When the Oxidant is Onion, and Effect is Anaemia

The humble onion has always seemed the most unlikely of toxins but, it’s true, they can kill  both dog and cat.   Yet many pet owners have long fed onion-bearing leftovers to dogs with no apparent ill effect. So how real are the risks? Here’s a story of a Border Collie pup, and a not-so-near-death experience of onion ingestion, which demonstrates mechanism, diagnosis, treatment and prognosis.

Tristan’s new dog, Tiger, a 3-month-old Border Collie, was typically excited by the smells emerging from the kitchen. The  Sunday night casserole ingredients, amongst others, included 4 red onions, several leeks, and a couple  of cloves of garlic.  You can imagine the shock of the realisation that onions were a toxicity risk, dawning only moments after the meal was finished. Fatally poisoning a precious, new family member, irrespective of species, is not a pleasant experience.

Delayed only by a minor car collision, brought-on by the frantic urgency of dash to the hospital, Tiger’s stomach was purged of contents within an hour of eating.  And the yield was spectacular. The volume of  vomitus appeared more than three-quarters of the volume of casserole consumed, but was enough onion absorbed to kill a Tiger?

Luckily this disease is not immediately life-threatening and smoulders over a week or two.  Tristan could return home with Tiger, sort the car crash, rest up, and await the results of blood testing over the following days. As a preventative measure, antioxidant doses of Vitamin C were commenced.

Tigers baseline percentage of red blood cells in circulation was 37%, within the normal range of 35 -50%. 24-hours later the serum was clear, not stained by heamoglobin released from destroyed red blood cells.

Comforted by these results, Tiger never needed to return to hospital, and was simply observed for symptoms of anaemia: weakness, lethargy, pale gums, and breathlessness. Urine dipsticks, easy for an owner to check at home, remained reassuringly clear of haemoglobin over the following days, and Tristan could relax that a potentially fatal mistake was averted.



Organosulphide compounds found in the Allium family of vegetables: onion, shallot, garlic, leek and chive. Flesh, juice, leaves, raw, cooked, spoiled, and even processed powders, all contain the toxin. Both cats and dogs are susceptible. In fact, cats are more sensitive to the poison, but dogs are more famous for falling victim virtue of their scavenging habits.  The toxic dose is  > 100g  (a medium onion) per 20kg of dog.


Once in the bloodstream, the toxin enters red blood cells and causes oxidative damage to haemoglobin, rendering the red blood cell useless for carrying oxygen.  Identified by the body and destroyed, the cells release their hamoglobin, initially into serum then filtered by the kidney and excreted in the urine, staining it pink in severe cases.


Within a day of ingestion, the percentage of red cells is falling, reaching it’s lowest point in 3 to 5 days.  The bone marrow responds and replaces damaged and lost red cells within a week. The whole episode is over in 1 to 2 weeks.


Is based on symptoms of anaemia, falling red cell counts, haemoglobin in urine, or identification of damaged red cells under a microscope.


Like most toxins, inducing vomiting is the best immediate preventative treatment if ingestion is identified early enough. If 3 hours have passed since an onion meal, there isn’t much point in provoking a purge.

If a large volume of onions has been eaten, and vets do hear horror stories of half-bags of rotten onions scavenged from compost, it may be worth going all-the-way with anaesthesia, cathartics, enemas and activated charcoal. More commonly smaller meals, containing some onion, like bolognaise or casserole, may not be toxic enough to cause any significant problem, and these drastic steps may not be necessary.

Once the toxin is in circulation, antioxidant treatment using Vitamin C (20mg/kg, three times daily) can minimise the toxic damage to red blood cells.


The vast majority of minor anaemias, associated with onion ingestion, go unrecognised by owner. Most animals can cope with minor falls in red cell count, typical of mild to moderate haemorrhage. If a large enough dose is absorbed, life-threatening anaemia may develop, falling to around 10% red cell count. Oxygen and blood transfusion may be required to sustain life until the bone marrow does it’s thing and replace the lost cells.

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3 Responses to “When the Oxidant is Onion, and Effect is Anaemia”

  1. Rich Ploeg says:

    Love ya work Matti.

    Gorgeous picci too 🙂

  2. Juud Addsion says:

    Hi, glad Tiger is ok, she is a treasure!
    Always thought that also garlic was really healthy for dogs, so thanks for passing on the message, can`t bear anything happening to our little ones, xox juud

  3. matt says:

    small amounts of garlic may well be therapeutic. its all a matter of dose. most things can be toxic to most species if consumed in sufficiently large quantities. It just seems cat and dog, and probably a range of other exotic carnivores, are uniquely sensitive to this family of plants.

    Its likely there would be other compounds, or even the organosulphides themselves, which may have helath benefit if consumed in small quantities. I’d feel comfortable recommending very small amounts of garlic intermittently for the dog, but maybe not the cat. They are about 3 times more sensitive to the toxic effects.

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