But is there truth in advertising?

Even producers who have eliminated antibiotics may be using other bacteria-killing compounds in the anti-microbial family, which have a murky definition under the law. Although not all anti-microbials are defined as antibiotics by the Food and Drug Administration, their use opens up a minefield of issues.

Among them:

-- Anti-microbials are not listed on labels.

-- They work the same way as antibiotics.

-- Many people consider them to be harmful to the environment.

"The only way to know if a producer is using anti-microbials is to call the manufacturer and ask them," said Urvashi Rangan, a scientist for the nonprofit Consumers Union.

"A manufacturer doesn't have to list it on the label even if they're making an antibiotic-free claim. And as far as we're concerned, if you're using a drug to kill a parasite or a micro-organism and you're making a 'no antibiotics' claim, you're being misleading."

Some antibiotics when overused can breed resistant bacteria in livestock, in turn threatening the future success of these drugs in humans. But many farmers and ranchers argue that other anti-microbials, specifically anti-parasite medications, are harmless to people.

While scientists debate the pros and cons of using any bacteria-killing drug in food animals, most agree that the only difference between antibiotics and anti-microbials is purely one of semantics.

According to the FDA, an anti-microbial is an umbrella for microbe-killing products that include antibiotics. Antibiotics are produced from living micro-organicisms such as fungi, while anti-microbials also include synthetic products.

Calls for transparency

"Consumers shouldn't need a degree in zoology to read labels," said Charles Hansen, the executive director of the Truthful Labeling Coalition, an organization representing the natural chicken industry. The group spearheaded a successful lawsuit against Tyson Foods for advertising that its poultry was raised without antibiotics, despite the fact that the company was injecting its chickens with ionophores before they hatched. Ionophores are a type of anti-microbial that combats intestinal infections and promotes growth, but is not known to impact antibiotic resistance in humans. In the end, Tyson was ordered to stop its advertising claims.

"If you're going to put something on a label, it should require a high degree of transparency," Hansen added.

Otherwise, said Marion Nestle, professor of food studies and public health at New York University and author of "What to Eat," it creates the "dread and outrage factor." Although there might not be enough evidence to say whether these drugs could be harmful to humans, consumers have a right to know what's going into their food, she said.

Consumers Union's Rangan is especially concerned about the effect anti-microbials have on the environment.

"These things are excreted from the animals, get into the ground and the water and become pollutants," she said.

Supporting anti-microbials

Michael Apley, a clinical pharmacologist, veterinarian and associate professor at Kansas State University, believes that using any kind of anti-microbial and then calling the product antibiotic-free is "disingenuous," although he is in favor of using certain anti-microbials in food animals.

"The ones they are using do so much good," he said. "And there is no data to show a negative effect on humans. They really are a sound practice for preventing stress on animals."

Michael Payne, also a veterinarian and pharmacologist who teaches at UC Davis, agrees.

"The judicious and responsible use of anti-microbials as preventive medicine may not only guard against livestock illness, it may actually prevent the necessity of treating animals with larger amounts of more powerful antibiotics," he said.

"Our experience in veterinary medicine is that the use of measured, extremely small amounts of these compounds actually lowers mortality rates and reduces pain and suffering of livestock and poultry."

Good animal husbandry

It's just good animal husbandry, argues Brian Kenny of Hearst Ranch (owned by the same company that owns The Chronicle), and John Tarpoff of Niman Ranch. The two California producers advertise their beef as antibiotic-free, but use anti-parasite drugs. Hearst uses Dectamax and Niman Deccox and Corid to rid their cattle of tiny parasites they pick up on the range.

Although Kenny and Tarpoff said they occasionally treat sick animals with antibiotics because "it's the humane thing to do," they won't sell those cattle under their label. They do, however, sell the livestock that has been treated with other anti-microbials.

Tarpoff said the medications are not absorbed into the steer's system and that the ranch doesn't slaughter its animals until two to four months after treatment.

Kenny says that to not use them is untenable on his scale of ranching - between 2,500 and 3,000 head of cattle on two Central Coast ranches 10 times the size of Manhattan.

"We operate under the purview of the FDA and the USDA, and both those agencies say these are not antibiotics," Kenny said. "If consumers want meat that hasn't been treated with anything, they're going to go hungry.

"If you're raising five head of cattle, you can do it," Kenny continued. "If you want to eat something that's never been treated for illness, that's what hunting is for."

But organic meat and poultry companies such as Humboldt County's Eel River Organic Beef and Petaluma Poultry are forbidden by the USDA's certified organic program from using anti-parasitic drugs as well as antibiotics. The National Organic Program allows only the anti-parasite Ivermectin to be used in the emergency treatment of dairy and breeder stock when organic remedies fail to prevent infestation, according to the USDA.

However, even some nonorganic certified producers, such as Coleman Natural, based in Colorado, and Soul Food Farm in Vacaville, don't use anti-microbials, according to company officials.

But if they needed to use them to treat a sick animal, Rangan of Consumers Union said she wouldn't have a problem with that.

"Just tell us about it."

E-mail Stacy Finz at sfinz@sfchronicle.com.

This article appeared on page A - 1 of the San Francisco Chronicle