INTRODUCTION

Immune-mediated haemolytic anaemia (IMHA) is characterized by the immune destruction of red blood cells that have been coated with immunoglobulins, complement, or both, resulting in direct destruction or phagocytosis and removal from systemic circulation. Antibodies may be directed against normal erythrocytes (primary or idiopathic) or against erythrocytes that have been antigenically altered through interaction with secondary causes. The antibodies are directed against a protein or glycoprotein that is a normal component of the red cell membrane. Any biologic organism has the ability to tolerate and recognize its own tissue; however, if that system became intolerant, an autoimmune disease would ensue. The term autoimmune disease implies a primary immune response or intolerance to a self-antigen, whereas, immune-directed diseases have either a primary or underlying disease (secondary) responsible for the immune reaction. Immune-mediated haemolytic anaemia more accurately describing a process by which red blood cells are destroyed and has replaced the term autoimmune haemolytic anaemia.

Immune-mediated haemolytic anaemia is a common and often fatal haemolytic disorder in the dog; whereas it is uncommon in the cat. It is, thus, important that causes for IMHA be aggressively looked for and treated before a diagnosis of primary IMHA is made.

Pathophysiology

Immune-mediated haemolytic anaemia arises from development of IgG or IgM antibodies directed against the red blood cell membrane. The degree of red blood cell lysis depends on the type and amount of antibody that binds to the red blood cell and the involvement of complement fixation. Most canine cases of IMHA are the result of a combination of IgG and complement binding to the red blood cell.

When the red blood cell attaches to an IgG molecule, it is phagocytosed by macrophages possessing multiple receptors for the constant fragment (Fc) portion of the IgG molecule and as these macrophages are found primarily in the spleen active erythrophagocytosis results in splenomegaly. As immunoglobulin production increases and more molecules are bound to the red blood cell, erythrophagocytosis occurs within the liver, which can result in hepatomegaly.

Once the immunoglobulin is attached, activation of the complement system occurs with binding of complement proteins to the red blood cell. Complement activation can cause immediate intravascular lysis or enhance extravascular lysis and produce spherocytes. CI, a serine protease from the liver, enters the complement cascade and generates a "membrane attack complex" that attaches to the red blood cell membrane. The membrane attack complex then causes "holes to be punched" in the red blood cell membrane, allowing an influx of water and electrolytes, cell swelling, and lysis. The membrane complex can be removed by macrophages within the spleen (partial erythrophagocytosis), producing spherocytes.

IgG possesses only two antigen-binding sites (monomeric) and usually cannot directly agglutinate red blood cells unless a large amount of antibody is present. On the other hand, IgM is a large pentameric molecule that can directly agglutinate red blood cells. IgM may also detach after complement activation and bind to other red blood cells. The red blood cell-IgM complex may bind complement and be phagocytised by macrophages that possess the Fc receptor. This most commonly occurs in the liver with about 20% of receptors in the spleen.

Causes of secondary IMHA

Immune mediated haemolytic anaemia is defined as destruction of red blood cells by immune processes that are either idiopathic (primary) or have an underlying disease (secondary). The important causes of secondary IMHA are drugs, chemicals, neoplasia, parasitism, infectious diseases, and microangiopathies. In patients with secondary IMHA the underlying disease results in a triggered for the body's immune reaction to produce antibodies against the red cell.

Causative factors that can be associated with secondary IMHA in the dog and cat include the following:

 Infectious

 FeLV

 Bacterial endocarditis

 Ehrlichiosis

 Leptospirosis

 Haemoplasmas

 Babesiosis

 Chronic bacterial infections

 Systemic fungal infections

 Intestinal helminths

 Neoplasia

 Lymphoma

 Haemangiosarcomas

 Other

 Drugs

 Penicillins

 Cephalosporins

 Endogenous oestrogen

 Vaccines

 Blood transfusions

 Toxic

 Beestings

 Snake bites

In the cat the majority of cases of IMHA are secondary and are associated with feline leukaemia virus infection, lymphoproliferative disease, haemoplasma infections (formerly Haemobartonella), or drug reactions.

Patient presentation

The classic presentation of primary IMHA in dogs is reported to be young to middle-aged female spayed dogs. Feline IMHA has been predominantly reported in younger male animals. Breeds reported to have a higher incidence are Doberman Pinschers, Cocker Spaniels, Miniature Poodles, Irish Setters, Collies, English Springer Spaniels, and Old English Sheepdogs.

Clinical examination may show pale mucous membranes, icterus, hepatosplenomegaly with abdominal discomfort, tachypnoea, water-hammer pulse, and a soft systolic heart murmur.

Patients with IMHA commonly present with a regenerative anaemia, although there may be a delay of 2-5 days before a regenerative response is apparent in the peripheral blood. Immune mediated haemolytic anaemia may also be non-regenerative if antibodies or complement is directed against erythrocyte precursors within the bone marrow.

Diagnostic workup

The presence of IMHA is confirmed of demonstration of in-saline agglutination (macroscopic and microscopic evaluation), Coomb's test, or flow cytometry. These tests will, however, not differentiated primary from secondary IMHA.

A detailed history remains the cornerstone for an accurate diagnosis. The patient may have been recently vaccinated, traveled to tick/parasite-infested regions, or given anti biotics.

Severe regenerative anaemia, haemoglobinaemia, and spherocytosis are hallmarks of IMHA, but their presence depends on how acutely the haemolytic crisis occurred. The release of reticulocytes into the blood stream does not occur until several days after the acute haemolytic episode. An absolute reticulocyte count and a corrected reticulocyte percentage are essential for the assessment of red blood cell regeneration. Patients with IMHA may typically exhibit a mild to marked leukocytosis and often have a left shift with toxic changes in the neutrophils. Although thrombocytopaenia is a common finding, only a small percentage (<20%) may show signs, such as petechiae, ecchymosis, epistaxis, and melaena.

Serum biochemistry will often reflect hypoxic changes such as azotaemia (prerenal or renal) and increased proteins, liver enzymes, and total bilirubin. Coagulation profiles serve as an early indicator of disseminated intravascular coagulopathy (DIC) and thromboembolism, both of which occur in primary and secondary IMHA patients.

A complete evaluation of the IMHA patient should include thoracic and abdominal radiographs and abdominal ultrasound. Evaluation of the bone marrow may be also needed, especially if the patient's anaemia is non-regenerative, a destruction process directed at red cell precursors is suspected, or another cytopaenia is present. Bone marrow aspirates may be helpful in diagnosing red blood cell aplasia or hypoplasia, neoplastic infiltration, or immune destruction of erythrocyte precursors.

In summary a full investigation into the aetiology of a patient with IMHA would include a combination of the following, depending on the initial assessment:

 History

 Clinical examination

 Urine and faecal analyses

 Serum biochemistry

 Haematology

 Urine and blood culture

 Serology and/or PCR (leptospirosis, toxoplasmosis, FeLV, FIV, Ehrlichia)

 Survey thoracic radiographs

 Abdominal ultrasonography

 Echocardiography

 Bone marrow aspirate/biopsy

THERAPY

In secondary IMHA both the primary disease and the red blood cell destruction needs to be aggressively treated with immunosuppressive therapy. Immunosuppressive drugs that can be used include corticosteroids, azathioprine, cyclophosphamide, cyclosporine, danazol, intravenous immunoglobulins, mycophenolate, and leflunonamide. The goal of therapy is the initial stabilisation of the haematocrit, followed by the normalisation of the haematocrit over a period of time.

Supportive care with patients with IMHA includes anticoagulant therapy, gastric protectants, fluid therapy, and blood transfusions.