Diseases of the Australian Freshwater Fish Silver Perch (Bidyanus bidyanus) - Part 5

EPIZOOTIC ULCERATIVE SYNDROME (EUS, red spot disease) 

The aquatic fungus, Aphanomyces invadans, causes EUS on silver perch farms located in coastal regions in northeastern NSW and southeastern Queensland. The fungus is known to cause serious losses of farmed fish in Asia and estuarine fish in Australia; the latter usually following periods of high rainfall and subsequent decline of water quality (low salinity, low pH, high organic load). The disease can occur periodically on silver perch farms that have a surface water supply. Epizootics can cause high morbidity, particularly in juvenile (<100 g) fish; 100% infection rates are common. Mortalities in juvenile silver perch can be significant when the fish develop large, deep, necrotic ulcers. The disease is less of a problem in larger silver perch that often recover from infection, with whorls of scales a hallmark over the repaired area. 

Pathogen 

Atypical water mould, Aphanomyces invadans; differs from typical mould Saprolegnia parasitica, in having hyphae that penetrate skin and musculature; causes small to large red lesions and ulcers (4–25 mm diameter); severe and chronic inflammatory reaction; generally on body and caudal regions (Figs. 71 and 72); sloughing of necrotic tissue often leaves a deep, cratershaped cavity; significant potential for secondary bacterial infections. 

Signs 

Focal areas of pale skin, raised scales (early signs) 

Small red lesions and ulcers (10 mm), superficial, causing muscle tissue inflammation (myositis) 

Deep, red ulcers 

Fish mortality and/or morbidity 

Abnormal swimming (fish at pond edges, in currents, on surface, isolated) Loss of appetite (heavy infections)

Diagnosis 

Initially, macroscopic examination; pale patches of skin, small, single scalesized red lesions, small ulcers to large, deep (exposing skeletal muscle) necrotic ulcers in advanced cases. Microscopic examination (x100 mag.) of ‘squashed’ muscle tissue underlying ulcers; presence of fungal hyphae, 12–18 µm in diameter, amongst muscle fibre (Fig. 73). 

Treatment 

No known treatment of infected fish. Recovery has been recorded in large silver perch after improvement in water quality. 

The following assist in reducing new infections through killing the infectious zoospore stage. 

Tanks: 

formalin, 25 mg/L continuous bath and 50 mg/L, 1 h bath; or salt (NaCl) 10 g/L 1 h bath, then 5 g/L indefinite bath. transfer of fish to clean tank, lower stocking density (chlorinate destocked tank). 

Ponds: 

Emergency harvest, treatment (as above) and restocking to new ponds proven beneficial in recovery of advanced cases in fingerlings; restocking of large fish having early signs (small lesions to medium sized ulcers) has helped fish heal naturally; improve water quality. 

Prevention 

A void excessively high stocking densities and high biomasses. Outbreaks known to occur under one or combinations of the following predisposing conditions; infestations of ectoparasites, poor water quality, especially high or low pH, high organic matter, days of extreme hot weather, high rainfall leading to a ‘fresh’ in source water. Do not pump water from rivers and creeks with a ‘fresh’, in flood or containing infected fish. Maintain good water quality; dry ponds regularly. Spelling surface water in a fish-free environment for 7–10 days is likely to be sufficient to eliminate infectious zoospore stages of the fungus.

BACTERIAL DISEASES

Bacteria form a large, diverse group of small, ubiquitous organisms that play a key role in nature. Bacterial cells are simple, do not contain complex organelles and can divide rapidly. Some bacteria cause common and damaging diseases in fish. Most bacterial diseases result directly from environmental stress such as poor water quality and/or rough handling. To date, seven bacterial diseases have been reported in silver perch. However, other than conditions related to poor handling, silver perch appear to be robust and largely resistant to bacterial infections. Systemic infections are very uncommon in silver perch. 

