Onions, leeks, and garlic are all members of the Allium family, and are common toxins in dogs and cats, along with chives, scallions, shallots and the many wild native and domesticated ornamental Allium species. Alliums contain a variety of organosulfides, chemicals which, when metabolized, are converted to highly reactive oxidants. These lead to oxidative hemolysis, or break-down, of red blood cells.
Organosulfides may be found in all parts of the fresh plants, in raw or cooked foods, or in liquid, powdered, or dehydrated preparations. As little as 5gm/kg of onions in cats or 15-30 gm/kg of onions in dogs may result in clinical signs of toxicosis. That’s about 1 ounce in a 10 lb cat or 5 to 10 oz in a 20 lb dog. Poisoning may occur from a one-time binge, or the effects may be the result of cumulative ingestion if there is a constant source available (i.e., a dog who regularly “grazes” on Allium species found in the yard or garden.) Some dogs, notably Japanese breeds, have an inherited defect making them more susceptible to onion toxicosis, and certain medications can also increase the risk of toxicity.
Signs of onion toxicosis are related to the anemia which results from hemolysis (destruction) of red blood cells: rapid heart beat and respirations, weakness, exercise intolerance, pale or yellow color to the gums, depression, decreased appetite or activity, or a dark color to the urine. These symptoms may be noted within one day if a large amount was ingested, or it may take several days for signs to develop in cases of chronic exposure.
There is no specific antidote for this poisoning. Treatment is primarily decontamination (inducing vomiting and administering activated charcoal) in cases where ingestion is known to have taken place in the last 1-2 hours and supportive care. Severe cases of anemia may require a blood transfusion.
Note: Some people give their pets garlic pills or supplements as a flea preventative. This practice a) could potentially lead to toxicosis and b) doesn't work for fleas anyway. (Flea prevention post is coming up.)
Note 2: Some people sprinkle onion or garlic powder on their pet's food as an appetite stimulant. This practice a) sometimes works, but b) should be used with caution - tiny amounts, infrequently, and not in pets who are increased risk for toxic effects (Japanese breeds, on sulfa drugs, has other risk factors for hemolytic anemia.)
Note 3: Baby food is sometimes used to encourage a pet's appetite, especially in cats, and baby food often has onion powder in it. Whether or not this is enough to cause any toxicity is a matter of debate. I personally think it would only be a problem if the cat ate nothing but jars of baby food for some time.
Aside: In vet school, we called this "White Castles' Syndrome." If you aren't from Ohio, you might not get this joke. If you have eaten White Castle burgers (aka "sliders") you have my sympathy.