Plant intoxications in wildlife are difficult to diagnose, are overlooked, or are sometimes even neglected. Hence, factors that induce wild animals to ingest poisonous plants have not been sufficiently documented. An outbreak of glycoprotein storage disease in sambar deer (Cervus unicolor), induced by ingestion of the swainsonine-containing plant, common wireweed (Sida carpinifolia), is reported. Nine out of 55 deer held by a zoo in Brazil were affected. The poisoning was characterized by emaciation and neurologic signs followed by unexpected death in some of the animals. Animals presented abnormal consciousness, posterior paresis, and musculoskeletal weakness; less evident were vestibulo-cerebellar signs. Histologically, there was vacuolation of neurons and epithelial cells of the pancreatic acines, thyroid follicules, and renal tubules. Furthermore, in the central nervous system were axonal degeneration, necrosis, and loss of neurons. Three factors may lead to the ingestion of S. carpinifolia by sambar deer: 1) A grazing field with only S. carpinifolia as a source of forage; 2) a large number of animals kept in this field; and 3) a hierarchy within a cervid group in which dominant males isolated and displaced juvenile and weaker adult males, leaving them with access to only S. carpinifolia.