Sharon, I am so pleased you asked this question. You gave me the opportunity to brag about the department I was honored to lead from 1995 to 2006.
Faculty in the department Veterinary Biosciences at the University of Illinois, Urbana-Champaign formed the original Animal Poison Control Center, long before I arrived. We were fortunate in convincing the American Society for the Prevention of Cruelty to Animals (ASPCA) to take over and grow that Center. Veterinarians in the Poison Control Center started to notice an increase in reports of dogs showing signs of toxicity after the ingestion of both grapes and raisins in 1999. From April, 2003 to April, 2004 the Center logged 140 cases of dogs consuming either grapes or raisins, 50 of these dogs developed signs of toxicity and 7 of the dogs died.
Drs. Carla M. K. Morrow, a toxicology graduate student in the department, Victor E. Valli, Dean of the College of Veterinary Medicine, Petra A. Volmer, a faculty member of in the department, and Paul A. Eubig, another toxicology graduate student published a definitive report on grape and raisin toxicity in the Journal of Veterinary Diagnostic Investigation in 2005. They studied ten dogs that suffered acute renal (kidney) failure after ingesting 6.72 grams or more of raisins per pound of dog. That calculates to about 9.5 ounces of raisins for a forty-pound dog. Other studies reported toxic doses of 6.8-18.9 ounces for raisins and 19.6 ounces of grapes for a forty-pound dog (sixteen ounces to a pound).
Usually within twelve hours after ingesting either grapes or raisins, dogs will start vomiting, become lethargic and develop diarrhea. If not treated, they become increasingly lethargic, dehydrated and refuse to eat. The dog may urinate more frequently than normal early but progresses to a decrease in urine production and eventually to anuria (no urine formation). The primary lesions of this toxicity are in the kidneys with significant degeneration or necrosis (death) of the proximal renal tubules (very bad). Morrow, et al. also reported pathological changes in arteries supplying the large colon in the dogs they studied. Unfortunately nobody has yet been able to determine the toxic substance in grapes, raisins, or grape pressings from wineries (yep, at least one dog ate grape pressings). It is difficult to obtain funding when the scientists must feed animals potentially toxic substances isolated from the grapes to be able to determine what it is that is toxic.
If your dog has eaten grapes or raisins and is showing signs of illness, or if s/he vomits and you see pieces of grape or raisin in the vomitus, get the poor guy to your veterinarian within two hours. The treatment involves inducing vomiting, possibly treating with activated charcoal and administering large doses of intravenous fluids in an effort to flush the kidneys. If the kidneys are badly damaged it may be necessary to use either peritoneal or hemo-dialysis. With fast, effective treatment, about half of the dogs showing severe signs of grape or raisin toxicity can be saved.