Ulcerative disease outbreak after buying river-caught fry
Prafulla Sahoo has a 0.6 ha pond at Kendrapara. In July he stocked 5,000 fingerlings, of which 2,000 were bought at half price from a river-side vendor selling mixed wild-caught fry. Three weeks later 20–30 fish are dying each day. Rohu and mrigal show deep red necrotic ulcers on the head and flank, and some lesions carry cottony growth; gills are pale. Only two fish inspected carry a few Argulus. Water tests: pH 7.8, dissolved oxygen 5 mg/L, total ammonia 0.8 mg/L, temperature 29 C. Two neighbouring ponds that bought from the same vendor also report deaths. Prafulla has started mixing oxytetracycline powder into feed at 500 mg/kg without any diagnosis, and plans to sell the surviving fish in the local market next week.
1. Give your most probable diagnosis and justify it against the alternatives.
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The picture fits epizootic ulcerative syndrome (EUS): deep, necrotic ulcers on the head and flank of rohu and mrigal, often with secondary cottony Saprolegnia, occurring in Kendrapara in the monsoon period three weeks after stocking, with the same seed source implicated in neighbouring ponds. EUS is a seasonal, water-borne problem in this part of Odisha, and a shared seed source with simultaneous outbreaks in three ponds is strong evidence of a common-source introduction.
Alternatives are weaker. Argulosis can cause ulcers, but only two fish carried a few Argulus - far too few to explain 20–30 deaths a day across the pond. Ammonia at 0.8 mg/L is elevated and worth correcting, but ammonia damage shows as gill and skin darkening, not deep craters on the flank. A bacterial ulcerative condition is possible as a secondary invader, but the cottony tufts and the pattern of spread point to the primary agent being fungal/oomycete.
Action: send two moribund fish, packed cool and wet, to the nearest Fisheries College or CIFRI centre for confirmation. But do not wait for the report to start control measures - the pond should be managed as EUS from today.
2. Critically evaluate Prafulla's response - the medicated feed and the plan to sell survivors next week - and outline what he should do instead over the next ten days.
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Both actions are wrong. Feeding oxytetracycline without diagnosis treats a fungal/oomycete infection with a drug that does not touch it. It selects for resistant bacteria, leaves residues in fish that may be sold, and adds medicated feed to a pond where sick fish are barely eating - so the feed decays and worsens oxygen demand. There is also no withdrawal period being observed.
Selling survivors next week is worse: moribund and freshly recovered fish move the infection to the market chain and to any water they contact, and they will fetch poor prices anyway.
Better ten-day plan:
- Stop all water exchange with neighbouring ponds and stop sharing nets, buckets and baskets. Disinfect gear that must be shared.
- Remove dead and dying fish daily and bury them away from the pond with lime - not in the canal.
- Reduce or stop feeding for 3–4 days; resume lightly.
- Apply agricultural lime at roughly 200–300 kg/ha in the pond and common salt at 200–500 kg/ha, which are the standard supportive measures; maintain oxygen with pumping at night.
- Get laboratory confirmation and follow the state advisory for the area.
- Harvest only after mortality has stopped, and sell only firm, lesion-free fish. Then dry the pond, lime the bottom at 600–1,000 kg/ha before the next stocking.
3. What biosecurity rules should he and his neighbours adopt before the next season, and how should the seed-purchase decision be handled?
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The outbreak entered through cheap, unexamined wild fry. The rules should close that door first:
- Buy seed only from a registered hatchery, with a health certificate and a known spawn date; keep the bill.
- If wild or unknown seed must be used, quarantine it for 15–20 days in a small separate nursery pond with its own water supply and nets, and inspect it daily before it goes anywhere near the main pond.
- Never mix quarantined seed and farm-reared seed until the quarantine period is over clean.
- Keep one set of nets, buckets and baskets per pond; screen the inlet; restrict visitors and stray animals.
- Maintain a daily mortality log with a simple count and note the water temperature; a jump from 2 to 20 deaths a day is the earliest warning available.
- Never dispose of dead fish or pond water into a channel that feeds another pond, and never sell visibly diseased fish.
The economic argument matters here: he saved perhaps Rs 2,000 on fry and lost many times that in fish, feed and reputation. The cooperative should fix a minimum seed standard for all members so that no single farmer's cheap purchase can infect the group.