Page 1424 - Small Animal Internal Medicine, 6th Edition

1396 PART XIII Hematology

and have a negative test result because they have not yet effects on dogs with vector-borne diseases, but not all of
mounted an appropriate immune response. them are doxycycline responsive.
VetBooks.ir breed distribution (i.e., leishmaniasis in Foxhounds in the needed (see Chapter 82). However, the transfusion of WFB,
As discussed in Chapter 95, some of these diseases have
Blood or blood components should be transfused as
United States) or geographic distribution (e.g., leishmaniasis
malization of the platelet count or even in increases in the
in Mediterranean countries). As a general rule, if the patient platelet-rich plasma, or platelets rarely, if ever, results in nor-
is asymptomatic other than the bleeding, the thrombocyto- platelet count to safe levels. In addition, in most dogs, plate-
penia is not likely caused by sepsis or vector-borne diseases, let transfusions are cost-prohibitive. Symptomatic prohemo-
although occasionally asymptomatic thrombocytopenic static treatment with aminocaproic or tranexamic acid, or
dogs have subclinical vector-borne diseases, such as anaplas- Yunnan baiyao can be used in patients with life-threatening
mosis or rickettsiosis. If sepsis is suspected on the basis of bleeding.
clinical signs and clinicopathologic findings (e.g., fever,
tachycardia, poor perfusion, toxic leukocyte changes, degen- Immune-Mediated Thrombocytopenia
erative left shift in the leukogram, hypoglycemia, hyperbili- IMT is the most common cause of spontaneous bleeding in
rubinemia), urine and blood should be obtained for bacterial dogs but is rare in cats. It affects primarily middle-aged
cultures; as discussed earlier, cystocentesis should be avoided female dogs; Cocker Spaniels and Old English Sheepdogs are
in bleeding patients. overrepresented. The clinical signs are those of a primary
The presence of spherocytic hemolytic anemia or autoag- hemostatic defect and include petechiae, ecchymoses, and
glutination in a dog with thrombocytopenia is highly sug- mucosal bleeding. As discussed earlier, a bleeding score for
gestive of Evans syndrome (combination of IMT and immune thrombocytopenic dogs was proposed for dogs (Makielski
hemolytic anemia [IHA]). A direct Coombs test is usually et al., 2018). Acute collapse may occur if bleeding is pro-
positive in these cases. On rare occasions, a direct Coombs nounced; if the anemia is mild, most dogs are fairly asymp-
test is positive in a dog with IMT and borderline anemia, tomatic. IMT is acute or peracute in onset in most dogs.
further supporting a diagnosis of Evans syndrome (see During physical examination, signs of primary hemostatic
Chapters 73 and 82). bleeding (e.g., petechiae, ecchymoses, mucosal bleeding),
A hemostasis screen should always be performed to rule with or without splenomegaly, may be found.
out DIC in a thrombocytopenic animal found to have RBC The complete blood count (CBC) in dogs with IMT is
fragments in a blood smear or evidence of secondary bleed- characterized by thrombocytopenia with or without anemia,
ing (e.g., hematomas, bleeding into body cavities). The rest depending on the degree of spontaneous bleeding and pres-
of the hemostasis screen is usually normal in dogs and cats ence or absence of concurrent IHA; the anemia can be regen-
with selective thrombocytopenia. erative or nonregenerative, depending on the time of onset
Several tests are available to evaluate antiplatelet antibod- of the bleeding. Mature leukocytosis may also be present.
ies (see Chapter 71). However, most of these are not clinically However, as a general rule, in dogs with IMT hematologic
reliable, and a diagnosis of IMT can be made only after other changes are limited to the thrombocytopenia. If IHA is asso-
causes of thrombocytopenia have been excluded, regardless ciated with IMT (i.e., Evans syndrome), a Coombs-positive
of the results of the antiplatelet antibody tests. regenerative anemia with spherocytosis or autoagglutination
Abdominal radiographs and ultrasonograms may reveal is present. Bone marrow cytologic studies typically reveal
an enlarged spleen not evident during physical examina- megakaryocytic hyperplasia, although megakaryocytic
tion. Diffuse splenomegaly (splenic sequestration of plate- hypoplasia with free megakaryocyte nuclei is occasionally
lets) may be the cause of the thrombocytopenia, or it present. In addition to the thrombocytopenia, the bleeding
may reflect work hypertrophy (mononuclear phagocytic time is the only other abnormal test result (ACT, aPTT,
system hyperplasia) and extramedullary hematopoiesis in OSPT, FDP–D-dimer, and fibrinogen concentration are
a dog with IMT. Splenic nodules are usually an inciden- normal). An inverse linear correlation is usually present
tal finding in dogs with thrombocytopenia and may repre- between the platelet count and BMBT (i.e., a longer BMBT
sent extramedullary hematopoiesis or hyperplasia; FNA of with lower platelet counts). Ideally, vector-borne diseases
the nodules should establish a cytologic diagnosis. Despite and drug-induced thrombocytopenia should be ruled out
the low platelet counts, clinically relevant bleeding is before establishing a definitive diagnosis of IMT.
extremely rare. My approach is as follows: If the index of suspicion for
Often a specific diagnosis of IMT is obtained only after a IMT is high—that is, a fairly asymptomatic dog with sponta-
therapeutic trial with corticosteroids (see later and Chapters neous primary hemostatic bleeding and thrombocytopenia
72 and 73) results in resolution of the thrombocytopenia. If as the sole hematologic abnormality—I institute a therapeu-
the clinician is in doubt regarding whether the thrombocy- tic trial with immunosuppressive doses of corticosteroids
topenia is caused by a vector-borne diseases or IMT (in (equivalent to 2-8 mg/kg/day of prednisone). Responses are
dogs), immunosuppressive doses of corticosteroids can be usually seen within 24 to 96 hours. No clinical evidence
administered in conjunction with doxycycline (5-10 mg/kg exists that dexamethasone is more effective than prednisone
PO q12-24h) until serologic or PCR test results become in controlling IMT. In my experience, acute gastrointestinal
available. This combination of agents has no deleterious (GI) tract ulceration is considerably more prevalent in dogs