Does your dog sit in the kitchen while you cook, nibbling away as you chop onions with wild abandon? Next time be careful not to let stray onions make it past your dog's lips. Dogs and cats lack the enzyme necessary to properly digest onions and this could result in gas, diarrhea, or severe gastrointestinal distress. If large amounts of onion are ingested or onions are a daily part of your dog's diet, the red blood cells may become fragile and break apart. Severe anemias and even death can occur if the dog ingests lots of onions and receives no treatment.
The most common source of onions for cats is in human baby food. Some baby foods have onion powder added for taste. When consistently fed baby food with added onion powder, signs of toxicity can develop.
For more information on the anemia that is caused by onion toxicity, please read Hemolytic Anemia