Growing Resistance

November 28, 2011 10:30 AM
Growing Resistance

As human resistance to antibiotics grows, you pay the price

The trend lines are clear: Across the globe, there are disease-causing bacteria in humans that are becoming increasingly resistant to antibiotics.

To be sure, overuse and inappropriate use by doctors and their patients is largely to blame. But a finger is also increasingly being pointed at agriculture—particularly the industry’s nontherapeutic use of antibiotics for growth promotion and feed efficiency.

While there is little such use in milk production, especially in lactating cows, the dairy industry is not immune from pressure. Routine use of antibiotics in replacement dairy animals is one target of new drug regulations.

The fear of resistance is both real and growing, said microbiologists speaking at the Antibiotic Use in Food Animals conference in Chicago in October. The conference was sponsored by the National Institute for Animal Agriculture to promote dialogue between groups that support and oppose antimicrobial use in agriculture.


Bonus Content

More on antibiotic resistance:

In the U.S., methicillin-resistant Staph. aureus (the dreaded MRSA) has grown from 27% of samples in 1997 to 51% in 2009. Over the past decade, resistance to penicillin in Europe has jumped from 10% of samples to 15%. In the Asia-Pacific region, it has almost doubled, from 18% to 32%, reports Robert Flamm, a microbiologist at JMI Laboratories in North Liberty, Iowa.

Antimicrobial resistance is complex, and even microbiologists admit they don’t fully understand all the underlying mechanisms. In its simplest form, however, resistance is merely selection. Antimicrobials kill susceptible bacteria, allowing resistance bacteria, though small in number, to survive and multiply.

More complex resistance occurs when resistant bacteria share their DNA with other microbes, which can be totally unrelated.

Livestock is in the crosshairs because of the amount of antimicrobials used on farms. On a tonnage basis, livestock producers use the majority of antibiotics.

In the U.S., for example, 28.8 million pounds of antibiotics were sold for animal use in 2009, according to a survey by the Food and Drug Administration (FDA). Roughly 11 million pounds of that amount, or 38%, were ionophores, which are not considered a threat in human drug resistance.

Even so, the remaining 17 million to 18 million pounds represent 60% to 70% of all drugs sold that are a concern on the human health side. That’s a huge number.

As a consequence, FDA is moving forward with guidance documents that will eventually eliminate the use of "medically important" antimicrobials for growth promotion and feed efficiency. Ionophores will not be included in this group, says William Flynn, deputy director for science policy at FDA’s Center for Veterinary Medicine in Rockville, Md.

But the new guidelines will mean much more judicious use of antimicrobials that are administered through feed or water. The drugs will be used only for the treatment, prevention or control of disease, and a licensed veterinarian will have to be involved.

"FDA acknowledges that this could result in an increase in animal disease, and we therefore need to be able to address this concern," Flynn says.

The move will likely mean relabeling and the moving of antimicrobials that are now sold over-the-counter to prescription-only or Veterinary Feed Directives.

"This would not occur overnight, but over a number of years and likely in a phased-in approach," Flynn says.

Presumably, medicated milk replacers will still be allowed. But in order to use them, veterinarians will have to prescribe their use to treat, prevent or control a specific disease.

Antimicrobials that are used only to promote growth or feed efficiency in heifer-raising programs will also be subject to the new initiative. A veterinarian will have to be write a prescription, and their use will be allowed for only a short duration for a prescribed disease threat.

Dual Risks

Antibiotics in livestock is a case of damned if you do, damned if you don’t.

If you use antibiotics, there is a risk of antibiotic resistance. And if you don’t use them, you have the risk of sick animals, which can lead to human health concerns.

growing resistance cows
Subtherapeutic use of antibiotics to promote growth or feed efficiency is being phased out by the U.S. Food and Drug Administration.

But neither risk is automatically a danger to human health. In order for that to occur, you must have both a hazard and exposure to that hazard, explains Scott Hurd, an associate professor at Iowa State University’s College of Veterinary Medicine and former deputy undersecretary for food safety with USDA’s Food Safety and Inspection Service.

To be a danger, in other words, resistant bacteria must be present in a food as a result of drug use. And people must then ingest the resistant bacteria in the food. And the resistant bacteria must be present in sufficient numbers to cause disease that is untreatable by antibiotics.

"If you remove any one of these conditions, the risk to human health goes to zero," Hurd says. "Most risk assessments of anti-biotic use in livestock show the risk to be very low."

The risk of not using antibiotics is that untreated animals can become sick, and sick animals can cause human health problems. "If we decrease animal health, there will be human health concerns," Hurd says.

Look to Denmark for evidence. In 1999, that country banned subtherapeutic uses for growth promotion and feed efficiency. In the year prior to the ban, Danish farmers used 108,000 lb. of antibiotics subtherapeutically, and another 125,000 lb. on a therapeutic basis.

In 2009, Danish farmers reported using 282,000 lb. of antibiotics to treat and control disease, which is more than the total for subtherapeutic and therapeutic uses combined in 1998. Therapeutic use more than doubled.

Denmark has also not seen a dramatic decrease in antimicrobial resistance in humans since the feed ban was put in place, says Tom Shryock, senior research adviser of microbiology for Elanco Animal Health.

In 2009, for example, Denmark reported 61 cases of Campylobacter resistance per 100,000 humans and 39 cases of Salmonella resistance per 100,000 humans. In the U.S., where no feeding bans currently exist, there were 13 cases of Campylobacter resistance per 100,000 humans and 15 cases of Salmonella resistance per 100,000 humans.

Shryock says no direct cause/effect relationship can be drawn from these numbers. "But it’s also true that Denmark has not seen much difference in human resistance rates since the country changed antibiotic use on its farms," he says.

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