Ulcers and deaths after unquarantined yearlings are added
A 0.35 ha grow-out pond was stocked with 4,200 advanced fingerlings of catla, rohu and mrigal. Two months later, 12–18 fish die each day. Affected fish have red ulcers with grey margins on the flank, frayed fins, and a few have a swollen abdomen with raised scales. Gills are brownish. Water quality is dissolved oxygen 4.8 mg/L, pH 7.4, total ammonia 0.03 mg/L and temperature 24°C. Eight hundred yearlings from another hatchery were added two weeks earlier without quarantine. The farmer applied 100 kg of lime and an unknown 'antibiotic powder' three days ago; deaths continue. No laboratory report is available. The farmer wants to double the antibiotic dose and add formalin.
1. What are the likely causes of the mortality, and why is a single diagnosis unsafe from these signs?
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The signs point to an ulcerative disease complex, but several causes can look similar:
- Bacterial septicaemia: red ulcers, frayed fins, ascites and raised scales are consistent with motile Aeromonas infection, especially after new fish are introduced.
- Epizootic ulcerative syndrome (EUS): deep ulcers with grey margins can occur in cool weather; it is not cured by antibiotics.
- Parasitic or fungal damage: untreated external parasites or injuries can produce ulcers and secondary bacterial infection.
- Water-quality stress: oxygen at 4.8 mg/L is not an emergency, but it is not ideal; handling and crowding during introduction may have triggered disease.
The new yearlings are a key risk factor because they were not quarantined. A single diagnosis from visible signs is unsafe: bacterial, viral-like and parasitic problems can overlap, and the unknown antibiotic may already be selecting resistant bacteria. Laboratory examination of gills, skin and internal organs is needed before choosing treatment.
2. Set out the immediate steps, including samples to collect and changes to treatment.
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Immediate steps:
- Stop the unknown antibiotic and do not double the dose or add formalin blindly. Formalin can damage gills and is not a cure for bacterial ulcers.
- Remove dead fish promptly and dispose of them away from the pond. Do not throw them into the canal.
- Reduce feeding by about half and stop manure. Improve aeration and, if possible, exchange 20–25% of the water.
- Do not apply more lime until the cause is clearer; lime at 100 kg in 0.35 ha is not a specific treatment and may not help if the problem is bacterial.
- Collect samples: 3–5 live, affected fish in pond water, plus water samples. Send them to a fish health laboratory or veterinary officer for skin/gill microscopy and bacterial culture with sensitivity testing.
- Isolate the remaining new stock if possible, and do not move fish to other ponds.
Record deaths daily. If fish are still eating, medicated feed should be used only on laboratory advice.
3. What should the farmer be told about antibiotic use and prevention?
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The farmer should be told that antibiotics are not a substitute for diagnosis and good pond management.
- Use only a registered antibiotic prescribed after sensitivity testing, at the correct dose and for the full course, with the required withdrawal period before sale.
- Never use chloramphenicol, nitrofurans or other banned drugs. Do not use unknown powders; they leave illegal residues and encourage resistance.
- Antibiotics do not treat EUS, fungi or parasites, and they will not work if water quality and stocking stress are not corrected.
Prevention for the next cycle:
- Quarantine new fish for at least 2–3 weeks in a separate pond and examine them before mixing.
- Disinfect nets and equipment between ponds; avoid sharing gear with affected farms.
- Keep stocking density and feeding within capacity, maintain oxygen above 5 mg/L, and remove dead fish daily.
- Keep records of seed source, mortality and treatments.
The key judgement is to stop guessing and get a diagnosis before any more chemical treatment.