Common Breed-Specific Abnormalities in Dog Hematology
A long-term client brings in their newly adopted adult greyhound “Millie” for her first physical examination after picking her up from the rescue. Everything looks great; Millie gets her annual vaccines, and survey labwork, including blood samples, is run in house.
Your technician comes up to you with concern because Millie has a very high PCV when comparing the result to the generic reference interval for dogs. You take the time to look at a blood smear in-house, and some of the white blood cells are vacuolated in a way you have never seen before.
What does this mean? Does Millie need a bone marrow biopsy? What would you do next?
Why is breed-specific hematology data important?
Millie’s story is an important example of developing modern and breed-specific reference intervals. What may seem abnormal in some patients might actually be fairly innocuous in certain canine breeds.
There is no one-size-fits-all solution when it comes to accurately assessing hematological and biochemical parameters between dog breeds. The American Society for Veterinary Clinical Pathology has created a framework to help streamline studies. However, despite this helpful framework,, accurate, trustworthy data takes time to accrue. Previous studies for canine hematology have brought us far, but bringing information down to a breed-specific level will bring us even further with canine reference intervals.
Breed-associated cell morphology anomalies have the potential to result in hematologic abnormalities reported via automated hematology analyzer. These reported abnormal values may be of little clinical significance for the patient but lead the practitioner down the wrong path.
To prevent overinterpretation of a single value, assessing cell count and cell morphology together is important to obtain the whole clinical picture. Doing so will result in a complete hematologic evaluation and proper differential diagnosis construction. What may seem abnormal could be mistakenly attributed to pathologic disease processes, so keeping in mind breed-associated anomalies can help prevent misdiagnoses during hematologic evaluation.
The most common abnormalities in dog hematology
There are a number of inherited phenomena that manifest in various dog breeds that could lead to a misdiagnosis. Below we’ve broken down each abnormality along with the main and minor breeds that are affected during hematological analysis.
Inherited Macrothrombocytopenia
Main breeds affected: Cavalier King Charles Spaniels
Minor breeds affected: Jack Russel Terrier, Bichon, Mini Shwouzer, Boxer, Havanese, Shih Tzu, and mixed breeds
Most commonly found in Cavalier King Charles Spaniels, macrothrombocytopenia is an inherited disease that that changes the size of platelets in these dogs. Platelets should be smaller in diameter than a red blood cell. However, in dogs with the ano gene, platelets are larger. This is important clinically because automated hematology analyzers are unable to differentiate them from other, similar-sized cells. This results in low platelet counts but is not often representative of a true thrombocytopenia.
In these patients, assessing cell morphology is very important to ensure that the patient’s platelet levels are in fact normal.
Being heterozygous or homozygous for this gene affects Cavalier King Charles differently.1
Status & Platelet Count
Normal — 200,000 - 500,000/uL
Heterozygous — 100,000 - 250,000/uL
Homozygous — 30,000 to 100,000/uL
Heterozygous dogs will have a platelet count of 100,000 - 250,000/uL whereas homozygous dogs will be lower ranging from 30,000 to 100,000/uL. In normal dogs, the platelet count is 200,000 - 500,000/uL.
Although Cavalier King Charles is the most commonly known breed with this anomaly, other breeds including the Jack Russell Terrier, Bichon, Mini Schnauzer, Boxer, Havanese, Shih Tzu, and mixed breeds have also been cited.
It's important to note that even though the cells appear different, these patients exhibit normal clotting abilities.
Pelger Huët Anomaly
Main breeds affected: Australian Shepherds
Minor breeds affected: Cocker Spaniel, Basenji, Border Collie, English and American Foxhound, Samoyed, Australian Cattle Dog, Boston Terrier, German Shepherd, Coonhound, and mixed-breed dogs 2
Pelger-Huët anomaly (PHA) is a genetic variance that causes a reduction or elimination of segmentation in the nuclei of granulocytes. This means that basophils, eosinophils, and neutrophils can be affected. However, neutrophil changes are most commonly encountered.
Essentially, PHA results in little visual difference between mature neutrophils and immature neutrophils. The nuclei shape can be oval, bilobed, kidney, or round shaped. This can be very alarming as the clinical meaning of 15,000 mature neutrophils is completely different from that of 15,000 immature ones.
It is important to note that neutrophil hyposegmentation can also occur with myeloid neoplasms and inconsistently with the administration of a variety of different drugs.2 Therefore, differentiating patients with true PHA from those with pseudo- Pelger Huet Anomaly (PPHA) must be done with care.
PHA has been identified in dogs, cats, rabbits, horses, and humans alike. In veterinary medicine, it is most commonly noted in Australian Shepherds with an estimated incidence of 9.8%.2 Most animals are heterozygous for this gene as animals that are homozygous for it often do not make it through development in utero.
