Why are onions dangerous for dogs and cats?Is it true that they can cause red urine and kidney failure?

A deadly vegetable.

Whether fresh, cooked, or dehydrated, onions can be very toxic to all dogs and cats.

The active ingredient in onions is allyl propyl disulfide, a component of onion oil. This substance damages red blood cells, or RBCs.

Feeding your dog or cat onions can cause a condition called Heinz body anemia. Heinz bodies are small, round projections that extend out from the red blood cells. These projections are accumulations of damaged hemoglobin molecules. Allyl propyl disulfide irreversibly breaks down hemoglobin and permanently damages the RBCs. The oxidative effects of onion oil also damage the cells' surrounding membrane, further weakening the red blood cells. The body responds by destroying the damaged cells. As the number of red blood cells circulating in the body falls, Heinz body anemia results.

Anemia alone can have serious consequences. Clinical signs include pale mucus membranes, depression, increased heart and respiratory rate, weakness, fever, vomiting, and loss of appetite.

Hemoglobin from the destroyed red blood cells may appear in the urine, imparting a reddish-brown color. The loss of hemoglobin into the bloodstream, which is then filtered by the kidneys, can cause severe kidney damage, leading to kidney failure.

A veterinarian should immediately see any dog or cat that has eaten onions. In addition, dog and cat owners should read all food labels prior to feeding; even some baby foods contain onion salt.