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Resistance increases need for management

by Wyoming Livestock Roundup

“Antimicrobial resistance is an issue for everyone, both on the human and the animal health side,” said Bob Smith, feedlot veterinarian and vice-chair of the Beef Quality Assurance Advisory Board.

On Aug. 10, Smith spoke on antimicrobial resistance and its impact on the beef industry during a webinar hosted by the National Institute for Animal Agriculture.

“Antimicrobial resistance is present in every country, and it is one of the more important and complex topics facing animal and public health today,” he continued.

Increasing resistance

The bottom line for cattle producers when talking about antibiotic resistance is that antibiotics no longer work as resistance increases, said Smith.

“The more exposure that we have to an antibiotic, the greater the chance of antimicrobial resistance,” he commented, noting that fact is one of the reasons the Food and Drug Administration required the implementation of the Veterinary Feed Directive.

Smith continued, “Therefore, we’re no longer allowed to use those products contributing to antimicrobial resistance in the feed for growth promotion, for feed efficiency or to improve gain.”

In a study performed by Brian Lubbers, a microbiologist at Kansas State University, Smith noted the team found that five percent of organisms tested for antibiotic susceptibility were resistant to five antibiotics in 2009.

“Two years later, 35 percent of the organisms were resistant to antibiotics. We saw a big increase in resistance over that two-year period,” stressed Smith. “We know that incidence or prevalence of antibiotic resistance has increased in our respiratory bacteria since that time.”

According to Smith, evidence also suggests that livestock managers can select for antibiotic resistance when treating animals.

“If we use an antibiotic whether, the animal is sick or not, it’s possible that we can select for resistant organisms by wiping out those that are susceptible to that antibiotic,” he stated.


“An antibiotic can work in several ways, and how they work depends on which antibiotic group they are in. Antibiotics have some rather specific ways that they can inhibit the growth or reproduction of bacteria,” said Smith.

The four primary ways antibiotics can work is to target cell wall synthesis, DNA synthesis, protein synthesis or folic acid synthesis.

However, Smith explained that bacteria are “survivors.”

“Bacteria learn how to escape some of these things that are going to cause harm to them,” he continued. “Therefore, they can become resistant by a number of mechanisms.”

The first mechanism bacteria can utilize is to pump the antibiotic out of the cell back into the external environment.

“Another mechanism is bacteria can change their own cell wall permeability, so the antibiotic cannot get into the inside of the cell in the first place,” he commented. “They can also produce degrading enzymes that inactivate the antibiotic.”

Bacteria can also produce alternative proteins that compete with binding sites, so the antibiotic can’t attach to the cell, or bacteria may modify themselves in a manner that leads to decreased affinity of the antibiotic to the particular organism.

Smith explained bacteria can pass acquired resistance to other bacteria through a process called plasma mediation.

“That means that the bacterium that contains the resistance gene can share that resistance gene with another bacterium, and it doesn’t necessarily have to be the same species,” he stated.


When discussing how to sustain usefulness of antibiotics, Smith said, “It goes without saying that we can minimize use, but at the same time, we need to be treating animals that are sick, so we use antibiotics for prevention, control and treatment.”

Smith stressed the importance of using the most effective drug for a disease to achieve a treatment level in the target organ and using the correct dosage.

“A big part of dosage is knowing what the weight of our animal is,” continued Smith, noting that oftentimes weight is estimated in treating cattle.

In a study done where calves were treated based on the average weight of the group, Smith explained it was found that the heavier calves had a higher morbidity rate than lighter-weight calves.

“That’s because they were actually under-dosing the heavy calves and giving a good therapeutic dose to those lighter calves,” he said.

While reducing dosages from the labeled dosage to reduce cost may seem to be effective short-term, Smith cautioned it could have long-term consequences.

“If we do reduce our doses over a sustained period of time, we’ll end up with a resistance problem in that bacterial population that we have on the farm, ranch or feedlot,” he said.

Looking ahead

“Historically, the antibiotics we’ve used have bailed us out of some problems,” said Smith.

However, with no new antibiotics on the horizon and increasing resistance, Smith stressed that a lot of disease reduction will fall to husbandry practices rather than relying on new pharmaceutical interventions.

“That’s where we have to get to our old-fashioned animal husbandry practices,” he continued, citing nutrition, early castration and low stress weaning as examples.

“Another thing we found helpful was increasing space per calf,” commented Smith.

He noted producers should think about management systems that avoid treating animals “just in case.”

“In our practice, many times we don’t have to mass treat from the first of the year to late September,” concluded Smith. “We need to keep our eyes open to new technologies and new ways to do things.”

Emilee Gibb is editor of Wyoming Livestock Roundup and can be reached at

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