Part I: The basics of animal agriculture + antimicrobial resistance

Before antimicrobials, strep throat could be fatal. Nearly every child who had bacterial meningitis died. A small cut, once infected, could kill a person. In my mind, antimicrobials are neck-and-neck with vaccines and basic sanitation as the most important health and medical discoveries. And yet we are quickly losing our grasp on treating infections. Multi-drug resistant tuberculosis is on the rise, as is drug resistant gonorrhea, and MRSA strikes fear into anyone working or staying in a hospital.

Animal agriculture may have something to do with this. Antimicrobials are used extensively in the livestock and poultry industries. This piece is Part I of my exploration of the links between animal agriculture and the looming public health crisis of antimicrobial resistance.

The basics of animal agriculture and antimicrobial resistance

What is the difference between antibiotics and antimicrobials? Strictly speaking, an antibiotic is a substance produced by a microorganism that is used to kill or inhibit the growth of other microorganisms. Penicillin, grown from fungus, is an antibiotic.

An antimicrobial can be naturally-occurring, semi-synthetic, or entirely synthetic compound that it used to kill or inhibit the growth of other microorganisms. Antimicrobials include sulfonamides and amoxicillin. Antimicrobials can be used against bacteria, viruses, fungi, and protozoa such as malaria and toxoplasma gondii.

When discussing animal agriculture, the term antibiotic resistance is often used. However, because it doesn’t include synthetic or semi-synthetic antimicrobials, I’m going to follow the lead of the United Nations, the World Health Organization, and the World Organization for Animal Health and use antimicrobial resistance.

How are antimicrobials used in animal agriculture? Antimicrobials are primarily used as growth promoters and are given to livestock and poultry at sub-therapeutic levels, meaning that the levels at which the antimicrobials are administered are below the threshold that would fight off infection. Using antimicrobials as growth promoters is a direct result of the ever-increasing demand for meat and animal products.

Antimicrobials increase animal growth rate by 2-10% and feed conversion efficiency 3-9%. It’s unclear how or why this happens, but some researchers suggest that cytokines released when the immune system fights off infection may stimulate growth-inhibition hormones. Others suggest that antimicrobials keep animals’ gut bacteria in check, allowing the energy that would have been used to stave off infection to instead be used for growth.

Because nearly all animals raised for food are kept in cramped, stressful conditions, antimicrobials are also used for disease prevention and control (metaphylaxis). The animals live in such a way that makes infectious disease likely—packed in very closely, standing their own excrement—and rather than make changes to their living conditions, the various industries choose to feed the animals preventative antimicrobials.

Antimicrobials are also used when animals get sick, or after an injury or surgery. However, these uses make up just a small portion of the antimicrobials used.

Does animal agriculture really use 80% of the world’s antimicrobials? This statistic is often cited. However, there doesn’t seem to be much evidence to support it. However, this figure includes ionophores, which are not used in human medications but are used as growth promoters.

Which antimicrobials are used in animal agriculture? This table shows a selection of the antimicrobials identified as both critical to human medicine and regularly used in animal agriculture in the Congressional Research Service brief “Antibiotic use in agriculture: Background and legislation” by Geoffrey S. Becker. I added the columns “Common drugs in this class” and “Human infections treated by this class (selected).”

Antimicrobial class Common drugs in this class Human infections treated by this class (selected) Use in animal agriculture Level of importance for human medicine as defined by the FDA, based on level of difficulty of transmitting resistance across genera and species
Cephalosporin (3rd generation) Cedax, Fortaz, ceftriaxone Gonorrhea; urinary tract; respiratory; pelvic inflammatory disease; pneumonia Disease treatment in cattle and swine Critical
Fluoroquinolone Cipro, Floxin, Avelox Anthrax; hospital-acquired infections, especially pneumonia; urinary tract Disease treatment in cattle Critical
Penicillin penicillin, amoxicillin, flucoxacillin Meningitis; syphilis; Lyme disease; strep throat Disease treatment in cattle; growth and disease treatment in swine High
Macrolide Zithromax, erythromycin Legionnaire’s Disease; chlamydia Disease treatment and prevention in cattle; growth, disease treatment and prevention in swine Critical
Tetracycline doxycycline, tetracycline, Chlamydia; acne and rosacea; typus; plague Disease treatment and prevention in cattle; growth, disease treatment and prevention in swine High
Lincosamide clindamycin, lincomycin Toxic Shock Syndrome Disease treatment in swine High
Streptogramin pristinamycin, quinupristin Vancomycin-resistant Staphylococcus aureus (VRSA) and enterococcus (VRE) Growth, disease prevention in chickens High

How does antibiotic resistance happen? The National Institute of Allergy and Infectious Disease (NIAID) cites seven ways that microbes can become drug resistant:

Biological causes

  • Selective pressure: only the microbes with genes that make them resistant to antimicrobials are able to survive
  • Mutations: random changes in the genetic code protect some microbes from antimicrobials
  • Gene transfer: microbes can get genes from other, drug-resistant microbes

Human causes

  • Inappropriate use: prescribing antimicrobials for a disease that cannot be cured by them—for example, prescribing an antibiotic for a cold
  • Inadequate diagnostics: using a broad-spectrum antimicrobial when a specific one may be more effective, or being unsure of the underlying cause of illness and prescribing a drug “just in case”
  • Hospital use: hospital patients are susceptible to infections, but giving them high doses of antimicrobials puts them at risk for resistant infections
  • Agricultural use: NIAID states that agricultural use of antimicrobials is still debatable as a public health issue.

Now that we’ve covered the basics, check out Part II!