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June 8, 2011 -- An FDA study has found "very low levels" of a cancer-causing form of arsenic in the livers of chickens given the widely used feed additive 3-Nitro or Roxarsone.
Pfizer said today it would voluntarily suspend U.S. sales of the drug in 30 days. The FDA says sales will not resume until all its concerns have been addressed.
Giving arsenic to food animals doesn't seem like a great idea, but it's been going on since the 1940s. Although ostensibly used to fight a parasitic infection, the drug makes chickens grow faster and gives their meat a more attractive color. Roxarsone has FDA approval for all these uses.
While arsenic itself is a poison, the organic form found in treated chickens was generally considered safe at low levels. But recent studies show that organic arsenic can easily give rise to inorganic arsenic, which is known to cause cancer. With certain exceptions, U.S. law forbids carcinogenic substances in food.
It's not yet clear whether chicken meat contains substantial levels of inorganic arsenic, as the FDA tests for the substance work only for liver tissues. Bernadette Dunham, DVM, director of the FDA's center for veterinary medicine, estimates that chicken flesh contains about 40 times less arsenic than chicken liver.
"Levels in chicken livers are very low and represent a very low health risk to people who eat chicken," Dunham said at a news teleconference.
The FDA study was completed last February and made public only today.
3-Nitro-treated birds were found to have more than 800 times more total arsenic in their livers, and 14 times more total arsenic in their meat, than untreated birds. Even after the five-day washout period required before arsenic-treated chickens can be butchered, 3-Nitro-treated birds had 300 times more total arsenic in their livers and and ninefold more in their meat.
On average, total arsenic levels in the livers of Roxarsone-treated birds were below the FDA's approved tolerance level of 2,000 parts per billion. But some of the treated birds had total arsenic levels of 2,900 parts per billion, well above the FDA tolerance level.
Only a tiny portion of this arsenic appears to be the cancer-causing inorganic form. But the FDA study was not able to detect all of the most harmful forms of arsenic. The study raises -- but does not answer -- the question of whether the chickens' digestive processes form toxic compounds "potentially more toxic than the parent compound."
In a statement, Pfizer notes that the FDA finds no imminent public health risk from eating chicken treated with 3-Nitro.
Pfizer also notes that the "extremely low level" of inorganic arsenic the FDA detected in the liver of a 3-Nitro-treated chicken "is equivalent to the amount of inorganic arsenic found in an eight-ounce glass of drinking water."