Critical Management Point: Antibiotic Use

Critical Management Point: Antibiotic Use

Transcript (with additional commentary)

According to the USDA’s National Agricultural Statistics Survey, we raise 92 million cattle in the US, providing about 208,000 pounds of milk and about 24 billion pounds of beef per year.

Some of the most common illnesses that affect dairy cattle and calves are mastitis or inflammation of the udder, metritis, inflammation of the uterus, and pneumonia.

Beef cattle are raised in cow-calf operations, stocker or backgrounder facilities, and in feedlots. There are specific times during their life when they are most at risk of contracting bacterial infections – at weaning, shipping, and when co-mingled with animals from other herds.  For example, bovine respiratory disease complex is a major threat to their health and welfare, and a variety of viruses and bacterial pathogens contribute to disease.

Antibiotics are often prescribed by veterinarians to treat, control, or prevent these diseases, although some antibiotics are also sold over the counter.  Some antibiotics are also added to feed or used routinely to promote weight gain.

[Amy Pruden, Department of Civil and Environmental Engineering, Virginia Tech]: “The first critical control point is the use of antibiotics in livestock. The question is, if you use antibiotics, like we commonly do in livestock, is that selecting for resistant bacteria? We’re looking at dairy and beef cattle, and common antibiotics that are used.”

“…We’re looking to see if those antibiotics actually select for resistant bacteria, and then we’ll also be using DNA extraction and metagenomics to see if it affects the kinds of antibiotic resistance genes that are in the manure.”

Among commonly used antibiotics in the dairy and beef industries, macrolides–one class of antibiotics–are categorized by the World Health Organization as “critically important” in human medicine while cephalosporins, sulfonamides, and tetracyclines–three other classes–are “highly important.”

In this project, representatives from these antibiotic classes were dosed to dairy and beef cattle in accordance with common practice and compared to non-dosed controls.  Manure and urine samples were then collected regularly.  The manure samples were then analyzed for antibiotics, antibiotic break-down products, antibiotic resistant bacteria, and antibiotic resistance genes.
This study will provide guidance on manure management strategies for minimizing the spread of antibiotic resistance, and whether or not antibiotic use has a measurable effect on persistence of antibiotic resistance genes in the manure.

Glossary of Terms

  • Antibiotic metabolites – Break-down products that are formed as antibiotic parent molecules are degraded. Metabolites are generally not fully functional compounds, but are results of metabolism and other degradation processes.
  • Antibiotic residues – Antibiotic that remains in secondary material, either milk, meat, urine, or feces, after the antibiotic has been administered.
  • Antibiotic resistance – Antibiotic resistance is the capability of some bacteria to survive antibiotic treatments. Some bacteria are intrinsically resistant, for example an antibiotic that specifically targets Gram positive bacteria will not affect Gram negative bacteria. However, bacteria can also acquire resistance, either through mutation or through sharing of antibiotic resistance genes, in which much higher doses of the antibiotic are needed to have the desired effect.
  • Antibiotic resistance genes – Pieces of DNA carried by bacteria that encode functions that allow bacteria to survive and grow in the presence of antibiotics.
  • Antibiotic resistant bacteria – Bacteria that are not controlled or killed by an antibiotic with a spectrum that includes that type of bacteria.  An antibiotic spectrum is the range of bacteria susceptible to that drug. (CDC, Antibiotic Resistance, )
  • Metagenomic analysis – A new laboratory research technique aimed at sequencing all of the DNA present in a sample. Metagenomics can be used to compare kinds of antibiotic resistance genes present in a sample, as well as the types of bacteria and other functional capabilities. (National Research Council (US) Committee on Metagenomics: Challenges and Functional Applications. The New Science of Metagenomics: Revealing the Secrets of Our Microbial Planet. Washington (DC): National Academies Press (US); 2007. 1, Why Metagenomics? Available from: )


  • Collignon, P., Powers, J.H., Chiller, T.M., Aidara-Kane, A., Aarestrup, F.M. 2009. World Health Organization ranking of antimicrobials according to their importance in human medicine: a critical step for developing risk management strategies for the use of antimicrobials in food production animals. Clin. Infect. Dis. 49, 132-141.