Ingesting raw or dry plant material of some Trifolium species can cause photosensitivity and liver disease in grazing animals. The toxic principle is still unknown. There are currently two explanations: the plant contains an unidentified toxic metabolite, or the toxic agent is produced by fungi that infect clover (e.g. Cymodothea trifolii, Rhizoctonia leguminicola). Hop trefoil is often present in pastures, so grazing animals are exposed to its negative effects. Horses are especially sensitive, although symptoms of poisoning can occur in cattle as well. The symptoms develop after ingestion of raw or dry hop trefoil, and include excessive salivation, swollen tongue, lips, and eyelids, swelling of the skin, hair loss, inflamed skin that oozes serum, ulcers in the mouth, diarrhea, and colic. Liver damage occurs after prolonged ingestion of hop trefoil.