Anemias: Look out for vector-borne diseases
Q: Could you review anemias in cats?
A: Dr. Kristy Dowers gave an excellent lecture, "Causes of feline anemia: old and new," at the 2009 American College of Veterinary Internal Medicine Forum in Montreal. Here are some relevant points from it:
Anemia is a common blood abnormality in many species, including cats. Determining the regenerative nature of the anemia guides the work-up in cats. Regenerative anemias suggest blood loss or red-cell lysis. Red-cell lysis can be due to toxins, infectious agents, neoplasia (as a secondary immune-mediated phenomenon) or primary immune-mediated hemolytic anemia. Non-regenerative causes of anemia include iron deficiency, anemia of inflammatory disease, renal disease, bone-marrow disorders, neoplasia, infectious disease and immune-mediated disorders directed at erythrocyte precursors.Presentation, historical findings
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 anemia, fever may be present. Since many of the infectious agents are vector-borne, flea-and-tick exposure should be determined. Outdoor status increases the chance of contact with other cats and thereby exposure to the retroviral agents. Exposure to drugs or toxins such as onions and acetaminophen is important to ascertain. Other systemic signs, such as PU/PD with renal disease, can indicate the presence of other chronic diseases.
A complete blood count, including reticulocyte count, will determine whether regenerative or non-regenerative anemia is present. Acute blood loss and red blood cell lysis will not be regenerative for two to four days, so clinical and clinicopathologic parameters should be taken into account when making an assessment of the regenerative nature of the anemia. 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 can be diagnostic for regenerative anemia.
An iron-deficiency anemia will be characterized as microcytic anemia and FeLV-induced non-regenerative anemia will often be macrocytic anemia.
A thorough evaluation of blood smears can reveal intracellular or extracellular organisms. Presence of Heinz bodies may indicate an exposure to toxins that cause oxidative damage such as onions and propylene glycol. A direct Coombs' test may be performed, but a negative test does not rule out an immune-mediated anemia since its sensitivity and specificity are low in cats. An osmotic fragility test is indicated in breeds such as Abyssinians and Somalis which can have a hereditary erythrocyte membrane defect.
A serum chemistry panel is necessary to rule in or out other causes of anemia, such as renal disease and other systemic illnesses. Hyperbilirubinemia may be present if hemolysis is severe. Other abnormalities in the serum chemistry panel depend on the specific chronic disease, infectious agent or neoplastic process. FeLV antigen/FIV antibody testing should be done. Thoracic and abdominal radiographs (or abdominal ultrasound) should be performed to help rule out neoplasia.
Various toxins and drugs are associated with hemolytic anemia in cats. Zinc, onions, propylene glycol, acetaminophen, methylene blue, methionine, methimazole and benzocaine preparations are a few of these agents. Most toxins will induce Heinz body formation and some, such as acetaminophen toxicity, cause methemoglobinemia as well. Supportive care is the primary therapy for Heinz body anemia.