Aquaculture AMR: test before treating

Antimicrobial resistance in aquaculture cannot be reduced to a resistance gene found in water or a small inhibition zone on an agar plate. Before a group of fish is treated, evidence must show that a bacterium contributes to the disease and that the tested isolate represents the episode. Susceptibility testing can then support a clinical decision. Without diagnosis, quality control and appropriate interpretive criteria, it may instead create false confidence.

Start with the affected population

Increased mortality, skin lesions and reduced feeding do not prove bacterial disease. Hypoxia, temperature excursions, toxic exposure, parasites and recent handling may produce similar signs. The investigation should describe timing, distribution of affected fish, water variables, movements, previous medication and the dominant lesions.

Recently affected, preferably untreated fish provide the best specimens. A superficial swab from an old lesion may mainly recover environmental flora, whereas carefully chosen internal tissues can be more informative. The laboratory needs the host species, rearing temperature, sampled organs, transport time and treatment history. These details are part of the test, not optional paperwork.

Bacterial isolation alone does not establish causation. Recovery of a pure or dominant organism from several fish with compatible pathology is stronger evidence. Histology and molecular identification may add support. PCR can confirm identity or detect selected determinants, but it does not replace the viable isolate required for phenotypic susceptibility testing.

Know what the test measures

Broth microdilution estimates a minimum inhibitory concentration, while disk diffusion measures a zone diameter. Neither raw measurement is automatically “susceptible” or “resistant.” Categories require criteria valid for the bacterial species, antimicrobial, method and, in some settings, the treated host.

Aquatic bacteria create particular constraints. Some grow slowly or require different salinity, media and incubation temperature. Clinical breakpoints remain unavailable for many organism–drug combinations. A human breakpoint established at 35 °C should not be transferred automatically to an Aeromonas, Vibrio or Flavobacterium tested under different conditions. When no clinical breakpoint exists, the laboratory can report the measured value and relevant epidemiological distribution while stating what cannot be inferred.

Inoculum, medium, incubation, quality-control strains and reading rules must remain standardised. One result from an undocumented method should not drive a prescription. Repeated profiles among multiple isolates from the same event can, however, support epidemiological investigation and longer-term resistance surveillance.

Connect the result to veterinary prescribing

Treatment choice remains a veterinary judgement. It combines in-vitro activity with infection site, pharmacokinetics at the species and water temperature concerned, product authorisation, withdrawal periods, administration feasibility and environmental safety. An agent that inhibits bacteria in the laboratory may fail to reach the target tissue, especially when diseased fish no longer eat reliably.

WOAH principles call for the most accurate feasible diagnosis, targeted selection, suitable dose and duration, effectiveness monitoring and retained records. Its 2025 aquatic technical reference document lists antimicrobials of veterinary importance for food-producing fish and crustaceans, but explicitly is not a treatment guide. Inclusion on a list is therefore not permission or evidence for a particular prescription.

Monitoring begins before the first dose. Mortality, feed intake, water conditions and medicated-feed lots should be recorded. Failure to improve prompts a new assessment: wrong diagnosis, inadequate delivery, resistance, co-infection or a continuing environmental driver. Extending a course or switching agents without new evidence may increase selection pressure while leaving the cause unchanged.

Sampling after treatment also needs a defined purpose. A persistent isolate may indicate therapeutic failure, but recovery can be biased when antimicrobial exposure suppresses susceptible organisms. Laboratories should be told exactly when the last dose was given. Farm records should connect the isolate and susceptibility report to the treated population, prescription, clinical response and eventual outcome. Over time, this local dataset can reveal recurring pathogens or shifts in measured susceptibility, provided methods remain comparable. It should not be converted into a farm-specific clinical breakpoint without appropriate validation.

Prevention is the strongest stewardship tool

Biosecurity reduces both disease and the need for antibiotics: known health status of incoming stock, cohort separation, dedicated equipment, cleaning before disinfection and early mortality investigation. Vaccination, where a suitable product and protocol exist, and control of oxygen, stocking conditions and nutrition further reduce treatment opportunities.

EFSA’s 2024 technical specification proposed a harmonised EU baseline survey covering Aeromonas from salmonids and European seabass, alongside selected bacteria from mussels. The proposal illustrates why defined populations, sampling frames and laboratory methods matter. It does not mean that every resistant organism recovered from water is an immediate clinical hazard. Host, bacterial identity, phenotype, determinant and exposure pathway must remain distinct.

Environmental management belongs in the same plan. Uneaten medicated feed, faecal residues and poorly controlled discharge can expose microbial communities beyond the target fish. Accurate biomass estimates, supervised feeding and prompt removal of waste reduce both dosing error and avoidable release. These controls complement diagnosis; they cannot make an unnecessary treatment responsible.

Conclusion

Susceptibility testing extends a diagnosis; it does not replace one. Its value rests on representative fish, a relevant isolate, controlled methods and honest interpretation of the available criteria. Combined with monitored prescribing and prevention, it helps protect both the current group and future antimicrobial effectiveness.

Vetofish can support population examination, sampling design, bacteriology coordination and practical stewardship plans tailored to aquatic production systems.