Primary IMHA in Cats: More Common Than Thought
Primary immune-mediated hemolytic anemia (IMHA) is diagnosed when no cause can be found for the condition. Secondary IMHA in cats has been associated with FeLV, FIP, Mycoplasma infection, drugs, systemic lupus erythematosus, and lymphoma. Retrospective review of 19 cats with primary IMHA revealed the following trends: The age of onset varied widely from 6 months to 9 years of age (median, 2 years). Male cats were more commonly affected. Complete blood counts revealed that packed cell volume ranged from 6% to 22% (median, 12%), with no evidence of regeneration in 11 cats. Leukocytosis, lymphocytosis, hyperbilirubinemia, hyperglobulinemia, and elevated serum levels of alkaline phosphatase, alanine transaminase, and aspartate transaminase were common. Diagnostic imaging showed enlargement of the spleen (n = 6), liver (n = 2), and of both organs (n = 2). The results of Coombs' testing were positive in 15 cats (IgG only or IgG and IgM), but it could not be done in the other 4 cats because of persistent agglutination of red blood cells. All cats were treated initially with prednisone 1 to 2 mg/kg Q 12 H, and 10 of 19 cats needed at least 1 blood transfusion. Twelve cats received doxycycline and the other 7 received other antibiotics; 11 also received histamine blockers. Seventeen cats were monitored for varying periods; 2 cats were lost to follow-up. Overall mortality was 23.5% (day 9, 63, 8.5 months, and 5.9 years), and relapses occurred in 5 of 16 cats. This study found that IMHA in cats was more common than previously reported and had a better prognosis in cats than in dogs.
COMMENTARY: This article supports use of the differentiating Coombs' test to obtain a diagnosis of primary immune-mediated hemolytic anemia in cats. The differentiating Coombs' test detects erythrocyte-bound IgG, IgM, or C3b. A similar test available at some commercial laboratories is the direct antiglobulin test (DAT). The DAT reports findings on an increasing scale of 0 through 4+ instead of through a titer reading.
As with dogs, feline primary immune-mediated hemolytic anemia is a diagnosis of exclusion. Positive Coombs' tests can be seen with other diseases, such as chronic lymphocytic leukemia and systemic lupus erythematosus; in this study, 2 cats that were FeLV-positive and 1 cat with cholangiohepatitis had positive results.
The osmotic fragility test was mentioned in the article. It is conducted by subjecting a sample of red blood cells to decreasing concentrations of saline and noting the concentration at which 50% of the cells are hemolyzed. Osmotic fragility is increased in dogs with immune-mediated hemolytic anemia, but little is noted in the literature about cats. Osmotic fragility was measured in 10 of the 19 cats in this study, and all of the values were significantly elevated above the reference range. Osmotic fragility is not currently available in most commercial laboratories as far as I know, but more specialized laboratories may be able to run the test.
For most of us, this paper is an "abstract read," with the take-away point that we should add a few more tests and ruleouts to our workup of anemia in cats.
Primary immune-mediated hemolytic anemia in 19 cats: Diagnosis, therapy, and outcome (1998-2004). Kohn B, Weingart C, Eckmann V, et al. J VET INTERN MED 20:159-166, 2006.