For many people, it seems unbelievable that grapes and raisins can poison dogs. They’re harmless to humans. We’ve all seen dogs occasionally eating foods containing raisins with no apparent ill effects. How can they suddenly be poisonous?

Why are grapes and raisins not always poisonous to dogs, and never poisonous to humans?

First, like all poisons, the poisonous effect depends on the dose taken per kilogram of animal body weight. Large dogs can safely eat some raisins without problems.

Secondly, the toxic ingredient in raisins seems only to be present intermittently, so a dog may eat raisins without problems on several occasions, then fall seriously ill the next time.

What is the toxic ingredient in grapes and raisins?

The actual toxic ingredient is still a mystery. The fact that grapes and raisins can be poisonous has only been deduced by circumstantial evidence, with many dogs developing acute renal failure for no obvious reason, with the only common factor being the previous ingestion of grapes or raisins. Samples of the fruit in such cases has been analysed, but a toxic agent has not yet been isolated.

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The best guess so far is that it is a water-soluble substance, and that it’s in the flesh of the grape/raisin, but not the seed. One theory is that it is a mycotoxin (i.e. a poison produced by moulds or fungi on the grapes). The problem in dogs was first highlighted after a year with high levels of rainfall. This had led to damp grapes which were more likely to develop fungal growth.

But why should humans be safe from this toxin? It’s well known that cultured dog kidney cells in the laboratory are exquisitely sensitive to other types of mycotoxins. It makes logical sense that dog kidneys might also be more sensitive to damage by another mycotoxin, even its identity has yet to be established.

How many raisins or grapes can a dog eat?

If a terrier steals a few grapes off your plate, is a visit to the vet needed? If a Labrador has a slice of fruit cake, do they need to be taken to the emergency vet?

This is always a judgment that is not black and white. It seems sensible to look at the lowest recorded doses of grapes or raisins linked to acute renal failure in previous cases of poisoned dogs. This allows an estimate of the probable toxic dose depending on the animal’s body weight.

Grapes

The lowest toxic dose that has been reported is around 20g grapes per one kilogram of body weight. A typical grape weighs 2 – 5g, making a toxic dose is around 4 grapes per kg.

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Editors Note – these figures have changed since this blog was published, and we no longer do dose calculations because this is not a simple dose-response curve. And because we don’t know what the poison is, there’s no way of knowing if lower doses are safe, or if it’s just not been reported. This is why treatment is usually recommended, even though many dogs won’t display any symptoms.)

See our latest blog on raisins for a more up-to-date discussion! 

Raisins

The lowest poisonous dose in confirmed cases has been around 3g/kg. An average raisin weighs around 0.5g, making a toxic dose of approximately 6 raisins per 1kg.

Important note

Please remember that the above doses mention quantities that have definitely caused serious kidney failure in the past. The decision on whether or not to take a pet to the vet is a personal decision, taken after balancing the possible risks. Many people prefer to take a conservative approach, to be as safe as possible. For example, if a dog has eaten even half of the above quantities, it may be safer to take them to the vet for “just in case” treatment.

What do vets do for dog that have eaten grapes/ raisins?

1) If ingestion has happened in the previous hour

This is the ideal situation: the vet can give an injection to cause the pet to vomit, emptying the stomach and removing the grapes/raisins before any toxic ingredients have had a chance to be absorbed into the bloodstream.

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2) If ingestion has happened in the previous two days but the pet is still well

Depending on the situation, vomiting may still be induced, activated charcoal may be given to limit absorption of the toxin, and intravenous fluids may be given to flush fluids through the kidneys in an attempt to minimise any damage. Blood and urine tests may be recommended to monitor kidney function. If the dog is well after three days, then the high risk period is over.

3) If ingestion has happened and the dog is unwell (e.g. vomiting, dull, inappetant)

In such cases, the kidneys may have already been damaged by the toxin. Urine and blood tests will be carried out to assess the severity of the damage to the kidneys, and intensive care will be needed to save the pet’s life, including high levels of intravenous fluids. The prognosis is guarded: unfortunately, some affected dogs die, despite the vet’s best efforts.

 Conclusion.

  • Keep grapes and raisins away from dogs.
  • If any dog eats them accidentally, phone your local vet (even if it’s after-hours)
  • Tell your vet how many grapes/raisins were eaten along with the bodyweight of your pet.
  • Your vet will then advise you on the safest course of action.

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