Vincristine is given as a single dose of 0.02 mg/kg intravenously.^8,14 Increases in platelet numbers often occur within two or three days.⁸ Administration of vincristine as a constant-rate infusion or incubated with a platelet transfusion has also been reported, but these routes have not been thoroughly evaluated.^2,8 Vincristine is a strong vesicant if extravasated and should be given through a well-placed catheter; few other side effects are generally seen with a single dose.^8,14 In vitro vincristine decreases canine platelet function, but in vivo alterations in platelet function are not well-documented.⁸

Other immunosuppressants. For patients with primary immune-mediated thrombocytopenia that fail to respond to glucocorticoids, additional medications may be used concurrently. Dogs with intolerable side effects from glucocorticoids may be given additional immunosuppressive medications to allow tapering or discontinuation of the corticosteroids.^6,8 Azathioprine is the most common option.¹⁹ Azathioprine is an immunosuppressive agent that reduces the production of antiplatelet antibodies.⁸ It is given at a dose of 2 mg/kg orally every 24 hours. After two to four weeks, if the platelet count is normal, taper the azathioprine in tandem with the prednisone to the lowest effective maintenance dose.^2,8 Potential side effects of azathioprine include myelosuppression, pancreatitis, hepatopathy, and predisposition to infection.⁸ Dogs deficient in thiopurine methyltransferase may be more susceptible to side effects.²²

Alternatively, cyclophosphamide (50 mg/m² orally on the first four days of the week for a maximum of four to five months) may be used in conjunction with prednisone for immunosuppression. Cyclophosphamide is usually less well-tolerated than azathioprine for long-term use. Potential side effects that limit its long-term use include myelosuppression, susceptibility to infection, and hemorrhagic cystitis.⁸ Studies in dogs with immune-mediated hemolytic anemia suggest that cyclophosphamide may no longer have a role in treating this immune-mediated blood dyscrasia.^23,24

Other therapeutic options

In cases refractory to standard immunosuppressive medication, other therapeutic options include splenectomy, intravenous gamma globulin administration, plasmapheresis, or treatment with danazol.

Management of immune-mediated thrombocytopenia by splenectomy has not been thoroughly investigated in a large number of dogs, and published success rates are highly variable.^2,8 Blood smears and PCR testing should be done to monitor for Mycoplasma haemocanis (formerly Haemobartonella canis) and Babesia canis before and for several weeks after splenectomy.⁸ Immune-mediated thrombocytopenia can persist after the splenectomy through the actions of the hepatic mononuclear phagocytic system.⁸

Intravenous gamma globulins are given as a single infusion and are thought to competitively bind to macrophage receptors, decreasing platelet clearance by the mononuclear phagocytic system. But the effect is transient and this treatment is generally cost-prohibitive.⁸ In a recent report, human intravenous immunoglobulin was used at a dose of 0.28 to 0.76 g/kg intravenously.²⁵ Other published doses for human intravenous immunoglobulin have ranged from 0.5 to 1.5 g/kg.²⁶

Plasmapheresis is expensive and technically difficult, and its availability is limited in veterinary patients.⁸