Grape Expectations

It may seem like a long time ago for some of us, but to my generation it’s a relatively recent discovery that grapes are poisonous to dogs. It was somewhat of a shock when this first came to light, but the intervening decades have yielded enough data to help you confidently assess the threat posed by a handful of grapes or a slice of raisin toast.

At first, there were only case reports. And incredulity, often accompanied by tales of titanic grape eaters that ingested buckets and buckets of the stuff, year after year, without so much as a postprandial belch. Various theories were advanced about the toxic mechanism, ranging from genetic mutations to residual pesticides to mold. None of those ideas panned out.

It’s only been within the last five years that the poisonous compound in grapes was revealed to be tartaric acid. The toxin content per fruit varies widely based on variety, ripeness at harvest, and even the weather/soil conditions during grapevine husbandry. Sensitivity to the toxin apparently also varies from one individual to the next. It is therefore difficult to predict which pets are in the greatest amount of danger after eating grapes, so each and every case is treated with an abundance of caution. If your dog ingests more than one grape per ten pounds of body weight, he should seek veterinary care. Note that this chemical does not seem to affect cats to any great degree; even so, I wouldn’t recommend tempting fate.

Tartaric acid poisoning is not associated with grape juice, grape jelly, grape jam, grapeseed oil, or the grape leaves used for Middle Eastern cuisine. You shouldn’t be giving your dog wine, but there is almost no tartaric acid in it. This poison is present in grocery store and homegrown grapes of any color, with especially high concentrations in “cotton candy” grapes. It’s also found at dangerous levels in tamarinds and in Zante currants, which are actually a type of grape. True currants are a kind of berry and do not contain any appreciable level of toxin.

Don’t forget the raisins! They are simply dehydrated grapes, so each raisin carries the same danger as a single grape. On the other hand, a cup of raisins is far more poisonous than a cup of grapes because the dried fruit is much more densely packed. Cooking dampens the risk profile somewhat; each raisin baked into a cookie or bread (or otherwise exposed to prolonged heating) is rendered approximately half as potent as the one that comes straight from the box. Golden raisins are no less poisonous than the pigmented kind. Prunes bear a passing resemblance to raisins, but they are not made from grapes.

Too many raisins, or grapes, or Zante currants, cause kidney failure, so the first thing we want to do is get those back out before they can be digested. Fortunately, these items move very slowly through the gut, meaning they can sometimes still be removed (by vomiting, which your veterinarian will swiftly arrange) even several hours after ingestion. In general, dogs that steal grapes have a very good chance at full recovery as long as their theft is discovered in a timely manner.

Dr. M.S. Regan