I copied this from the following link:
http://www.petalia.com.au/Templates/...s&story_no=257 Onion and garlic poisoning Top
Onions and garlic are other dangerous food ingredients that cause sickness in dogs, cats and also livestock. Onions and garlic contain the toxic ingredient thiosulphate. Onions are more of a danger.
Hope this helps. This site has some great info about other foods that are toxic.