General Immunologic Principles

Immune-mediated diseases may be either primary or secondary. Primary immune-mediated hemolytic anemia (IMHA) and immune-mediated thrombocytopenia (ITP) are the most frequent forms in dogs, while secondary forms of each disease are most common in cats. In patients with IMHA, hemolysis happens either intravascularly or extravascularly. During intravascular hemolysis, large quantities of anti-erythrocyte antibodies bind to erythrocytes to induce rupture within the vascular space. During extravascular hemolysis, anti-erythrocyte antibodies bind to red blood cells, resulting in destruction by the mononuclear phagocytic system. In patients with ITP, antiplatelet antibodies coat platelets. Antibody binding ultimately results in decreased platelet survival time.

Immune-mediated Hemolytic Anemia: Clinical Signs & Diagnosis

Immune-mediated hemolytic anemia typically affects young adult and middle-aged animals, and is most commonly observed in Cocker Spaniels, English Springer Spaniels, Poodles, and Old English Sheepdogs. Clinical signs typically reflect the presence of both anemia and compensatory responses caused by tissue hypoxia and stimulation of the sympathetic nervous system. Some patients also show clinical signs of an ongoing immunological or inflammatory process. Definitive diagnosis of IMHA requires confirmation of anemia, hemolysis and immune-mediation. Confirming anemia is understandably straightforward, but confirmation of hemolysis and immune-mediation is more challenging. A thorough diagnostic investigation is essential to identify all potential secondary causes of disease. Therapeutic failure is likely if an underlying disease is not identified and treated appropriately.

Immune-mediated Thrombocytopenia: Clinical Signs & Diagnosis

Immune-mediated thrombocytopenia appears to occur in middle-aged female dogs, and Poodles, Old English Sheepdogs and Cocker Spaniels are over-represented. Cats of any age and sex can be affected. Clinical signs frequently include hemorrhage, lethargy, weakness, pyrexia, hyporexia/anorexia and vomiting. Common signs of hemorrhage include epistaxis, scleral hemorrhage, hematuria, melena, hematochezia, petechiae/ecchymoses, gingival bleeding, and hyphema. As with IMHA, diagnostic investigation should be exhaustive to rule out all potential secondary causes.

Immunomodulatory Therapy

Corticosteroids: The cornerstone of IMHA and ITP therapy is corticosteroid therapy given at immunomodulatory doses. Patients may benefit from a multimodal immunomodulatory therapy since many agents take several days to a couple of weeks to become fully effective. Prolonged therapy appears to be the best way of minimizing relapses, but long-term corticosteroids often cause unacceptable side effects. 'Steroid-sparing' combination therapy often enables reductions in corticosteroid doses.

Cyclosporine: Cyclosporine inhibits early T cell activation and prevents production of IL-2 and IFN- by T cells. These actions lead to a decrease in cell-mediated immunity and T cell-dependent B cell antibody production. Side effects include gingival hyperplasia, and gastrointestinal side effects, the latter of which can be reduced by placing capsules in a freezer for storage between dosing; there are two published reports in dogs of neoplasia occurring in animals on cyclosporine.

Azathioprine: Azathioprine is a cytotoxic purine analog that inhibits cell division of proliferating lymphocytes. Clinical response may take 1–6 weeks. Azathioprine's main side effects include bone marrow suppression, gastrointestinal upset, poor hair growth, hepatotoxicity, and idiosyncratic pancreatitis. Use of azathioprine in cats is not routinely recommended due to their markedly reduced thiopurine methyltransferase activity that dramatically increases risk for toxicity.

Mycophenolate mofetil: Mycophenolate mofetil blocks guanine nucleotide synthesis, thus resulting in a reduction of B and T cell proliferation. West LD et al. reported the use of this drug in five canine patients with IMHA. Prescribed doses were 10–15 mg/kg PO q8 hr, and all dogs experienced gastrointestinal toxicity. All dogs survived beyond two weeks of diagnosis, and three were alive more than one year after diagnosis. Median resolution of spherocytosis and anemia was 13 days and 44 days, respectively. Bacek et al. reported the use of mycophenolate mofetil in two cats with primary IMHA at dose of 10 mg/kg PO q12 hr. Both cats had improved blood counts follow therapy and no adverse effects were noted.

Leflunomide: Leflunomide is an inhibitor of pyrimidine biosynthesis. There are anecdotal reports of its successful use in dogs with IMHA. Bianco et al. published a case reported documented the successful use of leflunomide and human intravenous immunoglobulin in a diabetic dog with Evan's syndrome. The pharmacokinetics and pharmacodynamics of leflunomide in domestic cats has been investigated but reports of clinical use are not yet available.

Vincristine: Vincristine is a vinca alkaloid that disrupts intracellular microtubules and inhibits ribonucleic acid/ RNA, deoxyribonucleic acid/DNA and protein synthesis. Mackin et al. and Neel et al. have shown vincristine administration is associated with significant and transiently increased circulating platelets. An increase in circulating platelets is commonly documented within 3–5 days of administration. Rozanski et al. showed administration of combined vincristine and prednisone was associated with a more rapid increase in platelet numbers and shorted duration of hospitalization in dogs with ITP compared with administration prednisone alone.

