Page 1376 - Small Animal Internal Medicine, 6th Edition

1348 PART XIII Hematology

the two most common causes in cats, although IHA is now
more common in this species. Other causes of extravascular
VetBooks.ir hemolytic anemia in dogs and cats are listed in Table 82.4.
Intravascular hemolysis can occur as a consequence of
direct RBC lysis caused by antibodies that activate comple-
ment (e.g., immune-mediated hemolysis with a high concen-
tration of IgG or IgM), infectious agents (e.g., Babesia canis
infection), drugs or toxins (e.g., zinc in pennies minted after
1983, in pet carrier bolts, other hardware, and zinc oxide–
containing ointments), metabolic imbalances (e.g., hypo-
phosphatemia in dogs and cats with diabetes mellitus treated
with insulin), or increased shearing of RBCs (e.g., microan-
giopathy, DIC). Intravascular hemolysis is considerably less
common in dogs and cats than extravascular hemolysis, with
the notable exception of DIC in dogs with HSA, zinc toxicity,
and hypophosphatemia. Certain congenital enzymopathies
(e.g., phosphofructokinase [PFK] deficiency) in dogs also
result primarily in intravascular hemolysis.
Dogs with congenital (frequently familial) hemolytic
anemias may have relatively prolonged clinical courses at the FIG 82.5
time of presentation, with the notable exception of English Large numbers of B. gibsoni in Diff-Quik stained capillary
Springer Spaniels with PFK deficiency–associated hemolysis, blood of a 7-year-old, female spayed Pitbull shortly after
in which acute hemolytic episodes occur after they hyper- undergoing a splenectomy (×1000).
ventilate during excitement (e.g., going to the veterinarian
or field work) due to alkaline hemolysis.
Dogs and cats with acquired hemolytic anemias are
usually evaluated because of acute clinical signs consisting
of pallor, with or without icterus (in my experience, only
approximately 50% of dogs and a much lower percentage of
cats with hemolytic anemia are icteric); diffuse splenomegaly
may be a prominent finding. If the patient has associated
thrombocytopenia (e.g., Evans syndrome, DIC), petechiae
and ecchymoses may be present. Clinical signs and physical
examination findings associated with the primary disease
can also be present in cases of secondary hemolytic anemias;
however, as opposed to humans, they are extremely rare in
dogs and cats.
In the evaluation of dogs or cats with hemolytic anemia,
a careful examination of the blood smear is mandatory. Mor-
phologic abnormalities pathognomonic for or highly sugges-
tive of a particular cause are often detected with this method
(see Table 82.2). In specific circumstances, capillary blood
should also be evaluated. In dogs with B. gibsoni infection FIG 82.6
Marked saline autoagglutination in a dog with immune
(mainly Pitbulls), the organism is rarely visualized in periph- hemolytic anemia (IHA).
eral blood but is often obvious in capillary blood from the
pinna (Fig. 82.5). We collect blood from the tip of the pinna
after constricting it for a few seconds and applying alcohol RBCs, and cause agglutination; it is sometimes referred to as
to induce vasodilation. After puncturing the pinna with a Mother Nature’s Coombs test (see later). It can be distin-
20-gauge needle, we collect a drop of seeping blood with a guished from rouleaux formation by adding saline solution,
microhematocrit tube and use it to make a smear. In my which disaggregates rouleaux; rouleaux formation is
experience, B. gibsoni organisms are easier to visualize when common in cats but rare in dogs. An easy way to do this is
using Diff-Quik than Giemsa or Wright-Giemsa stain. to place a drop of anticoagulated blood on a glass slide,
The sample should also be tested for autoagglutination by adding a coverslip, and “shooting” saline solution under-
placing a large drop of anticoagulated blood on a glass slide neath the coverslip (by capillarity) using a pipette.
at room temperature and at 4° C (Fig. 82.6). Autoagglutina- A direct Coombs test to detect RBC-bound Ig should be
tion occurs when large amounts of Ig are bound to the RBC performed in dogs and cats with suspected hemolysis and
so that the molecules tangle with molecules in adjoining lack of autoagglutination (see later). As a general rule, the