We don't know the exact pathogen at this time.  Fungal, pesticide and heavy metal etiologies have been ruled out. From the most recent data, the toxic agent seems to be located to the flesh of the fruit, not the seed. After ingestion, dogs will go into severe acute renal failure.

How much is toxic?

The lowest recorded amount that cause acute renal failure are:

• raisins: 0.11 oz/kg
• grapes: 0.7 oz/kg   

**not every dog may be susceptible since there are many dogs that can tolerate grapes and raisins in large numbers.  There is some evidence that cats may be affected as well.

Treatment

Treatment can be attempted after acute ingestion: emesis (vomiting), activated charcoal followed by intravenous fluid administration for 3-4 days. Kidney values should be monitored every couple of days. If there are no changes in kidney values by day 3, then there is a good chance the pet will remain unaffected.

Prognosis

If early treatment and therapy is initiated, prognosis is good. If renal failure develops, then prognosis is guarded.

references:

1. Brandy Porterpan. Raisins and Grapes: Potentially Lethal Treats for Dogs. Vet Med 100(5): 346-350 2005

2. Jill Richardson, VMD, VIN