Eighty percent of the antibiotics sold in the United States goes to chickens, pigs, cows and other animals that people eat, yet producers of meat and poultry are not required to report how they use the drugs - which ones, on what types of animal, and in what quantities. This dearth of information makes it difficult to document the precise relationship between routine antibiotic use in animals and antibiotic-resistant infections in people, scientists say. Advocates contend that there is already overwhelming epidemiological evidence linking the two, something that even the Food and Drug Administration has acknowledged, and that further study, while useful for science, is not essential for decision making. At some point the available science can be used in making policy decisions," said Gail Hansen, an epidemiologist who works for Pew Charitable Trusts, which advocates against overuse of antibiotics. But scientists say the blank spots in data collection are a serious handicap in taking on powerful producers of poultry and meat who claim the link does not exist. "It's like facing off against a major public health crisis with one hand tied behind our backs," said Keeve Nachman, an environmental health scientist at the Johns Hopkins Center for a Livable Future, which does research on food systems. "The single biggest problem we face in infectious disease today is the rapid growth of resistance to antibiotics," said Glenn Morris, director of the Emerging Pathogens Institute at the University of Florida. "Human use contributes to that, but use in animals clearly has a part too."