We have long known that there’s something about grapes that is toxic for dogs. Until earlier this year, however, the source of that toxicity was a mystery.

In the April 1, 2021 issue of JAVMA, a Letter to the Editor titled “Unique sensitivity of dogs to tartaric acid and implications for toxicity of grapes,” written by a team of veterinarians from ASPCA Animal Poison Control and two different veterinary hospitals, presented a possible explanation.

These authors noted that Cream of Tartar (which contains tartaric acid) causes clinical signs and pathologic findings that are very similar to those associated with grape ingestion in dogs. These veterinarians also noted that grapes and raisins can vary significantly in tartaric acid composition, but some grapes or raisins do contain tartaric acid at levels that have previously been found to be toxic. This may not only explain why grapes and raisins are toxic but also why the toxicity is so unpredictable. 

More evidence is needed before we can say with complete certainty that tartaric acid is the cause of grape toxicity. In the meantime, though, it certainly sounds like a plausible theory! 

Diagnosis of Grape Toxicity 

There is currently no available test to diagnose grape toxicity. Instead, diagnosis is based on the patient’s history, clinical signs, and laboratory findings. Clinical signs often develop within 6-12 hours of ingestion and include vomiting, diarrhea, lethargy, dehydration, abdominal pain, weakness, polydipsia, and trembling. 

In the early stages of toxicity, the most pronounced bloodwork abnormality is elevated creatinine. Over time, however, BUN will also begin to increase. Elevations may also be seen in serum glucose, calcium, phosphorus, liver enzymes, and pancreatic enzymes. 

Oliguric or anuric renal failure typically develops within 24-72 hours. 

Decontamination

The first step in decontamination is to induce vomiting. Grapes and raisins tend to remain in the stomach for longer than other substances and are slow to break down, so emesis may be beneficial even several hours after ingestion. 

Emesis can be induced with apomorphine or hydrogen peroxide. Apomorphine is generally preferred, due to the risk of esophagitis and hematemesis associated with hydrogen peroxide. However, hydrogen peroxide can be recommended if apomorphine is not available or if the client will have a long drive to the nearest veterinary hospital. 

Once the dog has stopped vomiting, emesis should be followed with a single dose of activated charcoal. This will help bind any toxins that remain in the intestinal tract. If necessary, an antiemetic may be administered to stop vomiting and allow the administration of activated charcoal. 

Treatment 

If patients show any signs of toxicity (vomiting, diarrhea, etc.), the patient should be hospitalized for at least 48 hours of diuresis. Bloodwork, urine output, and blood pressure should be closely monitored during hospitalization, looking for an elevation of renal values and signs of decreased urine output. If oliguria occurs, it should be treated promptly with dopamine and/or furosemide. Anuria can only be treated with hemodialysis and peritoneal dialysis; alternatively, euthanasia should be considered.

In patients that present before signs of toxicity have had an opportunity to occur, consider hospitalization for a brief period of monitoring. If the patient’s renal values remain normal after 24 hours and there are no clinical signs of toxicity, the patient can likely be discharged. 

Prognosis

The most challenging aspect of grape toxicity is that its effects are so variable. Some dogs eat several grapes or raisins without showing any signs of negative effects, while others experience fatal toxicities at relatively low doses. If a dog develops oliguric or anuric renal failure, the prognosis is guarded to poor.

The tartaric acid theory does suggest that toxic effects are determined by the characteristics of the grapes or raisins that are ingested, rather than by some inherent metabolic characteristic of the dog. If this theory is true, it suggests that even a dog that has eaten grapes in the past without issue could be at risk of toxic effects with future ingestions.