Antimicrobial resistance in livestock and One Health
How antimicrobial resistance arises and spreads between animals, people and the environment, why livestock use matters, and what responsible use means.
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Antibiotics transformed both human and veterinary medicine. Their overuse is now undoing that: bacteria are evolving resistance faster than new drugs are found. A major global study estimated that in 2019, infections resistant to antibiotics directly caused about 1.27 million human deaths, and were associated with nearly 5 million.
Animals are part of the problem and part of the solution. Large quantities of antimicrobials are used in livestock, poultry and fish, and resistant bacteria and genes move between animals, people and the environment. This is why antimicrobial resistance (AMR) is the textbook example of a One Health problem, and a growing topic in veterinary, animal science and public health examinations.
How resistance arises
An antimicrobial is any substance that kills or inhibits microorganisms; antibiotics are those active against bacteria. Resistance is a matter of evolution:
- In any large population of bacteria, a few carry genes that let them survive a drug, by mutation or by acquiring genes from other bacteria.
- When the drug is used, susceptible bacteria die and resistant ones survive and multiply: selection.
- Every use, appropriate or not, applies some selection pressure; low doses, long courses and treating animals that are not sick apply it without benefit.
Bacteria resist drugs by a handful of mechanisms:
- destroying or modifying the drug (for example β-lactamase enzymes that break down penicillins and cephalosporins);
- changing the drug's target so it no longer binds;
- pumping the drug out (efflux pumps);
- blocking entry by changing the cell envelope.
Crucially, resistance genes often sit on plasmids and other mobile elements that can pass between bacteria of different species by conjugation, and less often by transformation or transduction. A gene that arose in a harmless gut bacterium of a pig can end up in a bacterium that infects people. The discovery in 2015 of a plasmid-borne gene for resistance to colistin, a last-resort antibiotic for people that was widely used in pigs and poultry, in bacteria from farm animals and people, made this link vivid and led many countries to restrict colistin in animals.
Why it is a One Health problem
One Health is the principle that the health of people, animals and the environment is interconnected and must be managed together. It is promoted by the Quadripartite organisations: FAO, UNEP, WHO and the World Organisation for Animal Health (WOAH).
Routes between sectors include:
- The food chain: resistant bacteria on meat, in milk or on produce irrigated with contaminated water.
- Direct contact: farmers, veterinarians and abattoir workers carry livestock-associated strains more often than the general public.
- The environment: manure and slurry spread on fields, farm and fish-pond runoff, and human sewage carry resistant bacteria, resistance genes and drug residues into soil and water.
Estimates suggest that, by weight, a large share of the world's antimicrobials, in some estimates more than is used in people, is used in animals, and use is projected to grow with livestock production, especially where intensive systems are expanding. Data are incomplete; WOAH collects national reports from most member countries.
How antimicrobials are used in animals
- Treatment of sick animals: essential for animal welfare and food production.
- Metaphylaxis: treating a whole group when some animals are sick and the rest are at risk.
- Prophylaxis: preventive treatment of healthy animals at high risk, for example around weaning or transport.
- Growth promotion: low doses in feed to improve growth and feed efficiency. The European Union banned antibiotic growth promoters in 2006, and a growing number of countries have followed; WHO recommends ending their use.
Not all antimicrobials carry equal risk. WHO publishes a list of medically important antimicrobials, ranking classes by their importance for human medicine; the highest-priority critically important classes include third- and later-generation cephalosporins, quinolones (fluoroquinolones), macrolides and polymyxins. WOAH keeps a parallel list of veterinary critically important antimicrobials. Many countries restrict the highest-priority human classes in animals to cases where nothing else works.
Measuring use
Comparing use between countries or farms requires adjusting for how many animals there are. A widely used indicator divides the mass of active substance by the population correction unit (PCU), an estimate of the weight of livestock at risk of treatment.
Comparing antimicrobial use in animals
Country A sells 600 t of veterinary antimicrobials a year, and its livestock population is estimated at 6 million t PCU. Country B sells 250 t for 5 million t PCU.
- Country A: 600 t ÷ 6 million t = 600,000,000 g ÷ 6,000,000 t = 100 mg per kg PCU.
- Country B: 250,000,000 g ÷ 5,000,000 t = 50 mg per kg PCU.
Country A uses twice as much per unit of livestock. Because drugs differ in dose, many monitoring systems also count defined daily doses or treatment courses, which describe how often animals are treated rather than how many tonnes are sold.
