Hemolytic anemia in cats: Infectious or immune-mediated? (Proceedings)
Classic signs of anemia include lethargy, weakness, inappetence, pale mucous membranes or icteric membranes if hemolysis is occurring. Given that infectious disease and neoplasia are two major rule-outs for hemolytic anemia, fever is often present. Exposure to drugs or toxins such as onions, methimazole and acetaminophen is important to ascertain. Other causes of anemia (without hemolysis) include chronic renal disease, FeLV/FIV, primary bone marrow disorders and anemia of chronic disease. This latter group of diseases should be considered when the anemia is non-regenerative.
A complete blood count, including reticulocyte count, will determine whether the anemia is regenerative or non-regenerative. Acute blood loss will not be regenerative for 2-4 days; clinical evidence of bleeding and a low plasma protein should differentiate acute blood loss from hemolytic anemia. Spherocytes are difficult to identify in feline blood smears and are therefore not a reliable means to diagnose hemolytic anemia. Presence of reticulocytes and/or nucleated red blood cells and macrocytosis with or without autoagglutination is diagnostic for regenerative anemia. A Coombs' test may be performed, but a negative test does not rule out hemolytic anemia. A biochemistry panel is necessary to rule in or out other causes of anemia and may demonstrate hyperbilirubinemia if hemolysis is severe. Depending on the infectious organism or neoplastic process, other abnormalities may be apparent. FeLV/FIV testing should be done and thoracic and abdominal radiographs (or abdominal ultrasound) to evaluate for neoplasia.
Specific Infectious Diseases
Several infectious diseases have been documented to cause hemolytic anemia in cats, though some are more common than others.
The most common of these are the hemoplasmas, recently reclassified from a Rickettsia (Haemobartonella) to the Mycoplasma genus: Mycoplasma haemofelis (large form) and Mycoplasma haemominutum (small form). Identification of the epi-erythrocytic organism on a blood smear is unreliable because of cyclic parasitemia, staining artifacts and sample handling. PCR for organismal DNA is the most sensitive and specific test for the hemoplasmas. Treatment includes doxycycline or the fluoroquinolones in certain circumstances.
Although generally a rapidly fatal disease in cats, recent case studies have demonstrated some success with therapy. "Signet ring"-shaped organisms are located within the red blood cell during the erythrocytic phase of the infection, but these may not be present in the early phases of the disease. PCR assays are also available at university research labs. Treatment in cats that have survived has included an anti-protozoal agent (parvaquone and buparvaquone) combined with imidocarb and aggressive supportive care. A promising combination of an anti-malarial drug (atovaquone) and azithromycin is currently being evaluated by several researchers.