The transmissible venereal tumor is a round cell tumor that mainly affects sexually active semi-domiciled and street dogs, in tropical and subtropical regions. The treatment of choice is vincristine sulfate, weekly, and the main side effects are gastrointestinal and myelosuppression changes, so it is important a hematological control before each application. The aim of this abstract was reported the rare hypersensitivity reaction, immediately after the first vincristine injection.

A 4-year-old Boxer female dog was admitted at the Veterinary Teaching Hospital presenting with a transmissible venereal tumor. The treatment was done with vincristine applications (0.7 mg/m², intravenously) and the patient presented with rashes immediately after the first injection, which was characterized like a type 1 hypersensitivity reaction. It was treated successfully with hydrocortisone (50 mg/kg) and promethazine (1 mg/kg). TVT totally regressed with seven applications and only in first the hypersensitivity reaction occurred. This kind of reaction may occur due to the high molecular weight of vincristine, because drugs composed of high molecular weight proteins can be directly processed, whereas those proteins with a low molecular weight may need to be haptenated to peptides and proteins, to become recognized as allergen. Thus, we concluded that vincristine can cause immediate skin reaction, and it's important to quickly identify and treat, because depending on the evolution, this side effect can be fatal.