A South African scientist fed daily doses of about 250 mg/kg to sheep, but most animals refused food and lost weight, and some of them died. Another South African investigator gave melamine doses to a single sheep, starting with 2,600 mg/kg. At such high doses, the sheep died within a few days from kidney damage, and the investigators could see crystals of melamine hanging from the animal’s prepuce. Lower doses of melamine caused sheep to stop eating, especially if their water intake was restricted. These studies suggested that a dose of about 250 mg/kg kills some— but not all—sheep over time.So, by 1968, melamine was known to induce kidney damage when fed to sheep over prolonged periods. Ten years later, American investigators tested melamine in cattle and found that a dose of about 100 mg/kg caused four out of six steers to refuse feed.
We think these studies are highly relevant to the pet food situation. The FDA reported that melamine could have accounted for 10 percent of the weight of the false wheat gluten, and the false wheat gluten could have accounted for as much as 10 percent of the weight of the pet food. If so, 100 grams—about 3 ounces—of pet food could have contained as much as a gram (1,000 milligrams) of melamine, and an average cat or small dog could have eaten an amount close to the 250 mg/kg level that proved toxic to sheep in the 1960s.
Recently, investigators from Georgia and the University of California, Davis, have shown that much smaller amounts of melamine can form crystals if cyanuric acid is also present. In cats, doses as low as 32 mg/kg each of melamine and cyanuric acid caused crystallization and kidney blockage.
We were surprised that neither the university or FDA veterinarians involved in the melamine investigations knew about this earlier work, but we think we can guess why. Papers in international journals are not readily accessible on the Internet, and the old animal feed literature is not likely to be studied these days.We had to discover the papers the old-fashioned way, by going to the library in person, sifting through reference lists, following up leads that sometimes required interlibrary loans and pursuing the reference trail back to its origins.We had the interest and time to pursue these questions. For the veterinarians and FDA officials caught up in the heat of the recall, a trip to the library might have seemed like a luxury they could ill afford.
We can’t say whether earlier suspicion of melamine would have hastened the recall or improved veterinary care of the sick dogs and cats.But we can say that the old experiments on animal feeding are well worth reading, that it’s best to read entire papers and not just their abstracts, and that libraries still have much to offer that the Internet cannot.
This article is based on Nestle M., Nesheim M.C. Additional information on melamine in pet food [letter]. JAMVA 231(2007):1647.