You may already know that chocolate can be lethal to your pet, but did you know that onions can be toxic to your cats and dogs too? And I'm not just talking about their breath.
You see, all forms of onions--cooked, raw, dehydrated--contain sulfur compounds. When your pet ingests onions, these compounds are broken down into what are known as disulfide compounds, and these are toxic to red blood cell membranes and cause them to form small bubble-like projections that are known as Heinz bodies. These bubbles make the red blood cell more rigid, and weaken its outer membrane, which means not only that the cell can't flex and twist as it circulates in the body, but that it is more likely to rupture.
Now, the main function of red blood cells is to circulate oxygen throughout the body. Without enough of these, the animal becomes anemic, and--if the bone marrow isn't given enough time to regenerate new red blood cells--it may even die. This also explains why the effects of onion toxicity can take a few days to show up, and why even very small amounts of onions can still have a cumulative damaging effect.
So what can you do to protect your dog or cat? Make sure to read labels. For example, baby food, which people use to entice their sick cats to eat, often contains onion powder. Also, do not feed your pet table scraps that contain onions in any shape or form. And--as always--if your cat or dog begins vomiting, acting depressed, and breathing rapidly, don't delay and take it to your vet.