Responsible use on the farm
Prudent use rests on a simple order: prevent disease first, diagnose before treating, and treat correctly.
- Prevention: biosecurity (controlling who and what enters the farm), vaccination, good nutrition, clean water, appropriate stocking density and ventilation, hygienic milking and calving, all-in all-out management. Healthier animals need fewer drugs.
- Diagnosis: a veterinary examination and, where possible, laboratory culture and sensitivity testing, so the right drug is used.
- Correct treatment: the right drug, dose, route and duration, prescribed by a veterinarian; no use of highest-priority human drugs where alternatives exist; no routine prophylaxis or growth promotion.
- Records: what was used, on which animals, why and with what result.
- Withdrawal periods: the time after the last dose before milk, meat or eggs can be sold, so that residues fall below the legal maximum residue limits (set internationally by Codex Alimentarius and nationally by regulators).
Observing a withdrawal period
A dairy cow is treated for mastitis with a product whose label states a milk withdrawal period of 84 hours (3.5 days) after the last treatment. The last dose is given at 06:00 on day 3.
The milk may enter the supply only from 06:00 + 84 hours = 18:00 on day 6. All milk from that cow before then must be discarded, and many dairies also test bulk milk for residues. Withdrawal periods differ by product, species and country, so the label in force where the animal is treated is the reference.
The wider response
- The Global Action Plan on AMR (2015), endorsed by WHO, FAO and WOAH, asks every country for a national action plan covering surveillance, awareness, infection prevention, optimised use, and research.
- United Nations General Assembly political declarations on AMR (2016, and again in 2024) committed countries to targets, including reducing deaths from AMR and the quantities of antimicrobials used in agrifood systems.
- Integrated surveillance systems combine data on use and resistance from people, animals, food and the environment.
- Research on vaccines, rapid diagnostics, alternatives such as bacteriophages and probiotics, and new antimicrobials continues, but none removes the need for prudent use.
Key points
- AMR arises by mutation and gene transfer and spreads under the selection pressure of antimicrobial use.
- Mechanisms: drug inactivation, target change, efflux and reduced entry; plasmids move resistance genes between bacterial species.
- One Health: resistance moves between animals, people and the environment through food, contact, manure and wastewater.
- Uses in animals: treatment, metaphylaxis, prophylaxis and growth promotion (banned in the EU since 2006 and increasingly elsewhere).
- Highest-priority critically important classes for people include later-generation cephalosporins, fluoroquinolones, macrolides and polymyxins.
- Prudent use: prevent, diagnose, treat correctly under veterinary prescription, keep records, observe withdrawal periods.
What the exams ask
- Veterinary and animal science courses: mechanisms of resistance, horizontal gene transfer, classes of antimicrobials and their importance lists, withdrawal periods and residue limits.
- Veterinary licensing examinations: prudent use principles, antimicrobial stewardship on farms, mastitis and respiratory disease treatment decisions, and the legal duties of the prescriber.
- Public health, One Health and civil-service examinations: the Global Action Plan, integrated surveillance, the Quadripartite, and policy options for reducing antimicrobial use in food animals.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Is meat from treated animals safe to eat?
Yes, when withdrawal periods have been observed, residues are below legal limits. The wider concern with antimicrobial use in animals is not residues but the resistant bacteria and genes it selects for.
Can farms really reduce antibiotic use without harming animals?
Many have, through better biosecurity, vaccination, housing and management, with stable or improved productivity. Reductions work best when the causes of disease are fixed rather than when treatment is simply withheld.
Do vaccines help against AMR?
Yes. Preventing infections in the first place reduces the need for antibiotics, and vaccines against bacterial and viral diseases of livestock have cut antibiotic use in several production systems.
Sources
- Antimicrobial Resistance Collaborators. "Global burden of bacterial antimicrobial resistance in 2019: a systematic analysis." The Lancet 399, 629–655 (2022).
- WHO. Global Action Plan on Antimicrobial Resistance. World Health Organization, 2015.
- WHO. WHO List of Medically Important Antimicrobials. WHO, 2024.
- WOAH. Annual Report on Antimicrobial Agents Intended for Use in Animals. World Organisation for Animal Health, recent editions.
- Liu, Y.-Y. et al. "Emergence of plasmid-mediated colistin resistance mechanism MCR-1 in animals and human beings." The Lancet Infectious Diseases 16, 161–168 (2016).
- Codex Alimentarius. Code of Practice to Minimize and Contain Foodborne Antimicrobial Resistance (CXC 61-2005, revised 2021).
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