Objectives

• Discuss the diagnostic workup of cats who present for anemia

• Focus on infectious causes of anemia

Key Points

• The complete blood count determines the course of the workup by dividing the possible rule-outs into those causing a regenerative vs. non-regenerative anemia

• Infectious diseases remain an important cause of anemia in cats

• Primary immune-mediated anemia may be more common than previously thought.

Introduction

Anemia is a common blood work abnormality in many species, including cats. Determining the regenerative nature of the anemia guides the workup of the case. 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.

Clinical Presentation and 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.

Diagnostic Evaluation

A complete blood count, including reticulocyte count, will determine whether the anemia is regenerative or non-regenerative. Acute blood loss and red blood cell lysis will not be regenerative for 2-4 days and so clinical and clinicopathologic parameters must 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 is diagnostic for regenerative anemia. An iron-deficiency anemia will be characterized as microcytic and FeLV-induced non-regenerative anemia will often be macrocytic. A careful evaluation of blood smears can reveal intra- or extracellular organisms. Presence of Heinz bodies may indicate an exposure to toxins that cause oxidative damage such as onions, propylene glycol, etc. 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 biochemistry 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 biochemistry panel depend on the specific chronic disease, infectious agent or neoplastic process. FeLV antigen/FIV antibody testing should be done and thoracic and abdominal radiographs (or abdominal ultrasound) should be performed to help rule-out neoplasia.

Toxins

A variety of toxins and drugs are associated with hemolytic anemia in cats. Zinc, onions, propylene glycol, acetaminophen, methylene blue, methionine, methimazole and benzocaine preparations are few of the most common agents. The majority of these toxins induce Heinz body formation and some, such as acetaminophen toxicity, cause methemaglobinemia as well. Supportive care is the primary therapy for Heinz body anemia.