Page 1423 - Small Animal Internal Medicine, 6th Edition

CHAPTER 87 Disorders of Hemostasis 1395

Once thrombocytopenia has been confirmed by a platelet
BOX 87.4 count, evaluation of a blood smear or analyzer graphics, its
VetBooks.ir Causes of Thrombocytopenia in Dogs and Cats pathogenesis should be identified. The absolute platelet
count may offer clues to its cause; for example, platelet counts
of <25,000/µL are common in dogs with immune-mediated
Decreased Platelet Production
Immune-mediated megakaryocytic hypoplasia thrombocytopenia (IMT), whereas platelet counts of 50,000
Idiopathic bone marrow aplasia to 75,000/µL are more common in dogs with ehrlichiosis,
Drug-induced megakaryocytic hypoplasia (estrogens, anaplasmosis, lymphoma with splenic involvement, or
phenylbutazone, melphalan, lomustine β-lactams) rodenticide toxicity.
Myelophthisis The patient’s drug history should be obtained from the
Cyclic thrombocytopenia owner. If the pet is receiving any medication, the thrombo-
Retroviral infection cytopenia should be considered drug-related until proven
Canine monocytic ehrlichiosis otherwise. The drug should be discontinued, if possible, and
Feline monocytic ehrlichiosis?
the platelet count reevaluated within 2 to 6 days. If the count
Increased Platelet Destruction, Sequestration, or returns to normal, a retrospective diagnosis of drug-
Utilization associated thrombocytopenia is made. Drugs that have been
Immune-mediated thrombocytopenia associated with thrombocytopenia in cats and dogs can also
Infectious (e.g., Anaplasma spp., Bartonella spp., sepsis) cause anemia and neutropenia (see Boxes 82.2 and 85.1).
Live viral vaccine–induced thrombocytopenia Because retroviral disorders commonly affect the bone
Drug-induced thrombocytopenia marrow and may result in thrombocytopenia in cats, feline
Microangiopathy leukemia virus (FeLV) and feline immunodeficiency virus
Disseminated intravascular coagulation tests should be performed first. A bone marrow aspiration is
Hemolytic-uremic syndrome, thrombotic thrombocytopenic indicated in a thrombocytopenic cat with no history of pre-
purpura vious medication and a negative retrovirus status. The risk
Vasculitis
Splenomegaly of bleeding during or after bone marrow aspiration in a
Splenic torsion thrombocytopenic animal is minimal. If determined by the
Endotoxemia laboratory, a mean platelet volume is high in most cats with
Acute hepatic necrosis FeLV infection (i.e., macrothrombocytosis); however, mac-
Neoplasia (immune mediated, microangiopathy) rothrombocytes are also seen in cats and dogs with periph-
eral platelet destruction, consumption, or sequestration, in
Note: Entries in boldface are common causes; entries in italics are which they may be analogous to reticulocytes (young, imma-
relatively common causes; entries in regular typeface are ture, large platelets) and in normal Cavalier King Charles
uncommon causes.
Spaniels.
Bone marrow evaluation may also be indicated for dogs
infectious mechanisms; increased platelet consumption with thrombocytopenia. Given the high prevalence of IMT,
occurs most commonly in dogs and cats with DIC (see later), at our clinic we usually elect to treat a dog with a presumed
and sequestration is usually caused by splenomegaly or, diagnosis of IMT. If the patient does not respond to immu-
rarely, hepatomegaly (see Box 87.4). nosuppressive drugs within 2 to 3 days, a bone marrow aspi-
ration may be performed.
Approach to the Patient With Hyperplasia of megakaryocytes occurs in response to
Thrombocytopenia peripheral destruction, consumption, or sequestration of
Before assessing a patient with primary hemostatic bleeding, platelets. Occasionally dogs and cats with IMT have decreased
the clinician must remember that in some breeds, platelet numbers of megakaryocytes and abundant free megakaryo-
counts below the reference range for dogs are common. cyte nuclei in the bone marrow. This is thought to be medi-
Platelet counts in Greyhounds typically range from 80,000 ated by antibodies directed against platelets that also destroy
to 120,000/µL, whereas in Cavalier King Charles Spaniels the megakaryocytes. Infiltrative or dysplastic bone marrow
with macrothrombocytopenia, platelet counts < 50,000/µL disorders causing thrombocytopenia are easy to identify on
are common. In the latter the global platelet function is a bone marrow smear.
normal. Because of the tendency of feline platelets to clump Because IMT is a diagnosis of exclusion, vector-borne
in the EDTA tube, we see a few cats per year referred for diseases (e.g., canine ehrlichiosis or anaplasmosis, Rocky
evaluation of asymptomatic thrombocytopenia. Evaluation Mountain spotted fever, leishmaniasis, babesiosis, bartonel-
of the blood smear or hematology analyzer graphics in those losis) should theoretically be ruled out by evaluating the
cats reveals platelet clumping, and hence, pseudothrombo- appropriate serology or PCR and a blood smear. In throm-
cytopenia. Iloprost, a prostaglandin I 2 analogue added to the bocytopenic dogs, we perform a SNAP-4DX Plus test
blood collection tube, can effectively decrease platelet aggre- (IDEXX Laboratories) as the first line of diagnostics to rule
gation in cats and eliminate or minimize thrombocytopenia out Ehrlichia, Anaplasma, and Borrelia as causative agents;
(Riond et al., 2015). however, acutely infected dogs may be thrombocytopenic