Diseases of the Australian Freshwater Fish Silver Perch (Bidyanus bidyanus) - Part 4

DISEASES AND PATHOGENS

Ectocommensal Ciliate Infestations 

Species of sessile, ectocommensal ciliates have rarely been recorded on silver perch; to date all cases have been identified as Ambiphyra spp. Parasites attach to skin or gill tissue by a holdfast (scopula) often in colonies (Fig. 48). 

The parasites feed on bacteria and organic matter in the water and use the host primarily for attachment. There have been no reports of problems with this parasite in the silver perch industry. Large numbers on the gills could potentially impede gas exchange, metabolic function and/or offer a site for bacterial and fungal infection (Fig. 49). 

Pathogen 

Bell or gobletshaped, elongated ciliates; attached either by stalk or scopula; spiral of cilia at posterior end (Fig. 50); some species having ciliary girdle midbody; most range 40–100 µm in length; some species solitary; some colonial. No obvious motility. 

Signs 

• Hyperventilation (heavy infestations) 

• Emaciation 

• Flashing 

Diagnosis 

Microscopic examination of gill (mainly), fin and skin tissue; easily recognisable at 100× magnification; identification to species not needed; treatment the same for all species 

Treatment 

Tanks: 

• formalin 25 mg/L, retreat after 2–3 days; aerate water; no feeding. 

• salt 10 g/L (NaCl) for 60 min or prolonged immersion at 5 g/L; some species may be resistant.

Ponds/cages: 

• formalin 15–20 mg/L, maintain 24 h aeration for 4–5 days, repeat treatment after 2–3 days if necessary. 

Prevention 

Presence is indicative of organicallypolluted water that could have a high concentration of bacteria; improve water quality (aeration and exchange) to alleviate the problem and minimise the risk of recurrence. 

Monogeneans 

Monogeneans (skin and gill flukes), in particular Lepidotrema bidyana (Dactylogyridae) are commonly found on silver perch in ponds, cages and tanks. They have become widely distributed in the industry over the last 10 years, and the parasites now occur on most farms. This rapid spread demonstrates the risk of moving pathogens with live fish and the need for stringent on-farm quarantine measures to be implemented to avoid the transfer of pathogens between farms and onto farms from wild fish. Gill flukes attach using their posterior, haptor organ to gills (adult flukes) and to skin (juveniles). Heavy infestations (>15 parasites per field of view) of flukes have not caused mortalities in silver perch; however, the parasite may cause stress, poor feeding response and growth, tissue damage and interference with gill function, predisposing the fish to fungal and bacterial diseases (Fig. 51). Eradication of L. bidyana from ponds and tanks is difficult with current therapeutic agents. Infestations of a similar parasitic group in the family Gyrodactylidae have also been recorded on silver perch (Fig. 52).

Pathogen 

Wormlike parasite, adults up to 600 µm in length; attached posteriorly; oviparous, producing eggs which are released to the aquatic environment; eggs hatch as a freeswimming infestive stage (oncomiracidium); juveniles can develop into gravid adults within 7 days; eyespots and head organs obvious; low numbers of parasites may be present without causing problems or stress. 

Signs 

• Flashing 

• Excessive mucus production on skin and gills 

• Gill hyperplasia 

• Loss of appetite 

• Emaciation (heavy, prolonged infestations) 

Diagnosis 

Easily identified; microscopic examination of gill at 40–100× magnification (Fig. 53); both adults (gill) and juveniles (skin) show characteristic stretch and recoiling motion. 

Treatment 

Monogeneans can reinfest silver perch between 5 and 30 days after treatment with formalin or trichlorfon (an organophosphate), depending on water temperatures and degree of pond contamination with eggs. Up to three treatments, 21 days apart may be required to eradicate or reduce monogenean numbers to an insignificant level. 

Tanks: 

• formalin 30 mg/L, aerate water, no feeding; or 

• formalin 150 mg/L, 30 minute bath; or 

• trichlorfon 0.25 mg/L active ingredient indefinite bath; or 

• salt 15 g/L, 1 h bath, repeat following day; or Ponds/cages: 

• trichlorfon 0.5 mg/L active ingredient, indefinite bath; 

• formalin 30 mg/L (when water <25°C), maintain 24 h aeration for 4–5 days; monitor DO daily. 

