Autoimmune hemolytic anemia
Anemia is a
clinical sign, not a disease, and is
defined as a decrease in the number
of red blood cells (RBCs) or the
amount of hemoglobin, resulting in a
decrease in the oxygen-carrying
capacity of the blood. Anemia can be
caused by blood loss, decreased
production of new RBCs, or an
increase in the rate of their
destruction, known as hemolytic
anemia.
In hemolytic
anemia, the RBCs become "defective"
in the eyes of the immune system by
acquiring markers on the cell
surface that are recognized as
"non-self." These markers can be
true autoantibodies, as in primary
AIHA, or can be secondary to drugs,
infectious disease, cancer, blood
parasites, or heavy metals.
Levamisole, certain antibiotics,
Dilantin (phenytoin), lead, and zinc
have all been implicated as
potential causes of hemolytic
anemia.
When the
spleen and the rest of the immune
system is working to rid the body of
old, diseased, or damaged RBCs, it
is doing its job appropriately. When
a large percentage of cells are
affected, and they are removed
faster then they can be replaced,
AIHA results and the animal shows
external signs of the disease.
The clinical
signs of AIHA are usually gradual
and progressive, but occasionally an
apparently healthy pet suddenly
collapses in an acute hemolytic
crisis. The signs are usually
related to lack of oxygen: weakness,
lethargy, anorexia, and an increase
in the heart rate and respirations.
Heart murmurs and pale mucous
membranes (gums, eyelids, etc.) may
also be present. More severe cases
also have a fever and "icterus"
(jaundice), a yellow discoloration
of the gums, eyes, and skin. This is
due to a buildup of bilirubin, one
of the breakdown products of
hemoglobin.
The
diagnosis is usually made on these
clinical signs as well as a CBC
documenting anemia, often with
misshapen or abnormally-clumped RBCs.
A Coomb's test may be done to
confirm the diagnosis.
Corticosteroids are the primary
drugs used to treat any autoimmune
disease. Very high immunosuppressive
doses are used initially to induce a
remission, and then the dose is very
slowly tapered over many weeks or
months to a low maintenance dose.
Most affected dogs must be kept on
steroids the rest of their lives and
are susceptible to relapses.
If steroids
alone are insufficient, more potent
immunosuppressive drugs such as
Cytoxan (cyclophosphamide) or Imuran
(azathioprine) may be added. These
chemotherapeutics are very
effective, but the dog must be
monitored closely for side effects,
including a decreased white blood
count.
Splenectomy,
the surgical removal of the spleen,
has also been recommended for
nonresponsive cases. This benefits
the dog in two ways: less antibodies
are made against the RBCs, and the
primary organ responsible for their
destruction is removed. An animal
can live quite normally without a
spleen.
Blood
transfusions are rarely used. Adding
foreign protein can actually
intensify the crisis state, increase
the amount of bilirubin and other
breakdown products the liver must
process, and suppress the bone
marrow's natural response to anemia.
In a life-threatening anemia,
cross-matched blood may be
transfused along with
immunosuppressive therapy.
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