[Drug interactions of phenylbutazone and phenprocoumon in a warmblood gelding]

A 15 year old Oldenburger gelding was treated during 3 weeks for laminitis with the anticoagulant phenprocoumone (27 mg orally, once daily) and concurrent administration of phenylbutazone (2-4 g orally, twice daily). After this treatment the animal was presented to the Equine Clinic University of Zurich with a history of acute colic and advanced symptoms of shock. On the basis of the clinical signs and laboratory values, a diagnosis of combined drug induced coagulopathy was made. The horse was treated with the antidote Vitamine-K1 (0.5 mg/kg, subcutaneously). Eventually, the general condition of the animal worsened and it was therefore euthanized. Necropsy revealed profound, multifocal hemorrhagic diathesis of the serosal surface of the viscera, as well as bleeding into the visceral cavities. This case shows that concurrent administration of phenprocoumone and phenylbutazone may lead to drug interactions that increase the anticoagulation effect of the coumarine-derivative. Simultaneous use of coumarine-derivatives and phenylbutazone is therefore contraindicated due to the higher risk of bleeding. A reasonable treatment of horses with anticoagulants requires regular monitoring with constant evaluation of coagulation status and special attention to potential drug interactions.