(several species, commonly known as oak)). The leaves and acorns of oak species are poisonous in large amounts to humans and livestock, including cattle, horses, sheep and goats, but not pigs. Poisoning is caused by the toxin tannic acid, which causes gastroenteritis, heart trouble, contact dermatitis and kidney damage. Symptoms of poisoning include lack of appetite, depression, constipation, diarrhoea (which may contain blood), blood in urine, and colic. Rarely fatal however, and in fact after proper processing acorns are consumed as a staple in many parts of the world.