This occurs from eating poorly preserved tuna, mackerel and related ‘scombroid’ fish. A toxic histamine-like substance is produced by the action of bacteria on histidine, a normal component of fish flesh. This toxin produces nausea, vomiting, headache, upper abdominal pain, difficulty in swallowing, thirst, itching and sometimes URTICARIA. The condition usually settles in 12 hours. ANTIHISTAMINE DRUGS may help.