Page 1374 - Small Animal Internal Medicine, 6th Edition

1346 PART XIII Hematology

TABLE 82.3
VetBooks.ir Criteria for Differentiating Blood Loss From Hemolytic
Anemias
VARIABLE BLOOD LOSS HEMOLYSIS

Serum (plasma) protein Normal-low Normal-high
concentration
Evidence of bleeding Common Rare
Icterus No Common
Hemoglobinemia No Common
Spherocytosis No Common
Hemosiderinuria No Yes
Autoagglutination No Occasional FIG 82.4
Direct Coombs test Negative Usually positive Abundant spherocytes in the blood smear of a dog with
(in IHA) immune-mediated hemolytic anemia (IHA). Some
Splenomegaly No Common erythrocytes and polychromatophilic erythrocytes contain
RBC changes No Common (see Howell-Jolly bodies.
Table 82.2)

IHA, Immune hemolytic anemia; RBC, red blood cell. the destruction of erythroid precursors in the bone marrow
From Couto CG et al.: Hematologic and oncologic emergencies. In
Murtaugh R et al., editors: Veterinary emergency and critical care results in a lack of regeneration (PRCA; see later).
medicine, St Louis, 1992, Mosby, p 359. On the basis of their pathogenesis, hemolytic anemias can
be classified as extravascular (i.e., the RBCs are destroyed by
the mononuclear-phagocytic cells) or intravascular (i.e., the
stopped; if the patient is bleeding as a result of a systemic RBCs are lysed by antibody and complement, drugs, toxins,
hemostatic defect, it should be identified and specific treat- or by hitting fibrin strands). In addition, on the basis of the
ment should be initiated (see Chapter 87). Aggressive intra- age of the animal at onset, anemias can be classified as con-
venous (IV) fluid therapy with crystalloids or colloids or the genital or acquired (Table 82.4). Most dogs and cats with
transfusion of blood or blood components is often required hemolytic anemia seen at our clinic have acquired extravas-
in patients with anemia caused by acute blood loss (see cular hemolysis.
p. 1355). In extravascular hemolysis, RBCs are phagocytosed by the
mononuclear-phagocytic system (MPS) in the spleen, liver,
Hemolytic Anemia and bone marrow. Stimuli that trigger RBC phagocytosis
In humans, the bone marrow is capable of undergoing consist mainly of intracellular inclusions, such as RBC para-
hyperplasia until its production rate is increased approxi- sites or Heinz bodies and membrane coating with immuno-
mately sixfold to eightfold; the same is probably true for globulin G (IgG) or IgM (common in dogs). Congenital RBC
dogs and cats. As a consequence, a considerable number of enzymopathies can also precipitate extravascular hemoly-
RBCs must be destroyed before anemia develops. As noted, sis. Once abnormal RBCs are recognized, the MPS rapidly
some dogs and cats with a normal HCT have high numbers phagocytoses them, resulting in a decrease in the number
of circulating reticulocytes; if the serum bilirubin concentra- of circulating RBCs and the generation of cells with specific
tion is marginally increased, or if there is hemoglobinuria morphologic changes (e.g., spherocytes). Anemia devel-
or bilirubinuria, hemolysis should be suspected in these ops if the destruction of RBCs continues. Spherocytes are
patients. Regardless, a blood smear should be evaluated in RBC “leftovers”; after a mononuclear-phagocytic cell takes
every dog or cat with reticulocytosis to identify morpho- “bites” off the cytoplasm and membrane, the membrane is
logic RBC changes, independently of whether the HCT is resealed. The RBC then loses its redundant membrane and
normal or low. As discussed earlier, the presence of reticulo- consequently its central pallor (Fig. 82.4). Spherocytes are
cytosis in dogs with normal HCT allows for early diagnosis characteristic of immune hemolytic anemia (IHA), although
of common diseases such as immune-mediated hemolysis they can be seen in other disorders, such as Babesia gibsoni
and HSA. infection, zinc toxicity, or hemophagocytic malignant histio-
As is the case in cats and dogs with blood loss anemia, cytosis; they also appear in the recipient after transfusion of
patients with peracute hemolysis can be in a nonregenerative stored blood. Immune hemolysis is the most common cause
state at the time of presentation because the bone marrow of extravascular hemolytic anemia in dogs in our clinic.
has not yet been able to mount a regenerative response. In Drug-associated hemolysis (e.g., β-lactam antibiotics) and
addition, in some dogs with immune-mediated hemolysis, mycoplasmosis (formerly known as hemobartonellosis) are