Columnaris 

The bacterium, Flavobacterium columnare, causes the disease columnaris. Flavobacterium columnare has a worldwide distribution, is common in the pond environment and can rapidly infect fish causing high mortalities. The disease is not common in silver perch, although it has caused mortalities in fingerling and mature silver perch, but usually following stressful precursors such as physical injury, other disease and crowded conditions associated with low DO and high organic loads. Flavobacterium columnare can be invasive once established, through production of a range of tissue toxins; pathogenicity is usually higher at temperatures >20°C. 

Pathogen 

Slender, filamentous gram negative rod; 0.4 µm diameter, 4 to 10 µm length; lacking flagella, motile by gliding mechanism; preference for aerobic conditions colonising gills and skin; prevalent at temperatures >20°C. 

Signs 

Chronic to acute fish morbidity or mortality 

Erosive and necrotic skin lesions on head and caudal regions (Figs. 74 and 75) 

Lesions ‘whitish’ colour, with inflamed, red periphery 

Frayed and eroded fins 

Gill infection, eroded lamellae with yellow to white necrotic edge 

Ulcers with ‘yellowish’ pigmentation

Diagnosis 

Microscopic examination of wet mounts of lesions (skin and gill); 1,000× magnification; bacteria as slender rods having characteristic gliding motion; bacteria usually congregate into colonies giving a ‘haystack’ appearance hence the name columnare; lesions may have secondary fungal infection. 

Treatment 

Tanks: 

Oxytetracycline 20 mg/L active ingredient, 7 days at 20–30°C or 10 days at <20°C, continuous bath, maintain low light levels, good aeration, water exchange to ‘dilute’ bacterial load and retreat daily; This organism is commonly sensitive to this antibiotic; however, laboratory sensitivity testing is recommended. 

Salt (NaCl) 2 g/L continuous bath may assist in recovery by preventing fungal infections. 

Lesions on individual fish can be swabbed with 20% iodine solution. 

Ponds: 

potassium permanganate (KMnO4) 2 mg/L a.i., higher levels may be required in ponds with high organic matter; total KMnO4 not to exceed 6 to 8 mg/L, KMnO4 becomes inactive when water colourless or light brown; in systemic infections, oxytetracycline as feed additive for 10 days, 50–100 mg/kg fish/day. 

copper (as copper sulfate) 0.2 mg/L 

prolonged immersion, maintain level; higher CuSO4 dosages dependant upon total alkalinity, alkalinity must be >50 mg/L; continuous aeration during treatment. 

Prevention 

Avoid poor handling of fish, especially exposure to any periods of low DO; avoid harvesting at high temperatures (>28°C); avoid overstocking/high organic loading during harvest procedures; maintain good water quality. Ensure rapid solids removal in tank systems and maintain appropriate stocking levels (e.g. up to 40 kg/m3 <18°C; up to 30 kg/m3 >18°C).

‘TAIL ROT SYNDROME’ 

Tail rot syndrome or fin rot, is a relatively common bacterial disease in juvenile silver perch. It occurs mid to late summer when fingerlings are harvested and transported from ponds to quarantine tanks, or when fingerlings are transported from hatcheries and growout farms. The disease is characterised by necrosis of the caudal fin and distal part of the caudal peduncle and usually occurs as a result of rough handling, overcrowding, low DO, high temperatures, high organic loading and inappropriate quarantine procedures between harvest and transport periods. Mixed infections by the bacteria belonging to the Flavobacterium, Pseudomonad, Vibrio or Aeromonad bacterial groups most likely cause the disease. 

Pathogen 

Flavobacterium spp. as for columnaris; Aeromonas spp. (e.g. A. hydrophila), 0.8 to 1.0 µm diameter and 1.0 to 3.5 µm long, short, motile rods by a single polar flagellum, ubiquitous found in most freshwaters; Pseudomonas spp. (e.g. P. fluorescens); similar to Aeromonas; these bacteria are often natural inhabitants of fish mucus. 

