Page 1266 - Small Animal Internal Medicine, 6th Edition

1238 PART XI Immune-Mediated Disorders

immunosuppressive drugs. The remaining dogs require PURE RED CELL APLASIA
long-term immunosuppressive therapy. Factors that clini-
VetBooks.ir cally appear to confer a good prognosis in dogs with IMHA PRCA is a rare disorder characterized by severe, nonregen-
erative anemia with marked depletion or absence of ery-
include a rapid response to treatment with glucocorticoids,
ability to maintain the packed cell volume above 30% with
evidence of concurrent peripheral hemolysis is present on
glucocorticoids alone, and identification of a treatable sec- throid precursors in the bone marrow. In some cases,
ondary cause. the basis of the presence of spherocytes and a positive direct
The prognosis is guarded in dogs that require multiple Coombs test. Other cell lines are usually normal. The ery-
drugs to control the disease and those with persistent auto- throid aplasia in PRCA is in contrast to the nonregenerative
agglutination, azotemia, an elevated bilirubin concentration, form of IMHA, in which there is erythroid hyperplasia or
marked thrombocytopenia, and severe leukocytosis. In a sometimes maturation arrest of the erythroid maturation
multicenter study, prognostic factors for canine IMHA were sequence at the rubricyte or metarubricyte level. PRCA is
investigated, and a model using the presence of SIRS, bili- likely one end of the spectrum of IMHA, with acute periph-
rubin, urea, and creatinine concentrations, and ASA score eral hemolysis at the other end of this spectrum (Table 73.4).
was predictive of outcome in 80% of cases. In this model, The affinity of circulating antibody for different erythroid
bilirubin concentration and azotemia were independently precursors likely influences the level at which damage occurs
associated with death. Reported mortality rates of dogs with in the bone marrow. As with IMHA, both primary and sec-
primary IMHA range from 26% to 70%; thromboembo- ondary forms of PRCA are recognized. Secondary causes of
lism is the cause of death in 30% to 60% of cases. Other PRCA include treatment with recombinant human erythro-
common causes of death include infection due to immu- poietin and parvovirus infection in dogs. Infection with
nosuppression, DIC, and failure to control anemia. Inter- feline leukemia virus subtype C is a cause of PRCA in cats.
estingly, patients that are hypercoagulable as assessed by Dogs with PRCA have a similar signalment and present
TEG have a better prognosis than those that have normal with similar clinical signs as dogs with IMHA. As is the case
coagulability; it has been proposed that, in these dogs, there with primary IMHA, cats with primary PRCA are typically
is an underlying consumptive coagulopathy. If a major TE younger than dogs, with an age range of 8 months to 3 years.
does occur in a dog with IMHA, particularly if the blood Dogs and cats with PRCA have severe, nonregenerative
supply to a major organ is disrupted, the long-term prog- anemia, whereas the platelet count and leukogram are typi-
nosis is poor. Contrary to popular opinion, the prognosis cally normal. In contrast to IMHA, the biochemical panel
in Cocker Spaniels with IMHA does not differ from that of and urinalysis are also usually unremarkable, with no
other breeds. evidence of peripheral hemolysis or inflammation. Low
numbers of spherocytes are sometimes present in dogs with
PRCA. The Coombs test is usually negative.
FELINE IMMUNE-MEDIATED Diagnosis of PRCA is made by evaluation of a bone
HEMOLYTIC ANEMIA marrow aspirate and bone marrow core biopsy. In PRCA,
erythroid precursors are rare or absent and the M/E ratio is
Cats with primary IMHA tend to be younger than dogs, with high (>99 : 1). In contrast to nonregenerative IMHA, severe
a median age of 2 years. Males are slightly overrepresented, myelofibrosis is rare.
with no influence of neuter status. Secondary causes for Treatment of PRCA is similar to IMHA. In the authors’
antierythrocyte antibody formation are identified more fre- experience, most dogs with PRCA respond to prednisolone
quently in cats, with feline leukemia virus and Mycoplasma alone; however, other drugs are commonly added to the
haemofelis being common infections associated with IMHA. treatment protocol because of the length of time it takes to
Primary IMHA in cats is more likely to present with a non- see a response in some dogs. Adjunctive immunosuppressive
regenerative anemia, accounting for about 50% of cases. medications have been used with success in dogs that have
Most cats with IMHA respond to prednisolone alone, and an incomplete response to prednisolone alone. These adjunc-
the adverse effects of glucocorticoids are less severe. In cats tive medications include azathioprine, cyclophosphamide,
that need an additional immunosuppressive drug to treat cyclosporine, and mycophenolate. The time taken to achieve
IMHA, treatment with chlorambucil, cyclosporine, or myco- complete remission (2-6 months) is longer in dogs with
phenolate should be considered. Not enough published PRCA compared with IMHA, and it is sometimes difficult
information exists to recommend one drug over another. to judge whether a particular protocol is failing or whether
Azathioprine is not recommended in cats because of the risk inadequate time has been allowed for the bone marrow to
of unacceptable adverse effects (see Chapter 72). The authors respond to treatment and begin to produce and release RBCs
typically use chlorambucil or cyclosporine in cats that need into the circulation. Sequential bone marrow evaluations
an additional immunosuppressive drug. There is an increased should ideally be used to determine when to change the
risk of diabetes mellitus in cats treated with glucocorticoids; treatment protocol. A repeat bone marrow aspirate should
in these cats a second immunosuppressive drug should be be considered after 2 months of treatment if no improvement
added to allow tapering and ultimately discontinuation of in the anemia is observed. Repeated transfusion of pRBCs
glucocorticoids. or whole blood is necessary while waiting for a response to