The evidence is building that a combination of two adulterants in pet foods, melamine and cyanuric acid, contributed to the deaths of hundreds of cats and dogs earlier this year.
The complex situation led to a massive recall of pet foods and caused a great deal of public concern. The AVMA hosted a conference call on April 30 to facilitate the exchange of information among representatives of the AVMA, American College of Veterinary Internal Medicine, Food and Drug Administration, and American Association of Veterinary Laboratory Diagnosticians.
On Sept. 24, the AVMA hosted a follow-up conference call. Some participants presented preliminary results from studies of melamine and cyanuric acid in several species of animals. Two of the same toxicology studies were the subjects of presentations during the AAVLD annual meeting from Oct. 18-24 in Reno, Nev.
During the meeting, the AAVLD also released the results of the "AAVLD survey of pet food-induced nephrotoxicity in North America, April to June 2007" and presented a preliminary case definition.
Facts and theories
The AAVLD survey found 347 cases that met diagnostic criteria for "pet food-induced nephrotoxicity" from April 5-June 6. The cases involved 235 cats and 112 dogs, with 61 percent of the cats and 74 percent of the dogs having died.
Dr. Barbara Powers, AAVLD president and director of the Colorado State University Veterinary Diagnostic Laboratory, said the survey probably found only a percentage of the actual cases. She said the mortality rate is not likely to be representative of all cases, however, because survey respondents had more information to submit for animals that died.
Dr. Wilson Rumbeiha, associate professor at the Michigan State University Diagnostic Center for Population and Animal Health, organized and presented the results of the AAVLD survey. He said more cats than dogs might have become ill because of cats' relatively small sizes.
"Even among dogs, small- to medium-size dogs were affected the most," he said.
Dr. Rumbeiha said the problem with melamine and cyanuric acid both having been in the adulterated pet food is that they can combine to form crystals in animal bodies. The crystals apparently can impair renal function.
According to the FDA, melamine and cyanuric acid were present together in wheat flour from China that went into pet food in the United States. Adding melamine could throw off a test for the protein concentration of an ingredient, allowing flour to pass for a costlier high-protein ingredient. The U.S. manufacturers believed they were buying the high-protein ingredients of wheat gluten and rice protein concentrate.
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Dr. Rumbeiha said the prevailing theory for how cyanuric acid, ammelide, and ammeline adulterated pet food is that they were co-contaminants. Incomplete reactions during melamine production could lead to the formation of these co-contaminants.
Case definition, studies
Four cats and one dog from the AAVLD survey formed the basis of a preliminary case definition, while a much larger sample of cases is still under review.
All five animals ingested pet food that manufacturers recalled because of adulteration with melamine, according to the proceedings from the AAVLD meeting. Results of kidney or urine tests from each animal were positive for the presence of one or more of the four contaminants. All animals had markedly high concentrations of BUN and creatinine. Urine was isosthenuric.
Casts, leukocytes, erythrocytes, and yellow-to-brown crystals with radial striations were often, but not always, in the urine sediment. Histologically, the most common finding was characteristic yellow-brown crystals in the renal distal tubules and collecting ducts—with or without microscopically visible tubular necrosis, degeneration, or sloughed epithelium. It also was common to see signs of chronic interstitial nephritis, which may have predisposed the animals to intoxication and death.
During the AAVLD meeting, Dr. Birgit Puschner of the California Animal Health and Food Safety Laboratory presented results from a pilot study on the effects of adding melamine and cyanuric acid experimentally to the diet of several cats. The study found that neither melamine nor cyanuric acid alone had an observable effect on renal function, but the combination caused acute renal failure.
Dr. Steve Ensley of the Iowa State University Veterinary Diagnostic Laboratory summarized a similar study in pigs. During the recalls, pigs and chickens ate some of the pet food that contained adulterants. Preliminary results from the pig study also suggested that the combination of melamine and cyanuric acid is a more potent nephrotoxin than either is individually.
Toxicology response system
Dr. Powers said the adulteration of pet food demonstrated a broader need for veterinarians to create a system for evaluating any toxins that may contaminate the food supply in the future.
The AAVLD formed a work group that wrote a white paper in May proposing to expand the focus of the National Animal Health Laboratory Network beyond infectious disease to include toxins. According to the paper, the NAHLN Veterinary Analytical Toxicology Response and Surveillance system would complement the Food Emergency Response Network by concentrating more on animal health than on food.
Dr. Stephen Hooser, assistant director of the Purdue University Animal Disease Diagnostic Laboratory, chaired the work group. He said the white paper proposed additional federal funding for existing NAHLN veterinary toxicology laboratories. Funding would go toward equipment, personnel, and information technology.
"We need to make sure that all of the testing capabilities are similar between all the labs and to make sure they can communicate with each other," Dr. Hooser said.
Dr. Hooser also hopes for contingency funds to cover analytic testing in situations such as the contamination of pet food. He encouraged veterinarians to support the National Agriculture and Food Defense Act of 2007 in the U.S. Senate, which would authorize appropriations for NAHLN from 2008-2012.
Methods for analyzing melamine and co-contaminants also were the subjects of presentations during the AAVLD meeting. A complete copy of the proceedings is available at www.aavld.org under Annual Meeting.