Feline Anemia (General) Study Guide

Overview and Clinical Importance

Anemia is defined as a reduction in packed cell volume (PCV), hematocrit (HCT), hemoglobin concentration, or red blood cell (RBC) count below the species-specific reference range. In cats, anemia is not a diagnosis but rather a clinical sign of an underlying disease process. Understanding the classification, pathophysiology, and diagnostic approach to feline anemia is essential for success on the NAVLE, as it represents a frequently tested topic in the hemic/lymphatic system category.

The majority of anemias in cats are non-regenerative, which differs from dogs where regenerative anemias are more common. This distinction has significant diagnostic and therapeutic implications. Feline red blood cells have a unique lifespan of approximately 70-80 days (compared to 110-120 days in dogs), and cats possess distinctive hemoglobin with eight reactive sulfhydryl groups, making them more susceptible to oxidative damage.

Normal Feline Hematologic Values

Understanding normal reference ranges is essential for identifying anemia and assessing its severity.

Classification of Feline Anemia

Primary Classification: Regenerative vs Non-Regenerative

The most important initial step in evaluating any anemic cat is determining whether the anemia is regenerative (bone marrow responding appropriately) or non-regenerative (inadequate bone marrow response). This classification directs the entire diagnostic workup.

Feline Reticulocytes: A Unique Feature

Cats are unique among domestic species in having two types of reticulocytes, which is critical for NAVLE questions on regeneration assessment.

Causes of Regenerative Anemia

Regenerative anemias indicate blood loss or red blood cell destruction. The bone marrow response (reticulocytosis) takes 3-5 days to become evident after acute blood loss or hemolytic crisis.

Hemorrhagic Anemia (Blood Loss)

Key Distinguishing Features of Hemorrhagic Anemia: Decreased total protein (TP) due to concurrent plasma loss, evidence of external or internal bleeding, normal bilirubin (unless chronic with hepatic involvement).

Hemolytic Anemia (RBC Destruction)

Hemolytic anemia results from premature destruction of red blood cells, either intravascular (within blood vessels) or extravascular (in spleen, liver, bone marrow).

Infectious Causes

Hemotropic Mycoplasmas (Hemoplasmas): These are the most common infectious cause of hemolytic anemia in cats. Three species are recognized:

Diagnosis: PCR is the gold standard (blood smear is unreliable due to cyclic parasitemia and staining artifacts).

Treatment: Doxycycline 10 mg/kg PO q24h for 2-4 weeks (up to 8 weeks for clearance). Marbofloxacin 2 mg/kg PO q24h is second-line if doxycycline fails. Always follow doxycycline with water to prevent esophageal strictures.

Other Infectious Causes: FeLV (can cause both regenerative and non-regenerative anemia), Cytauxzoon felis (tick-borne, severe acute disease in endemic areas of southeastern US, often fatal), Babesia felis (rare, primarily in South Africa).

Immune-Mediated Hemolytic Anemia (IMHA)

IMHA is less common in cats than dogs. Most cases are secondary to underlying disease (FeLV, hemoplasma infection, drugs, neoplasia). Primary (idiopathic) IMHA is rare.

Clinical Signs: Lethargy, anorexia, pale mucous membranes, icterus (variable), fever, splenomegaly.

Diagnosis: Positive direct Coombs test (direct antiglobulin test), autoagglutination on blood smear (less reliable than dogs), spherocytes (difficult to identify in cats due to small RBC size and minimal central pallor), hyperbilirubinemia.

Treatment: Prednisone 2-4 mg/kg PO divided BID (higher doses than dogs required). Cyclosporine may be added for refractory cases. Blood transfusion if PCV critically low. Address underlying cause.

Oxidative Injury: Heinz Body Hemolytic Anemia

Cats are highly susceptible to oxidative damage because feline hemoglobin has 8 reactive sulfhydryl groups (vs 4 in dogs) and the feline spleen is non-sinusoidal and less efficient at removing damaged RBCs.

Heinz bodies are precipitated, denatured hemoglobin that attach to RBC membranes. Up to 5-10% Heinz bodies can be normal in healthy cats.

Treatment: Remove source, supportive care, N-acetylcysteine for acetaminophen toxicity (140 mg/kg loading dose, then 70 mg/kg q6h for 7-17 treatments). Heinz bodies resolve over 1-4 weeks after removing the oxidant.

Causes of Non-Regenerative Anemia

Non-regenerative anemia is more common than regenerative anemia in cats. The bone marrow fails to respond adequately due to decreased production or ineffective erythropoiesis.

Feline Blood Types and Transfusion Medicine

Understanding feline blood types is essential for safe transfusion and prevention of neonatal isoerythrolysis.

Transfusion Reactions

Unlike dogs, cats have naturally occurring alloantibodies that can cause severe transfusion reactions even with a first transfusion. Blood typing and crossmatching are essential before any transfusion.

• Type A blood given to Type B cat: Acute, severe, potentially FATAL hemolytic reaction (half-life of transfused RBCs is 6 hours)
• Type B blood given to Type A cat: Milder reaction, shortened RBC survival (half-life 2 days)

Neonatal Isoerythrolysis (NI)

NI occurs when Type A or AB kittens nurse from a Type B queen within the first 16-24 hours of life. The queen's high-titer anti-A antibodies are absorbed from colostrum and destroy the kitten's RBCs.

Clinical Signs: Healthy at birth, then fade within hours to days; stop nursing; dark red-brown urine; icterus; tail tip necrosis; sudden death.

Prevention: Blood type breeding queens in high-risk breeds. Remove kittens from Type B queen for first 24 hours if sire is Type A. Hand-rear with milk replacer or foster to Type A queen.

Diagnostic Approach to the Anemic Cat

Step-by-Step Diagnostic Algorithm

• Confirm Anemia: PCV less than 25%, HCT less than 25%, or Hb less than 8 g/dL
• Classify Regeneration: Reticulocyte count (aggregate only), blood smear for polychromasia
• Assess Total Protein: Low TP suggests hemorrhage; normal/high TP suggests hemolysis or decreased production
• Evaluate Blood Smear: Look for organisms, Heinz bodies, spherocytes, agglutination, RBC morphology
• Test for FeLV/FIV: MANDATORY for all anemic cats
• Chemistry Panel/Urinalysis: Assess kidney function, bilirubin, organ involvement
• Additional Testing: PCR for hemoplasmas, Coombs test, coagulation panel, bone marrow aspirate as indicated

Treatment Overview