Onions are one of the many foods we shouldn't feed our pets. Looking it up, I learned that onions cause hemolytic anemia in MANY species: cattle, sheep, horses, dogs, cats, and even some primates. If anything, WE'RE the odd ones out. A few sulfur-containing organic compounds found in onions were fingered as the causative agents, and oxidative damage to red blood cells following decreased activity of glucose-6-phosphate dehydrogenase was the most mentioned mechanism. How do humans avoid this?