Watch out with mixing dogs and onions!
While the occasional leftover Spaghetti Bolognese or curry is unlikely to do your dog any harm, in high doses onions and garlic are quite poisonous to him. It doesn't matter if they are cooked or raw,

Onions and garlic both contain thiosulphate. This chemical interferes with the hemoglobin in the red blood cells, and results in the red blood cells being broken apart inside the blood vessels.  Hemoglobin is involved in carrying oxygen around the body.

Approximately 15-30g of onions per kilo bodyweight is needed to poison your dog; most dogs wouldn't eat that much at once, so it's not a particularly common poisoning.

Poisoning can either occur with one large meal of onions, or with multiple smaller amounts over a period of several days. It can also occur with fresh onions, dried onions or even onion powder.

Garlic also contains thiosulphate, however it seems that garlic is less toxic, and large amounts would need to be eaten to cause poisoning.

Initially your dog will show signs of gastroenteritis, he won't want to eat, and he may vomit and have diarrhea. Other symptoms develop up to five days later, and relate to a drop in the number of red blood cells in the bloodstream.

This results in reduced oxygen delivery to the organs and tissues of the body. Sometimes there is enough damage to the hemoglobin to result in the brown colored methemoglobin, a form of hemoglobin which cannot carry oxygen. We never thought dogs and onions could be such a terrible mix.

Your dog will be lethargic, he'll have pale gums, and will be breathless. Hemoglobin from the damaged red blood cells will pass out in his urine, which will make it reddish in color.

Treatment is directed at managing your dog's symptoms, as there is no specific treatment for onion poisoning. Your vet will make your dog vomit, and may use activated charcoal to reduce further absorption of thiosulphate.

An intravenous drip helps to support your dog as it recovers, and helps the hemoglobin flush through the kidneys. If poisoning is severe, oxygen therapy or a blood transfusion may be necessary. The outcome varies depending on how much onion your dog has eaten.

To prevent poisoning due to onions or garlic, keep them stored in a cupboard out of your dog's reach. Also, its recommended that you avoid giving your dog leftovers which may contain onion or garlic; while poisoning isn't all that common, it still occurs and it can kill.

ALWAYS consult your Veterinarian!!

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