Written by Margaret A. Wissman

(This article was in the Simian, the SSA's monthly newsletter, some time ago.)

As a veterinarian, I have known for many years that onions, whether fresh, cooked or dehydrated, can cause anemia in pets. I remember reading a case report years ago concerning a small dog that ate a good portion of a bag of fresh, whole onions one day while his owner was at work, and the dog was near death when discovered by his frantic owner. However, I continually read stories (including one in the most recent Simian Society Newsletter) about owners feeding onions to their monkeys, and I realize that most people have no idea of the dangers of feeding onions. So, I thought it would be a good idea to educate owners about onions.

Onions can cause Heinz-body hemolytic anemia. What is anemia? Anemia is any condition in which the number of red blood cells is decreased. Red blood cells carry oxygen to the tissues on a molecule called hemoglobin, and the number can be measured by counting the number of red blood cells (RBCs) or quantity of concentration of the blood (this is called PCV or packed cell volume.) RCBs are also known as erythrocytes. I know this is all technical, but bear with me just a bit longer. Now let's go through that hemolytic means. Hemolysis is the destruction of RCBs, which liberates hemoglobin. So hemolytic refers to something that destroys RBCs in the blood stream. The last term that we need to understand is Heinz-body. This is a term used to describe changes that occur to RCBs when they are exposed to oxidants, including ingested onions. Heinz-body from when hemoglobin molecules are adversely changed and the hemoglobin coalesces in the RBCs. Heinz-body changes to RBCs are irreversible. These changes to the red cells make them more rigid, which is not good. Red Blood cells must bend and flex as they twist and turn on their course through the blood vessels of the body, and if they cannot, they may rupture or be filtered out of the bloodstream.

Let's now cut through all this medical jargon. What exactly does happen when onions are consumed? Simply put, onions cause red blood cells to become unable to function properly, then causing them to rupture. This causes anemia.

What clinical signs will occur with onion toxicosis? The signs that you see will depend on the amount of onions consumed, the length of time they have been fed, and the size of the animal. Gums (mucous membranes) may appear pale if there is moderate or severe anemia. If lots of RBCs have ruptured, there may be hemoglobin in the urine, causing it to appear reddish or brown, or the tissues may become jaundiced. Weakness, depression, rapid heart rate and rapid respiratory rate may be observed as a result of there not being enough oxygen reaching the tissues (hypoxia). Vomiting, decreased appetite and diarrhea may also occur. Blood test taken by your veterinarian will show certain characteristics. Heinz-bodies can be seen on blood smears and the packed cell volume will be decreased, indicating anemia. As the body tries to replace the damaged blood cells, certain characteristics may also be seen in the blood. Hemoglobin may be seen in the urine.

Many case reports have been published concerning Heinz-body hemolytic in dogs and cats. Most clinical cases have occurred in small dogs. One report describes Heinz-body hemolytic anemia in two cats fed onion soup! I know that many research facilities that house non-human primates will not feed them onions because of problems with anemia. One lab had anemia problems in baboons that were frequently fed onions. Since we know that onions can cause fatal hemolytic anemia, it is best to never feed them to our pet monkeys. However, there can be hidden onion in some products that we feed as well. For example, many veterinarians feed sick cats and kittens all-meat baby foods, and some cases of Heinz-body hemolytic anemia have occurred after these have been fed. Why? Check the label. You may be surprised to find that many baby food manufactures add onions or onion powder to increase palatability. There can be enough onion products in these little jars to cause serious problems in these small creatures. A jar of chicken baby food that I have right here has this list of ingredients: chicken, water, modified corn starch, onion powder and extractive of celery. Another product that I like to feed, which is a good first solid for my baby monkeys are chicken sticks. However, let's go down the list of ingredients: chicken, water calcium reduced dried skim milk, salt, sugar, onion powder and garlic powder. I only offer this very infrequently to my toddler now! Onions are often added to products to enhance the flavor of many foods, especially for human babies, so be sure to read the label carefully before feeding any prepared food to your monkeys.

What about onion toxicity in humans? Since damage from feeding onions seems worse in little dogs and cats, it may be that the amount of onion in baby foods in not enough to cause problems in humans, in relation to their size. But certainly, the amount of onion in baby foods can be potentially harmful to little marmosets, tamarins and other monkeys. And fresh or cooked onions will certainly cause a degree of Heinz-body formation in the blood of primates. Based on the information that we now have, I would recommend not feeding onions to monkeys at all. I would like to see labs that have experienced Heinz-body hemolytic anemia in monkeys fed onions write up the cases to present to other vets, so that others may learn the hazards of onions. So, please spread the word to other owners that onions should not be fed to non-human primates or other species of animals, as well.

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