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Ask a Vet: What is the Latest on Grape Toxicity in Dogs?

Are grapes poisonous to dogs? It's still a veterinary mystery, but knowledge has improved.

 |  Feb 4th 2014  |   2 Contributions

The grape is an emblem of change in veterinary medicine. As recently as the 1990s grapes were recommended by many vets as healthy, low calorie treats for dogs. Then the pendulum of thought on grapes swung abruptly. In fact, the opinion changed so fast that it was more like a trap slamming shut than a pendulum swinging. It was discovered that grapes can be toxic to dogs. Suddenly grapes were bad.

Note that I said can be toxic, rather than are. Grape toxicity is one of the great mysteries of veterinary medicine. Nobody knows what the toxin is. Nobody knows why some dogs can consume large quantities of grapes without suffering any adverse effects, while other dogs are much more prone to toxicosis.

When I last wrote about grape ingestion in dogs about a year ago, I discussed the case of a 90-pound dog who had eaten two raisins. He received immediate veterinary attention, and my staff administered a medication that caused him to vomit both raisins unchewed and undigested. To be safe I spoke with a specialist in veterinary toxicology and she stunned me. She stated that the dog needed to be hospitalized for several days of intravenous fluids and treatments. The owners were even more incredulous than I was, but they ultimately decided to follow the specialist's recommendations.

I learned after that to have the owners call the toxicologists directly; owners seemed more likely to believe the seemingly outlandish recommendations if they heard them straight from the horse's (so to speak) mouth.

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They seem harmless, but they might not be. Fresh grapes by Shutterstock.

Fortunately, it turns out that the recommendations that sometimes seemed outlandish may have been in some instances just what they seemed: outlandish. Veterinary science has made some progress at chipping away at the mystery of grape toxicity.

Now, don't get your hopes too high. Grape toxicity is still very poorly understood in general. But things have improved somewhat. At the time of the 90-pound dog with two raisins incident, grape toxicity was so mysterious that toxicologists recommended behaving as if any grape exposure whatsoever had the potential to kill a dog. Grapes might as well have been made from polonium-210. Fortunately such recommendations are not likely to be made very often in the future.

Here is what we know. Grapes, or perhaps some grapes but not others, contain a toxin. The toxin likely is found in the flesh of the grape, rather than the skin or the seeds. It is considered unlikely by experts that the toxin is a fungus or pesticide residue.

Raisins are dried out grapes, and they have even greater potential to cause toxicity because the toxin appears to be concentrated in them. There is no information about whether grape juice and wine contain the toxin, but for now owners should consider them potentially toxic (and I should note that feeding good wine to a dog is a waste of good wine). Grapeseed oil appears to be safe.

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"The grapes are for me. I brought kibble for you." Woman having a picnic with her dog by Shutterstock.

Some dogs can eat grapes without getting sick, whereas others aren't so lucky. Experts do not know whether this is because some dogs are resistant to the toxin, or whether the toxin is not present in all grapes, or both.

The most serious consequence of grape toxicity is acute kidney failure. The Veterinary Information Network (an outstanding online resource for veterinarians) states that the lowest recorded dose of grapes that has caused kidney failure is 0.7 ounces of grape per kilogram of dog (oz/kg). For raisins, the figure is 0.11 oz/kg. For a 30 kilogram (66 pound) Labrador Retriever such as my pal Buster, this translates to more than a pound of grapes or about three ounces of raisins. According to these figures, a much smaller dog could be sickened by just one or a few giant grapes. And remember that these figures are simply the lowest recorded ones at this time. As our knowledge evolves, they may change. So don't start giving your dog grapes as treats.

It also turns out that dogs that suffer grape toxicosis don't just suddenly develop kidney failure and die. They usually suffer from vomiting and diarrhea first. These gastrointestinal problems appear to contribute significantly to the toxicity. Vomiting and diarrhea cause dehydration. Dehydration causes concentration of the fluid that is destined to become urine in the kidneys. And the concentration of the fluid, and therefore the toxin, appears to be a significant step in the process that leads to kidney failure.

This feature of grape and raisin toxicity -- that dehydration generally plays a role in the development of kidney failure -- means that dogs that are treated early for grape ingestion generally have good prognoses. The mainstay of treatment is administration of intravenous fluids that prevent dehydration and dilute the fluid in the kidneys. Grape toxicity is highly treatable before kidney failure develops. Unfortunately, once kidney failure occurs the outlook darkens significantly.

I look forward to the day when the toxin (or perhaps toxins) in grapes is identified, and when the mystery of grape toxicity is fully solved. I am glad that my future is unlikely to involve the hospitalization of 90 pounders who have eaten (and subsequently vomited) two raisins. But for now it is important to remember that grape toxicity is a mystery. It is possible that our understanding of grape toxicity will be revised again, and that some day I'll be writing another piece emphasizing that even a single grape can kill a large dog. I hope that day never comes. In the meanwhile, don't let your guard down. Keep grapes away from your dog.

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