Danazol (5 to 10 mg/kg orally every 12 hours) is an androgen that may decrease production of antiplatelet antibodies and decrease macrophage receptors, impairing mononuclear phagocytic system clearance of platelets.^2,8 It is used concurrently with prednisone for managing immune-mediated thrombocytopenia.^6,8 It may take three to six months to reach peak effect. Once the prednisone has been reduced to alternate-day dosing, the danazol dose is gradually tapered to the minimum effective dose.⁸ A small number of dogs with refractory immune-mediated thrombocytopenia have responded to danazol.²⁷ Liver enzyme activities should be monitored monthly. Danazol's expense may preclude its use in large-breed dogs.⁸

Monitoring

Initially, monitor the platelet count every one to two weeks for the first two months. After that, recheck the count before and after any reduction in drug dose. For patients in complete remission, recheck the platelet count at three-month intervals. Monitoring the platelet count is critical because clinical signs of thrombocytopenia do not become evident until platelet numbers are severely decreased.⁸

In patients in which immune-mediated thrombocytopenia is suspected to be secondary to vaccine administration, avoid the potentially causative vaccine or vaccines in the future. Administering distemper-hepatitis vaccines may lower platelet counts, with the peak effect one week after vaccination, so vaccination should be avoided in thrombocytopenic patients.²⁸ For patients that have recovered from immune-mediated thrombocytopenia, we recommend administering only core vaccines and administering the vaccines at separate times. Monitoring the platelet count before and for a few weeks after vaccination is warranted.

SUMMARY

Immune-mediated thrombocytopenia is a common acquired coagulation disorder in dogs resulting from antibody-mediated clearance of platelets though the mononuclear phagocytic system. It is mainly a diagnosis of exclusion. Primary immune-mediated thrombocytopenia is confirmed by ruling out secondary disorders that can mediate platelet destruction by the immune system.

Primary immune-mediated thrombocytopenia is treated with immunosuppressive therapies, whereas secondary immune-mediated thrombocytopenia is treated by addressing the underlying disorder, often with concurrent immunosuppressive therapy. The prognosis for this disorder is guarded to good. If dogs with primary immune-mediated thrombocytopenia fail to respond, additional immunosuppressive therapy may be required, although reevaluation for an underlying cause that was not identified initially should also be considered.

Johanna Heseltine, DVM, MS, DACVIM*
Department of Small Animal Clinical Sciences
College of Veterinary Medicine
Oklahoma State University
Stillwater, OK 74078

Anthony Carr, Dr. med. vet., DACVIM (internal medicine)
Department of Small Animal Clinical Sciences
Western College of Veterinary Medicine
University of Saskatchewan
Saskatoon, SK S7N 5B4 Canada

*Dr. Heseltine's current address is Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, SK S7N 5B4, Canada.

REFERENCES

1. Mackin A. Canine immune-mediated thrombocytopenia: Part I. Compend Contin Educ Pract Vet 1995;17:353-362.

2. Lewis DC, Meyers KM. Canine idiopathic thrombocytopenic purpura. J Vet Intern Med 1996;10:207-218.

3. Scott MA. Immune-mediated thrombocytopenia. In: Kristensen AT, ed. Schalm's veterinary hematology. 5th ed. Baltimore, Md: Lippincott Williams & Wilkins, 2000;478-486.

4. Topper MJ, Welles EG. Hemostasis. In: Latimer KS, Mahaffey, EA, Prasse KW, eds. Veterinary laboratory medicine: clinical pathology. 4th ed. Ames: Iowa State University Press, 2003;99-135.

5. Boudreaux MK. Platelets and coagulation. An update. Vet Clin North Am Small Anim Pract 1996;26:1065-1087.