Signs 

Eroded tail fin 

Necrotic tissue on caudal peduncle 

Dark colour, sometimes blotchy – fry/fingerings 

Lethargic, often near tank surface or facing current or airstones 

Chronic mortality following harvest/ transport 

Diagnosis 

Gross appearance; fish with frayed edges and reddened caudal fin in early stages (Fig. 76); skin of the distal caudal peduncle having pale, mucoid appearance with focal haemorrhages; more advanced cases, caudal fin eroded to peduncle, ulcerated tissue often exposing skeletal structure (Fig. 77); secondary fungal infection common. Definitive diagnosis by fish necropsy, cultures and isolation of bacteria. Mortality increases significantly once bacteria enter circulatory system. Fish having mild cases can completely regrow the soft fin ray tissue if infection resolves. 

Treatment 

Tanks: 

Oxytetracycline bath, 20 mg/L active ingredient, 7 days at 20–30°C or 10 days at <20°C; maintain low light levels, good aeration; water exchange to ‘dilute’ bacterial load and retreat; this syndrome often responds to this antibiotic; however, laboratory cultures and sensitivity results are recommended to ensure appropriate antibiotic usage, plus salt (NaCl) 2 g/L continuous bath may assist in recovery by preventing fungal infection; – remove infected, moribund fish promptly, particularly advanced cases. 

Restock into recentlyfilled pond with good water quality – fish can recover once restocked into the lowstress environment of a pond. 

Prevention 

Minimise stress during harvest procedures; use only knotless nets; avoid harvesting during hot weather, when water quality is poor and DO low; avoid overcrowding; always use bottled oxygen during harvest procedures (pond and fish transporter). Quarantine fingerlings 4–5 days post harvest and prior to transport; provide salt bath (2–5 g/L) for 24–48 hrs postharvest, clean water and aeration. 

Ulceration caused by Aeromonas salmonicida nova; (‘Goldfish Ulcer disease’, GUD)

An atypical strain of the bacterium, Aeromonas salmonicida, the subspecies nova causes this ulcerative disease. There has been only one reported outbreak involving silver perch in an earthen pond on a commercial farm that also produced ornamental goldfish (Carassius auratus). 

The typical strain of this bacterium, Aeromonas salmonicida subspecies salmonicida, causes the serious disease, furunculosis, in salmonids. GUD was probably introduced into Australia by the importation of infected goldfish (Fig. 78). A. salmonicida has been identified in common carp (Cyprinus carpio) and the pathogen can be carried by infected fish without showing symptoms. The disease expresses itself as skin and/or fin ulcers and systemic infections are probably likely late in the epizootic; mortality rates in the absence of secondary infections are usually low. Murray cod have shown resistance to GUD, but may act as asymptomatic carriers.

Pathogen

Aeromonas salmonicida nova; nonmotile, gramnegative bacterium; isolates grown on agar media are typically grey, small and circular colonies; pathogen may survive long periods off the host fish. 

Signs 

Chronic fish morbidity/mortality 

Lethargic swimming and inappetence 

Lesions and ulcers on skin and fins, ranging from superficial, reddened lesions to craterlike ulcers, may have pale or red centres with a peripheral ‘pale’ halo (Fig. 79) 

Fluids in abdominal cavity (ascites) 

Diagnosis 

Definitive diagnosis requires laboratory examination of infected fish; necropsy, culture and bacteria isolation; isolation of bacteria from internal organs (kidney and spleen) in systemic infections is more revealing than sampling superficial ulcers. 

Treatment 

Tanks: 

oxytetracycline (pure), bath, 20 mg/L active ingredient, 7 days, at 20–30°C or 10 days at <20°C; maintain low light levels, good aeration; water exchange to ‘dilute’ bacterial load and retreat daily; antibiotic choice should be based on 

laboratory sensitivity testing 

Ponds: 

prompt oral administration of appropriate antibiotic, determined by laboratory sensitivity testing; oxytetracycline, 75 mg/kg fish for 10 days; antibiotics can only be used under the supervision and prescription of a registered veterinarian; Withdrawal times apply prior to sale of fish. 

Prevention 

Prevention requires elimination of A. salmonicida from the farm and its water supply; goldfish and carp are possible sources of infection and should be eliminated from ponds, drains and water supplies; maintain good water quality; reduce stresses; regularly dry and lime ponds with Ca(OH)2 or CaO (calcium hydroxide or calcium oxide).