4 Equine Erythrocytes and Bloodwork

• Equine bloodwork interpretation and anemia are important topics in veterinary medicine

Polycythemia

• Polycythemia refers to an increase in red blood cell mass
• Relative erythrocytosis is a type of polycythemia caused by dehydration or splenic contraction
• Absolute erythrocytosis is a type of polycythemia caused by hypoxia (high-altitude), chronic pulmonary disease or inappropriate RBC production, like hepatocellular carcinoma.

Equine Erythrocytes

• Equine erythrocytes have marrow retention until maturation is complete
• Polychromasia and macrocytosis can be observed in equine erythrocytes
• Howell-Jolly bodies can be found in low numbers in healthy horses
• Rouleaux formation is common in equine erythrocytes
• Agglutination can be differentiated from rouleaux formation using a 1:4 saline solution

Reticulocytes

• Reticulocytes, immature RBCs, normally have a reference range of 0 in horses
• They have a fine reticulum of nucleotide material that stains positive with methylene blue
• In other species, increased presence indicates regeneration
• In horses, they stay in the marrow until mature

Thrombocytes

• Thrombocytes reference range is 117-256 X 103/µl
• Decreased thrombocytes may indicate DIC, IMT, endotoxemia, EIA, equine ehrlichiosis, or lymphoma
• Pseudothrombocytopenia, clumping in EDTA, or use of citrate or heparin can lead to decreased thrombocytes
• Increased thrombocytes may indicate chronic inflammation or Rhodococcus equi

Summary

• Clinical pathology is useful in case management
• Serum chemistry and CBC tests are commonly used for evaluating equine patients
• Serial monitoring can determine the course of disease
• A repeat analysis is indicated if there are any unexpected results
• Repeating analysis can help in ruling out laboratory errors and artifactual results

Equine Anemia

• Equine Anemia involves a decrease in circulating RBC mass, decreased packed cell volume (PCV), and decreased hemoglobin (except with intravascular hemolysis)
• Determining if anemia is regenerative or non-regenerative is important
• Polychromasia can indicate regenerative anemia
• Horses do not release reticulocytes into circulation so bone marrow aspirate/biopsy from the rib or sternum must be performed to determine the cause of anemia
• Monitoring RBC increase over time can help determine the response to anemia, 3 days is needed for response

Causes of Anemia

• Anemia is caused by blood loss, destruction or reduced production

Hemorrhage

• Acute blood loss results in clinical signs earlier
• Acute blood loss will cause a transfusion PCV of 15-20%
• Chronic blood loss causes a transfusion PCV of 10-12%
• External blood loss occurs through wounds or from other external sources, like Guttural pouch mycosis or middle uterine artery rupture
• Internal blood loss occurs into a cavity such as the thoracic or peritoneal cavity due to splenic trauma or hemangiosarcoma

Clinical Signs

• Clinical signs are determined by the magnitude of blood loss
• Pale mucous membranes (pallor) can indicate clinical signs
• Tachycardia (> 60 bpm), and tachypnea (> 24 bpm) are vital signs of anemia
• Weakness and syncope is an additional indication of anemia

Assessment of Blood Loss in Horses

• Splenic contraction during blood loss may result in normal PCV despite marked loss, decline may be seen in 4-6 hours
• Total solids/total protein generally falls with blood loss
• Heart rate increases (>60 bpm), increased lactate circulation, depressed demeanor, and reduced appetite indicate other clinical findings impacted by reduced oxygen carrying capacity

Treatments for Equine Anemia

• Treatment includes hemorrhage control (hemostasis)
• Volume depletion should be treated by intravenous fluids
• Blood loss should be treated with blood transfusions in cases of anemia, tachycardia, tachypnea, reduced appetite, and/or depressed demeanor
• A gelding or maiden mare can receive one-time transfusion due to no previous RBC antigen exposure
• A Universal donor is Aa and Qa blood cell antigen and antibody negative
• A broodmare is a candidate for cross matching for repeated transfusions

Equine Blood Transfusion

• The amount of blood to administer for a 500-600 kg horse is approximately 5-10 L of whole blood
• The volume to administer is given by the formula: VOLUME = (BW KG) (0.08) (PCV desired – PCV patient)/PCV donor
• Example calculation: 500 KG (0.08) (20% - 12%) / 32% = 10 L

Important Points

• Patients with chronic blood loss will tolerate lower red blood cell mass
• Vital parameters and overall clinical demeanor help determine the need for transfusion
• PCV is important, but it can be lower in a patient with chronic RBC loss

Erythrocyte Destruction

• Immune-mediated destruction is also known as IMHA (Immune-Mediated Hemolytic Anemia)
• Immune-mediated destruction can be extravascular (antibody target RBC destruction) or intravascular (complement mediated destruction)
• Toxins such as red maple toxicity can cause oxidative injury
• Infectious agents can cause RBC fragility
• Hypotonic solutions such as water, and hypertonic solutions such as DMSO can cause erythrocyte destruction

Intravascular Hemolysis

• Intravascular hemolysis is the acute and severe destruction of RBC
• Signs of intravascular hemolysis are pigmenturia (hemoglobinuria), pink plasma and icterus

Extravascular Hemolysis

• Extravascular hemolysis involves erythrocyte removal from an extravascular site through opsonization and splenic removal
• Icterus occurs with yellow plasma
• No hemoglobinemia is present

IMHA

• Antibodies coat erythrocytes in IMHA
• Autoantibody is rare
• Secondary immune-mediated is most common
• Foreign RBC antigens or neonatal isoerythrolysis
• Medications, especially beta lactam antibiotics
• Anemia viruses
• Neoplasia (Lymphoma and paraneoplastic syndrome) cause
• Anemia is caused by foreign RBC antigens, anaemia viruses, beta lactam antibiotics and neoplasia

Diagnosis of IMHA

• Diagnosis involves Identification of surface-associated erythrocyte by testing for autoagglutination, Coomb's testing, direct immunofluorescence/flow cytometry, and IgG, IgA, IgM
• False negatives occur with corticosteroid administration or massive hemolysis

Clinical Management of IMHA

• Discontinue medications
• Administer Corticosteroid therapy for immune suppression
• Whole blood transfusion if indicated
• Treatment should avoid pigment associated nephropathy using diuresis

Non-regenerative Anemia

• Anemia of chronic disease (ACD) is a common cause of mild anemia in the mid 20% range
• Resolution of primary disorder is needed by controlling a small protein called hepcidin that limits the availability of iron
• Equine example of ACD is chronic pneumonia where anemia will resolve once pneumonia is resolved
• Iron deficiency is an uncommon cause of anemia due to true loss of iron stores
• External hemorrhage, caused by a severe wound with blood loss, may result in iron loss and supplementation is indicated
• Bone marrow suppression involves medications that cause a direct suppression on bone marrow function
• Myelophthisis - bone marrow destruction
• Chronic renal failure leads to reduced erythropoietin secretion

Summary

• Anemia can result from hemorrhage, destruction, or chronic disease
• Determine severity with whole blood transfusion, PCV and careful and acute patient evaluation that will decompensate more rapidly
• Immune-mediated anemia requires determining the inciting cause
• External blood loss may require iron supplementation with whole blood transfusion, low PCV, and in critical patient conditions with high HR and/or worsening condition
• Anemia associated with ACD requires treating the primary disease to resolve it