Grape gripes

I recently saw a picture of a friends dog picking its own grapes from a vine in a back garden. I mentioned to the owner (who is a doctor) that grapes may be poisonous to dogs. He replied that his dog had been picking its own grapes from the garden for the last 10 years and to no apparent harm.
My friend the doctor said that he had looked into this matter and found that it was a chemical which is sprayed on to the grapes during the growing process or during the washing and getting them to the supermarket which is harmful to the dogs - not the grapes themselves! What do you think?
Sarah Morris
Hillingdon, Middlesex

Alison Logan, vet, advises:
The whole question of just how much of a danger grapes pose to dogs has not, to my knowledge, been answered. It was posed in the Dogs Today Think Tank in January 2009 where I described in detail the effects and what treatment can help.
It does concern me, not only as a practising veterinary surgeon but also as a dog-owner because my Lab Pippin is notorious for eating anything and everything, and especially fruit-wise. We do have three grape vines in the garden, which we planted when we moved here in 1992, when I was unaware of the potential danger to a dog of eating grapes.
In the last two years, there have been bunches of grapes within her reach. I was unaware of Pippin eating them until one day when I found grape skins in her faeces. She was not otherwise unwell. Does this mean that the toxin is not present in our grapes? Regardless, I no longer wait for them to ripen but pick them as soon as I find them but she does still help herself to a surreptitious snack when she can.
Why dogs should develop acute renal (kidney) failure from eating grapes, raisins and sultanas when we can eat them safely is still unclear. I have not managed to find an explanation anywhere: 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. As you rightly say, we are not even sure whether it is a substance occurring naturally within the grape or whether it may be something which is applied to the grape. Certainly, we do not apply any chemicals to the vines in our garden.
Sadly, though, dogs have died as a result of acute kidney failure so best advice is to avoid dogs having access to grapes, raisins and sultanas as much as possible.

Alex Campbell, Veterinary Poisons Information Service, advises:
It is a frustrating thing that the mechanism by which grapes, sultanas, raisins and even currants can cause toxicity in dogs and possibly other animals such as cats and ferrets is not fully clear. However, the fact that severe effects and even fatalities occur in some animals and not others, even when the doses ingested have been similar has led some authors to conclude that there may be a component of the fruits whose concentration varies markedly or that the effects may be due to an external compound that may occasionally be present. People have suggested various possibilities including cleaning agents,
pesticides, and heavy metals. Other hypotheses for the toxicity have included tannin intolerance, contamination of the fruit with mycotoxins, sugar overload, ingestion of excess vitamin D or even enzymatic or genetic variances in some animals. None of these have been proven in the published peer-reviewed literature.
What is certain, however, is that in some dogs ingestion of a small number of grapes can result in life-threatening renal failure. The signs of this may not be apparent until it is too late to respond and treat successfully. In a series of 180 cases reported to Veterinary Poisons Information Service (London); 13 dogs died and 4 were euthanased (death rate 10.5%).
The VPIS would advise that dog owners do not feed their animals ANY of these fruits. It is possible that some toxic effects may occur in dogs fed small amounts regularly that may not result in visible clinical signs. If your animal gets hold of the fruits take them to the vet as a matter of urgency.
NOTE: The VPIS is not a public access service and any queries should be referred to your vet in the first instance.