Pondering grapes

We are all meant to know that real dark chocolate is bad for dogs and that grapes are, too. But as I was eating a grape this morning I started to wonder what it is that is so different about us and dogs that makes grapes healthy for us but deadly for them. We're both omnivores, but exactly how do they differ from us digestion-wise? There are probably many things they can eat that would make us ill so we must differ in a few ways - just wondering anyone out there know exactly how?
Beverley Cuddy, Dogs Today

I first learnt about the dangers of fruits from the woody vine Vitis vinifens and their dried derivatives (grapes, raisins, sultanas) as a throw-away comment from the lecturer on a day course in August 2004. We were all veterinary surgeons in first opinion small animal practice, and none of us there was aware of there being a problem, which has only really come to light over the past ten years or so. Like you, I have wondered just why dogs should develop acute renal (kidney) failure from eating grapes, raisins and sultanas, and have not managed to find an explanation anywhere.
The main problem is that the source of the intoxication has not yet been identified and therefore a mechanism of action cannot be formulated. It does seem that a bigger quantity of grapes can be eaten than of raisins before signs of intoxication develop, suggesting that the toxin is more concentrated, present at a higher level, in raisins. A bunch of grapes has been said to be equivalent to a small snack size box of raisins, for example.
There are also conflicting reports on the amount of grapes or raisins which poses a toxic threat. I suspect this relates to the fact that the owner often has no idea how many have been eaten, unless a bunch of grapes has patently gone missing which no human has owned up to eating. This is especially the case if the patient has been helping himself direct from the vine. It does also make one wonder whether the toxic component occurs at highly variable levels, or needs another contributing factor present.
My Lab Pippin is notorious for eating anything and everything, and especially fruit-wise. She will delicately pick herself blackberries from the hedgerow, and was eating plums from the ground in our garden as a puppy if I did not manage to remove them first, which gave me many sleepless nights. I did often see her spitting out the stones, but otherwise they simply passed through!
We do have three grape vines in the garden, which we planted when we moved here in 1992 and hence before the intoxication problem had become apparent. In the last two years, there have been bunches of grapes within her reach. I no longer wait for them to ripen but pick them as soon as I find them because I did catch her helping herself to a surreptitious snack. The following day, there were grape skins in her faeces but no other signs of ill health. Does this mean that the toxin is not present in our grapes?
Renal failure has been reported to occur at a wide range of levels on a bodyweight basis so it is best to consider any amount eaten as a health risk. Clinical signs become apparent rapidly within 24 hours, manifesting as gastro-intestinal upset, loss of appetite, abdominal pain and lethargy. Development of kidney impairment manifests as increased thirst, leading on to failing urine production.
Treatment needs to be initiated urgently, including:
- Induction of vomiting;
- stomach lavage;
- activated charcoal may help limit absorption of toxins, provided there is normal intestinal function;
- anti-emetics to stop recurrent vomiting;
- aggressive intra-venous fluid therapy to support the kidneys and maintain normal hydration for at least 48 hours;
- diuresis if urine output is inadequate.
Sadly, though, dogs have died as a result of acute kidney failure so it is wise to avoid your dog having access to grapes, raisins and sultanas.
Alison Logan, vet