The saga of the pet food contamination in North America (‘The Recall’ as it is now being called) rumbles on with some murky new twists. The culprit seems to be a chemical called melamine, an industrial chemical that boosts the nitrogen content of food giving it the appearance of having higher protein content. There are reports from China that feed manufacturers regularly add melamine to animal feeds, and it seems that the melamine in the cat and dog food in the US and Canada came from contaminated of wheat gluten imported from China. It is not clear why Canada (the pet food processor was in Canada), a net exporter of wheat, needs to import a wheat product from China. Melamine has also recently been detected in pig feed and in poultry feed. The pigs and chickens feed the contaminated feed appear to be healthy, but they are being kept under quarantine and are being observed. The FDA regards to risk to humans from residues of melamine in meat from pigs and chickens to be very low, as the dosage of the chemical would be within the levels considered to be safe.
One interesting aspect of the recall is the lack of published toxicological data on melamine. Another is that some people, science journalists and others, who are writing about melamine toxicity, are ignoring some of the key published information. For example, the New Scientist (5 May 2007, p8) stated that a "literature search shows that the only ill effect of melamine so far identified is its ability to cause bladder cancer in animals given large doses over long periods". In the Guardian Unlimited (4 May) a list of questions and answers on melamine (since withdrawn) stated that "previously the only known risk was to rodents… When fed to male rats in high doses melamine caused tumours". Both of these stories ignored a key paper by R Clark in 1966 which describes poisoning in sheep after being fed melamine for 11 days. The clinical signs included nephrosis with crystals visible in the kidneys in postmortem examination. The abstract of this paper can be seen in the CAB Abstracts archive. Perhaps the journalists only searched Google or PubMed, or the current CAB Abstracts database and so missed this important piece of information. One of the main reasons that CABI decided to digitize its abstract journals was to ensure that users could check the older literature as so many searchers were reluctant to look through the printed archive. The need to search the older material was clearly demonstrated in the case, in 2000, of the a patient in a drug trial at the John Hopkins Hospital who died after being given hexamethonium. The researchers were criticised for not checking the older literature on the drug which would have shown it not to be safe.
The melamine abstract is:
Clark, R. 1966. Melamine crystalluria in sheep. Journal South African Veterinary Medical Assoc., 1966, Vol. 37, pp. 349-351
The toxic effects of melamine given directly or in the feed to merino wethers were studied. A single dose of 100 g increased urea in blood from 28 to 315 mg per 100 ml for a period of 11 days. There was complete loss of appetite and excretion of urine ceased on the tenth day. When the sheep was examined post mortem on the eleventh day the tubules of the kidney were packed with crystals. Nephrosis and erosive abomasitis were seen also. Daily doses of 50 and 25 g killed the sheep after 7 and 9 days, respectively. In those sheep the blood urea was high just before death and post mortem crystals in the kidney tubules, nephrosis, haemorrhagic cystitis and acute typhlitis were seen. The dose of 50 g also caused ulcers in the abomasum. With 10 g daily one sheep did not die but 2 did so after 16 and 31 days. The 2 sheep which died lost appetite and stopped urinating 3 days before death and urea and creatinine in blood then increased sharply. There were crystals in the kidneys and severe oedema of the lungs. In another experiment volume of urine was reduced by offering water freely for 1 h on alternate days. Melamine was mixed with maizemeal and offered to 3 sheep to supply 7 g per sheep per day. All sheep survived for 6 weeks and there was no excessively high value for urea in blood. When daily intake of water was restricted to 600 ml much of the maizemeal containing melamine was refused but the sheep showed no ill effect. The production of white crystals on cooling, the appearance of a white deposit on the addition of picric acid and absorbancy of acidified crystals at 235 m micro indicated the presence of melamine in the urine of treated sheep. Melamine had no effect on pH of contents or motility of rumen and there was no sign of damage to liver.