Page 1396 - Small Animal Internal Medicine, 6th Edition

1368 PART XIII Hematology

CHAPTER 84
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Erythrocytosis

DEFINITION AND CLASSIFICATION concentrations. Recently, JAK2 mutations were identified
in a dog with PRV (Beurlet et al., 2011); this mutation is
Erythrocytosis is defined as an increase in the circulating red common in humans with PRV. In a recent study, the clinical
blood cell (RBC) mass and is manifested hematologically as and hematologic features of 18 cats with primary erythrocy-
an increase in the packed cell volume (PCV) or hematocrit tosis were reviewed (Darcy et al., 2018).
(HCT) above the reference interval (RI). Because determina- Secondary erythrocytosis results from increased ortho-
tion of the RBC mass in a clinical setting is cumbersome and topic (produced by the kidneys) or heterotopic (produced
impractical, a diagnosis of erythrocytosis is typically made in sites other than the kidneys) Epo production. Ortho-
on the basis of the high HCT, not an increased RBC mass. topic (physiologically appropriate) Epo production occurs
Certain dog breeds, such as most sighthounds, have HCT in response to tissue hypoxia, such as that occurring at a
values above the RI for the species; this also occurs in dogs high altitude and in the settings of chronic cardiopulmonary
that live at high altitudes. For example, normal actively disease, right-to-left cardiovascular shunts, and carboxyhe-
racing or retired racing Greyhounds can have an HCT as moglobinemia. Tumor-associated erythrocytosis (hetero-
high as 70%. An increase in the RBC numbers may lead to topic or orthotopic Epo production) has been observed in
severe hemorheologic alterations, resulting in clinical signs humans with a wide variety of neoplasms, in dogs with renal
secondary to hyperviscosity. Although the term polycythe- masses, and in dogs with spindle cell sarcomas (e.g., nasal
mia is commonly used to refer to this hematologic abnor- fibrosarcoma, schwannoma, cecal gastrointestinal stromal
mality, it is incorrect because the term actually means an tumor). In a recent retrospective study we conducted,
increase in the numbers of all circulating cells (“poly” means 15/29 (51%) of dogs with renal lymphoma had erythrocy-
multiple). tosis. Hormonal stimuli may also trigger erythrocytosis in
On the basis of its pathogenesis, erythrocytosis can be animals with normal tissue oxygenation, such as in dogs with
classified as relative or absolute (Box 84.1). The term relative hyperadrenocorticism and cats with hyperthyroidism. At the
erythrocytosis refers to hemoconcentration (i.e., dehydra- author’s clinic, secondary erythrocytosis is more common in
tion) and is characterized by an increased PCV, usually in dogs, and PRV is more common in cats. However, erythrocy-
association with an increased serum or plasma protein con- tosis is rare in both species. Interestingly, although infiltrative
centration; in dogs and cats with relative erythrocytosis, the renal diseases (e.g., lymphoma, feline infectious peritonitis)
RBC mass is normal. Dogs with hemorrhagic gastroenteritis are common in cats, they rarely result in secondary eryth-
(HGE) frequently have relative erythrocytosis associated rocytosis. Recently a cat with a renal adenocarcinoma and
with normal serum or plasma protein concentration; the secondary erythrocytosis was reported.
reason for the lack of increase in the protein concentration
is unknown, but the erythrocytosis resolves with appropriate Clinical and Clinicopathologic Findings
fluid therapy. In absolute or true erythrocytosis, the RBC The clinical signs may occur acutely and consist primarily of
mass is increased; it can be classified as primary or secondary functional abnormalities of the central nervous system (e.g.,
depending on the pathogenesis and serum erythropoietin behavioral, motor, or sensory changes; seizures). In cats,
(Epo) concentration or activity. signs of a transverse myelopathy are common; in the retro-
Primary erythrocytosis (polycythemia rubra vera [PRV]) spective study of 18 cats with erythrocytosis, the most
results from an autonomous, Epo-independent proliferation common clinical signs were seizures and mentation changes
of RBC precursors in the bone marrow and is considered (Darcy et al., ePub ahead of print). A common manifestation
a myeloproliferative disease. As a consequence, most dogs of erythrocytosis in dogs is paroxysmal sneezing, likely
and cats with PRV have low to nondetectable serum Epo caused by increased blood viscosity in the nasal mucosa.

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