What is antimicrobial resistance (AMR)?

Antimicrobial resistance (AMR) is the ability of bacteria to survive a drug that once worked against them. When an infection becomes resistant, treatment fails: the disease drags on, welfare suffers, and the remaining options narrow.

One distinction matters throughout: it is the bacterium that becomes resistant, not the animal or the person. That is also why the subject spans animal and human health together.

How resistance arises

A bacterial population is never uniform. Small genetic changes occur at every division, and some of them — purely by chance — leave a bacterium better able to withstand a particular drug.

When an antibiotic is given, the susceptible bacteria are removed and the resistant ones remain, now without competition for space and nutrients. They multiply into the gap. This is selection: the antibiotic does not create resistance, it lets existing resistance come to the front.

The main strategies bacteria use are:

• Destroy the drug — as beta-lactamase enzymes do by opening the penicillin ring.
• Change the target — altering the shape of the binding site on a ribosome or enzyme so the drug can no longer attach.
• Pump the drug out — efflux systems that expel the molecule after it enters.
• Keep the drug out — narrowing or closing the channels in the outer membrane.

Why resistance spreads quickly

Resistance does not only pass from a bacterium to its descendants. Bacteria can hand resistance genes to each other — and to other species — on small rings of DNA called plasmids. This is horizontal gene transfer.

The practical consequence is that a resistance gene arising in a harmless gut bacterium can later end up in a disease-causing one. Resistance selected in a flock is therefore not confined to the disease that was being treated.

Plasmids often carry several resistance genes at once. That means using one drug can select for resistance to an entirely different class travelling on the same plasmid — known as co-selection.

The principles of prudent use

Resistance cannot be eliminated; the goal is to slow it down and keep the drugs we have working. The principles agreed by the international bodies (WOAH, WHO, FAO) are:

Prevent disease first. An antibiotic that is never needed selects for nothing. Biosecurity, vaccination, housing and stress management are the most effective resistance measures available.

Treat on a diagnosis. Identify the organism where you can and use susceptibility testing. That both directs the right drug and avoids unnecessary use.

Prefer a narrow spectrum. When the organism is known, a narrow-spectrum drug aimed at it causes less collateral selection than a broad-spectrum alternative.

Protect the critically important classes. Groups that are last-line in human medicine — fluoroquinolones and colistin among them — should be used only where genuinely necessary and where regulation permits.

Complete the course as prescribed. Under-dosing and stopping early create ideal conditions for resistant bacteria to survive. Dose and duration are the veterinarian’s decision.

Respect the withdrawal period. This is the time needed for residues to fall below the safe limit. For how it is determined, see The ABC of pharmacokinetics.

Keep records. What was used, when and why is the only way to see trends in a flock and notice unnecessary repetition.

Where we stand as a manufacturer

Medicavet manufactures veterinary medicines, antibiotics among them. That makes advocating prudent use a responsibility rather than a contradiction: in a world where resistance is widespread, the medicines we make stop working too. Long-term value in the field comes from a drug being effective when it is needed, not from it being used often.

If you are reviewing treatment protocols in your operation, do it with the veterinarian who knows the flock and within your country’s regulations.