Intravenous immunoglobulin: Intravenous immunoglobulin (IVIg) works by binding to the Fc receptors of macrophages, inhibiting the ability to bind to antibody-antigen complexes on RBCs. Scott-Moncrieff et al evaluated the efficacy of human IVIg in dogs with IMHA. No adverse effects with definitively attributed to IVIg infusion; eight dogs had significant increases in hematocrit and hemoglobin concentration, and five dogs can clinically meaningful responses to treatment. Whelan et al investigated use of human IVIg in 28 dogs with IMHA, and the addition of IVIg to corticosteroid treatment did not improve initial response or shorten hospitalization. Bianco et al studied human IVIg in dogs with presumptive primary ITP. Compared to corticosteroids alone, adjunctive use of human IVIg was associated with a significant reduction in platelet count recovery time and length of hospitalization in an emergency setting.

Miscellaneous Therapies

Blood product transfusion: Patients with clinical anemia secondary require increased oxygen carrying capacity. Packed red blood cells should be provided to target specific endpoints of resuscitation. Patients with concurrent hypoproteinemia may benefit from either whole blood or albumin infusions. Patients with severe thrombocytopenia can be treated with lyophilized platelets, platelet-rich plasma, and/or fresh whole blood.

Splenectomy: Feldman et al. showed splenectomy may be useful for treating ITP, IMHA and Evan's syndrome that were deemed refractory to medication. Toll et al showed splenectomized dogs with IMHA had a faster rate of return to normal hematocrit/HCT and longer survival.

Plasmapheresis: Plasmapheresis is an effective method of removing unbound anti-erythrocyte and anti-platelet antibodies. Crump et al published a case report of successful treatment with plasmapheresis of a dog with IMHA that was refractory to prednisone and cyclosporine therapy. The effect is transient, and multiple treatments may be necessary.

Melatonin: A few studies in human medicine have documented the therapeutic efficacy of melatonin for treating refractory thrombocytopenic purpura. Anecdotal reports of efficacy of melatonin in dogs and cats with both ITP and IMHA exist, but to date, the author knows of no prospective clinical research regarding this drug in patients with these diseases.

Thromboprophylaxis for IMHA

Hemolysis induces tissue factor on monocytes and endothelial cells with subsequent activation of coagulation. Damaged erythrocytes, activated platelets, and microparticles contribute to coagulation. Arterial and venous thromboembolic disease is a major factor affecting survival in dogs with IMHA. Heparin and antiplatelet medications are commonly used in dogs with IMHA. To date there is a lack of controlled studies effectively documenting validated therapeutic endpoints and survival benefit.

Prognosis

The mortality rate associated with IMHA has been reported to be 29–70%. The majority of deaths occurred within the first two weeks of diagnosis, and dogs that survived the first two weeks after diagnosis had a 6-month survival time of 92.5%.

Potential risk factors for death, including elevated serum urea, icterus/hyperbilirubinemia, spherocytosis, male sex, warm season, packed cell volume <20%, thrombocytopenia, total protein <6 g/dl, elevated creatinine, hyperlactatemia, prolonged aPTT, elevated ALP, monocytosis, elevated IL-18 and elevated monocyte chemoattractant protein-1/MCP-1. Results of these studies must be interpreted cautiously.

Immune-mediated thrombocytopenia is a serious but treatable disease. O'Marra et al. evaluated 73 dogs with ITP; 84% survived to discharge, and 9% had relapse of disease. Elevated serum urea concentration and melena at admission was associated with a poor prognosis.

References

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2.  Kohn B, Weingart C, Echmann V, et al. Primary immune-mediated hemolytic anemia in 19 cats: diagnosis, therapy and outcome (1998–2004). J Vet Intern Med. 2006;20(1):159–166.

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4.  Wondratschek C, Weingart C, Kohn B. Primary immune-mediated thrombocytopenia in cats. J Am Anim Hosp Assoc. 2010;46(1):12–19.

5.  Swann JW, Skelly BJ. Systematic review of prognostic factors for mortality in dogs with immune-mediated hemolytic anemia. J Vet Intern Med. 2015;29(1):7–13.

6.  Carr AP, Panciera DL, Kidd L. Prognostic factors for mortality and thromboembolism in canine immune-mediated hemolytic anemia: a retrospective of 72 dogs. J Vet Intern Med. 2002;16(5):504–509.

7.  Weinkle TK, Center SA, Randolph JF, et al. Evaluation of prognostic factors, survival rates, and treatment protocols for immune-mediated hemolytic anemia in dogs: 151 cases (1993–2002). J Am Vet Med Assoc. 2005;226(11):1869–1880.

8.  O'Marra SK, Delaforcade AM, Shaw SP. Treatment and predictors of outcome in dogs with immune-mediated thrombocytopenia. J Am Vet Med Assoc. 2011;238(3):346–352.