Ulcers, Night Mortality and a Barrel of Copper Sulphate
In a 1 ha pond in Hooghly district, a farmer stocks 6,000 rohu averaging 60 g and 3,000 catla averaging 70 g in June. By August, 40–60 fish are dying every night. Moribund fish hang at the margin with pale gills. About 30% of the surviving rohu have deep ulcers 1–2 cm across on the flank; the base of each ulcer is grey-white and necrotic with a dark rim, and some tails are eroded. Catla mostly show no ulcers but are lethargic and feeding poorly. Water readings: afternoon pH 8.8, total ammonia nitrogen 1.5 mg/L, dawn DO 2.9 mg/L, and 25 cm of black sludge. No lime has been applied for two years. The farmer feeds rice bran and oilcake at 6% of body weight. He has already broadcast 25 kg of copper sulphate and 20 kg of formalin over the pond in two rounds and now asks whether he should repeat the treatment with oxytetracycline added.
1. What is your working diagnosis, and what other possibilities must you rule out before committing to a treatment?
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The pattern - deep ulcers with a grey-white necrotic base and a dark rim, on rohu, with chronic nightly mortality and poor response to chemical treatment - is most consistent with epizootic ulcerative syndrome (EUS), caused by the oomycete Aphanomyces invadans. The exposed muscle with a granulomatous rim is characteristic; EUS typically appears in the cooler months after flooding or in ponds with low pH and heavy organic loads, and it does not respond to antibacterial drugs.
The key differential is motile aeromonad septicaemia (Aeromonas hydrophila), which is more acute: rapid mortality with dropsy, congested fins, shallow haemorrhagic ulcers and protruding eyes rather than deep necrotic craters. Mixed infections are common, so an acute septicaemia on top of EUS cannot be excluded clinically.
Also check: argulosis (Argulus visible on body and gills, focal red lesions), gill parasitosis or myxosporean infection behind the gill pallor, and chronic ammonia/oxygen stress as the underlying driver.
Confirming action: send 5–10 moribund fish, packed cool and wet, not frozen, to a diagnostic laboratory for wet mount, bacterial culture and sensitivity, and PCR or culture for A. invadans. Meanwhile, record gill and skin scrapings pond-side. Do not wait for the result to start water correction, but do not start antibiotics on a guess.
2. Critically evaluate the copper sulphate and formalin applications. What were the likely consequences, and what would a safe treatment plan look like?
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The two chemicals were the wrong tools applied at the wrong dose and timing. The pond holds roughly 15,000 m³ (1 ha × 1.5 m). 25 kg of copper sulphate is about 1.7 mg/L, well above the 0.5 mg/L ceiling usually quoted for ponds, and copper is more toxic in soft, low-alkalinity water. It kills phyto- and zooplankton, which collapses oxygen production - a serious problem in a pond already at 2.9 mg/L at dawn - and the decaying plankton release ammonia. 20 kg of formalin is only about 1.3 mg/L, far below the 20–30 mg/L used for parasite control, so it was useless against the disease while still irritating gills and further removing oxygen. Neither chemical has any action against an oomycete inside muscle tissue.
Safe plan
- Cut feeding to about half, or stop for three days, to reduce oxygen demand and waste load.
- Aerate at night or flush 10–15% fresh water daily; DO must be above 4 mg/L before any treatment.
- Remove and bury dead fish every morning; they are the main source of infection.
- Apply agricultural lime only after checking pH - with afternoon pH 8.8 and ammonia 1.5 mg/L, liming now would raise un-ionised ammonia; correct water first, and lime the dried bottom at the end of the cycle.
- For external lesions, a salt dip (about 3% for 3–5 minutes, with fish returned to clean water) or pond-side salt application is far safer than copper.
- Antibiotics: only if a veterinarian prescribes them on the basis of a sensitivity test, at the label dose for 5–7 days, and observe the withdrawal period of 21–30 days before sale. Never use oxytetracycline as a blind pond-wide drench, and never as a substitute for water management.
- Stop all recruitment of fish from ponds with ulcers and do not move water between ponds.
3. How far is water quality driving this outbreak, and what preventive measures would you build into the next cycle?
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Water quality is the engine of the outbreak. Dawn DO of 2.9 mg/L, TAN 1.5 mg/L at pH 8.8, 25 cm of black sludge and no liming for two years describe a pond with chronic organic overload. Continuous low oxygen and ammonia stress damage gills and skin barriers and suppress the immune response, which is exactly the condition in which opportunistic pathogens such as Aeromonas and Aphanomyces establish. The dense stock (9,000 fish in 1 ha) and 6% body-weight feeding of a low-quality mash add further waste. In practice, fish in such a pond will relapse whatever drug is used unless the environment changes.
Prevention checklist for the next cycle
- Drain and dry the pond until the bottom cracks; remove 25 cm of sludge; test soil pH and lime only if pH is below about 6.5.
- Stock only certified, disease-free fingerlings from a hatchery with a health record; quarantine and salt-dip new stock.
- Stock at a density the water can carry (6,000–8,000/ha for a 1 ha unacrated pond of this type) and use a pelleted feed at 2–3% body weight rather than a 6% rice bran mash.
- Keep dawn DO above 4 mg/L: night aeration in August–September, and fertilisation controlled by Secchi depth (25–35 cm).
- Apply lime and manure on a soil-test and water-test basis, keep a log of pH, ammonia and DO, and remove dead fish daily.
- Maintain a health calendar: monthly gill and skin scrapings, and an emergency plan that does not include copper sulphate.