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Onions are toxic to dogs. The toxicity is dose dependent, so the bigger the animal, the more onion need be consumed to cause a toxicity. Onion toxicity causes a Heinz body anemia. Heinz bodies are small bubble-like projections which protrude from a red blood cell and can be seen when the cells are stained. This "bubble" is a weak spot in the red blood cell and, therefore, the cell has a decreased life-span and ruptures prematurely.
If numerous red cells are affected and rupture, anemia can result. It is a form of hemolytic anemia. Onions are only one of the substances which can cause Heinz body anemia. Other substances such as Acetominophen (Tylenol) and benzocaine-containing topical preparations can also cause Heinz body anemia in the dog.
The toxic effect of the onions are the same whether the product is raw, cooked or dehydrated. The hemolytic episode usually occurs several days after onion ingestion (lowest hematocrit around day 5 post ingestion). Daily feeding of onions could have a cumulative effect due to ongoing formation of Heinz bodies versus a single exposure with a wide gap until the next exposure, allowing the bone marrow time to regenerate the prematurely destroyed red cells.
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