Purpura in serum sickness: a case of polyvalent snake antivenom reaction following Porthidium ophryomegas envenomation

This reports a case of a 68-year-old woman with a history of snake bite from Porthidium ophryomegas (commonly known as Tamagás Negro), treated with polyvalent snake antivenom. The patient presents to the internal medicine service eight days after the snake bite, with generalized urticaria and palpable non-thrombocytopenic purpura. Based on her clinical history, she was diagnosed with serum sickness secondary to the snake antivenom, a potential complication of its administration. Serum sickness is caused by a type III hypersensitivity reaction, characterized by urticaria, fever, malaise, polyarthralgia, or polyarthritis, and less frequently, purpura secondary to allergic cutaneous vasculitis. There is no consensus regarding the diagnosis, which is clinical, based on medical history and suggestive symptomatology. This case was treated with corticosteroids, antihistamines, and topical antipruritic agents, resulting in the resolution of signs and symptoms without further complications. Although purpura is an uncommon manifestation of serum sickness, its occurrence following antivenom administration for snakebite should prompt consideration of this diagnosis.