Copper Toxicity in Horses: Does it Exist?

Copper toxicity is thought to be a rare condition in horses. However, the number of cases diagnosed in Brazil is growing. This article aims to describe cases of copper toxicity involving horses from different geographic locations and discuss findings of physical examinations, differential diagnoses and potential causes. Five cases referred from 4 different properties where at least 15 other horses were affected were described. Hemolytic anemia and hemoglobinuria, presence of Heinz bodies and elevated aspartate aminotransferase and gamaglutamil transferase levels were detected in all cases. The diagnosis was based on clinical history and signs, laboratory tests results, copper level determination in feed and/or soil and histopathological findings. Two horses progressed to acute death; remaining horses responded to clinical management with or without blood transfusion, depending on disease severity. However, one of these horses, after several returns to the veterinary hospital, was euthanized due to complications. One horse was treated with ammonium tetrathiomolybdate. Two horses had several recurring episodes over the course of several months, an uncommon presentation in ruminants suffering from copper toxicity. Excess copper was associated with soil fertilization with poultry litter or treatment of previous or neighbor crops with copper-containing products. It can be concluded that copper toxicity does occur in horses and may arise from several sources and/or be associated with predisposing dietary factors. Given the growing number of cases, the condition should be included in the differential diagnosis list and proper preventive dietary and pasture fertilization measures adopted.