The question is, do neutrophils with PHA function normally? Pelger-Huët neutrophils were compared with normal neutrophils of dogs in various studies. No significant differences in neutrophil adherence, random movement, chemotaxis, phagocytosis, and bactericidal activity were noted.3 Therefore, animals with Pelger-Huët anomaly have fully functional neutrophils and do not exhibit clinical signs associated with this change in cell morphology.
Grey Eosinophils
Main breeds affected: Greyhounds
In most dog breeds, no matter what stain you use to evaluate a blood smear, eosinophils will exhibit light purple cytoplasm with a dark purple nucleus and pink intracytoplasmic granules. However, this is not true for almost 53% of the Greyhound population.
A study performed by Iazbikk and Couto in 2005 identified that eosinophils in this breed exhibit different staining characteristics.4 Their cytoplasm is a hue of purple/grey, and there are no pink granules. Rather there are vacuole-like structures that in reality are non-staining granules. This appearance could lead to misidentifying eosinophils as toxic neutrophils, which could result in miscategorization and the creation of a faulty differential list by veterinarians.
It’s not surprising that Greyhounds would be unique. Breed specific reference intervals have been published for Greyhounds as there are many other hematologic and biochemistry parameters that are breed-specific. For instance, the PCV of Greyhounds is above the upper reference limit used for other breeds of dogs.
Although research into Greyhound hematology has been done, it is still unclear why these cells are morphologically different in this breed.Possible causes for the nonstaining granules include an alteration in basic proteins that confer the pink-orange hue in eosinophil granules or a decrease in the pH of components of eosinophilic granules, resulting in less binding of the eosin in Romanowsky stains.5 Gratefully when noted, this unique morphology is not clinically significant.
Macrocytosis
Main breeds affected: Toy and Miniature Poodles
Minor breeds affected: Greyhounds, Toy Poodle mixes (Cockapoo, Yorkipoo, etc.)
Macrocytosis is defined as an increased cell size and is assessed using mean cell volume (MCV). In normal dogs, the MCV is 60-77fL. Differentials outside of a genetic anomaly for elevations in MCV include B-12 deficiencies, regenerative anemias, or agglutination.6
Status & MCV
Normal — 60-77fL
Macrocytosis — 85-105fL
Differentiating heredity macrocytosis from clinically significant cases is based on a combination of variables. Previous studies have shown that there is in fact congenital dyserythropoiesis from the bone marrow in animals partaking in these investigations.
For these animals, ranges from 85-105fL are often seen. Polychromasia is minimally evident to absent, and patients are not anemic. Howell-Jolly bodies are noted more regularly within red blood cells. Metarubricytes with variable nucleus to cytoplasma ratios are also seen. Hypersegmentation of the nucleus of neutrophils is also noted in some of these cases. 7
Despite these morphologic abnormalities, patients exhibiting hereditary macrocytosis are not clinically affected.
But what about Millie?
Now that you have some extra information regarding congenital differentials associated with hematologic abnormalities, a plan may be clearer for Millie’s case.
Taking into consideration that she is a greyhound and what you have learned, you are not surprised that her labwork is a little different than other breeds.
Millie’s PCV is expected for this breed despite being above the upper reference limit. After speaking with a clinical pathologist at your reference lab, you realize that what you were seeing were grey-eosinophils. This morphological characteristic of her white blood cells is often noted in greyhounds as well. Gratefully these findings are normal for Millie based on her breed and they will not negatively affect her.
Taking breed differences in dog hematology further
Considering breed-related differences in dog hematology can help veterinarians not only come to definitive diagnoses but also guide appropriate care for patients.
Due to the lack of outward clinical signs associated with these morphologic blood cell anomalies and high prevalence of mixed breed dogs, it is almost impossible to predict which patients should automatically have a blood smear evaluated when a complete blood count is submitted. Working with an animal diagnostics laboratory that specializes in and considers breed differences in canine hematology can enhance patient care.
With the advent and progress of computer vision, or artificial intelligence, the time-consuming process of predicting which patients need a blood smear evaluated is no longer necessary or separate from CBCs; Moichor’s ability to implement computer vision technology means CBC results are automatically confirmed, RBC, WBC, and platelet morphology are assessed, and evidence of infectious diseases is identified.
Moichor’s computer vision technology and inclusion of cell morphologic assessments in every complete blood count is improving veterinary practitioners' access to seeing the fuller picture when it comes to animal health. Moichor is dedicated to expanding reference intervals in general but also committed to creating breed-specific reference intervals.
Click here to see our full test and services menu and learn more about how computer vision can help your clinic take advantage of breed-specific differences in dog hematology.
References:
7. Other WBC changes. Accessed August 25, 2022. https://eclinpath.com/hematology/morphologic-features/white-blood-cells/other-wbc-changes/
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