:140–143.
Feature image credit: Cindy Frezon
Sullivan PS, Evans HL, McDonald TP. Platelet concentration and hemoglobin function in greyhounds. J Am Vet Med Assoc. 1994; 205:838–841. [PubMed: 7829376]
Zaldívar-López, S., Marín, L. M., Iazbik, M. C., Westendorf-Stingle, N., Hensley, S., & Couto, C. G. (2011). Clinical pathology of Greyhounds and other sighthounds. Veterinary Clinical Pathology / American Society for Veterinary Clinical Pathology40(4), 10.1111/j.1939–165X.2011.00360.x. http://doi.org/10.1111/j.1939-165X.2011.00360.x