No treatment for primary bone marrow megakaryocyte hypoplasia exists. (Treatment for DIC is discussed in: “Hemostatic Disorders.”) Treatment for IMTP is similar to immune-mediated hemolytic anemia. Medication withdrawal should be implemented with adjustment of antibiotic or drug therapy to a molecularly dissimilar agent. Attempts should be made to identify and treat potential underlying diseases. In life-threatening cases, whole blood or platelet rich plasma transfusion is indicated. Dexamethasone (0.05-0.1 mg/kg IV q24h) is generally indicated to decrease Fc binding, phagocytic removal, and antibody production. When the platelet count is greater than 100,000/μl, the dose of steroid should be reduced 0.01 mg/kg/day with close monitoring for disease recurrence. Prednisolone (1 mg/kg IM ql2h) may be attempted, but not all horses respond favorably to this protocol. Steroids should not be discontinued until the platelet count has been within normal limits for at least 5 days. If steroid therapy has been implemented for greater than 2 weeks, every-other-day administration should be implemented in the tapering dose protocol. Splenectomy has been reported in humans and dogs, but long-term outcome has not been reported in the horse. The vinca alkaloid, vincristine at a dose of 0.01 to 0.025 mg/kg IV weekly, has been used with steroid therapy to increase the peripheral platelet count with some success. Because vincristine has immunosuppressive activities, neutropenia may develop; this is an indication for drug discontinuation. Although anecdotal evidence suggests some improvement of thrombocytopenia with the use of azothiaprine (3-5 mg/kg PO q24h) and cyclophosphamide (300 mg/m2 body surface area), no data are currently available to support their routine use. Complications associated with immunosuppressive therapies may include laminitis and profound immunosuppression that lead to secondary sepsis. Concentrated immunoglobulin therapy has been used for human patients with profound IMTP. The mechanism is multifaceted; however, blocked Fc receptor binding, steric hinderance of immune complex adherence, enhanced T lymphocyte suppressor activity, and reduced B lymphocyte function are proposed actions of excessive Ig administration. The recommended dose is 200 to 1000 mg IgG/kg per day for 2 to 5 days or approximately 6 L plasma per dose for a 450-kg horse. Although equine plasma transfusion carries a reduced risk of adverse effects when compared with glucocorticoids, the cost associated with treatment may be prohibitive. A less expensive alternative to equine plasma is lyophilized equine IgG (Lyphomune, Diagnon Corporation; Rockville, Md.); however, its use in IMTP has been limited in horses.
Whole blood transfusion or platelet-rich plasma can be used to treat thrombocytopenic patients that have persistent hemorrhage. Platelet-rich plasma is harvested by centrifuging fresh blood for a short time at a slow speed (3 to 5 minutes at 250 g). Blood or plasma should be collected into plastic bags because glass activates platelets, and either product should be used immediately. Platelet transfusion is a transient life-saving procedure, and the ultimate outcome depends on the individual case.
Cases of secondary IMTP that result from drug administration often resolve rapidly after drug withdrawal, except in cases associated with chrysotherapy (gold salts), which may require months to years to resolve. Many cases of equine IMTP recover in approximately 3 to 4 weeks. Cases that are secondary to EIA or neoplasia carry a poor to grave prognosis. Reports exist in which chronic recurrent thrombocytopenia has continued for an extended period of time and required intermittent steroid administration.