Prevention 

Quarantine and treat all incoming fish prophylactically (2–5 g/L plus 30 mg/L formalin), especially fingerlings prior to stocking. Gill flukes are usually indicators of poor water quality, improve water quality. Frequently monitor gill fluke numbers. Monogeneans can be persistent in tank systems necessitating regular treatments and periodic drying. Dry ponds regularly and use calcium hydroxide [Ca (OH)2] or calcium oxide (CaO) liberally on any persistently, damp areas.

Copepods 

Copepods are crustaceans with a complex life cycle, developing through egg, nauplii and copepodid larval stages before attaching and maturing as adults on the host. The freshwater, parasitic copepods, Lernaea spp. and Ergasilus spp. have been recorded on silver perch. 

Lernaea spp. (anchor worm) Anchor worms possess anchorlike processes for securing themselves to the host (Fig. 54). Silver perch farms in the MurrayDarling Basin may have a high incidence of anchor worm because common carp (Cyprinus carpio) are often carriers of the parasite. However the parasite has also been recorded on silver perch (and other freshwater species) in the eastern drainage. Parasites can infest individual fish in high numbers (100’s) without causing mortality; however, poor feeding response and growth has been recorded. Attachment sites are often areas for secondary bacterial or fungal infections. Numbers of parasites may increase in slowflowing or static water sources when carp and native fish become concentrated. Anchor worm is more common in summer, but the parasite can occur year round. Marketability of fish infested with Lernaea is compromised due to the presence of small, red lesions. 

Pathogen 

Anterior end of adult female buried in flesh of fish (Fig. 55); body cylindrical, wormlike; cephalic segment with two to four soft horns; adult female up to 20 mm in length; paired egg sacs greenish, conical or ovoid at posterior end (Fig. 56); eggs hatch 1–3 days, nauplius metamorphose into copepodids 4–16 days, completion of several developmental stages prior to copulation, female attaches; male disappears, presumably dying; life cycle temperature dependant. 

Signs 

• Anchor worms clearly visible to naked eye 

• Haemorrhaging and red lesions at site of attachment (Fig. 57) 

• Emaciation and poor growth 

• Flashing 

Diagnosis 

Macroscopic examination of external features of fish; gravid females easily recognisable by eye attached to gills and skin, often on areas having ‘softer’ scale cover such as soft ray tissue, mouth and nares; Microscopic and macroscopic examination of gills; small immature stages, such as copepodids may not be grossly visible. 

Treatment 

Tanks: 

• removal of individual parasites with forceps; 

• trichlorfon 0.25 mg/L active ingredient, indefinite bath; 

• salt 10 g/L, 1 h bath, repeat daily. 

Ponds/cages: 

• trichlorfon 0.5 mg/L active ingredient, indefinite bath; repeat every 7 days for 28 days. 

• repeated treatments required to prevent reinfestation by emerging larval stages of Lernaea. 

Prevention 

Quarantine and prophylactic treatment prior to stocking; lowering of stocking density; improvement in water quality; ensure water source and storage reservoir free of carp and other fish.

Ergasilus sp. 

Infestations of the copepod, Ergasilus sp., have rarely been recorded on silver perch farms. Ergasilids are often described as ‘gill maggots’ due to the appearance of white egg sacs attached to the adult females. The parasite’s clasping attachment causes severe gill damage and interference with gill function (Fig. 58). In one case, pondreared, silver perch (>500 g) suffered a heavy ergasilid infestation (species unknown). Fish were nutritionally challenged, displayed long periods of inappetence, recorded relatively poor growth and were easily stressed during harvest. Parasites were attached to the gills. No mortalities were recorded in ponds. Damage to gill tissue caused by ergasilids can lead to secondary bacterial or fungal infections. 

Pathogen 

Distinct cephalothorax and abdomen, 1 mm length; large, paired, clawlike antennae (used for locomotion), length, 1 mm; female having posteriorly attached white egg sacs, length,1–2 mm; eggs ~100/sac, 0.05 mm diameter, colour darkens on maturity (Fig. 59). Life cycle, egg hatching to freeswimming nauplius; several copepodid developmental stages via moulting; copulation at freeswimming stage; females enter gill cavity and attach to rakers and gills. 

Signs 

• Flashing 

• Loss of appetite 

• Poor growth 

• Haemorrhaging of fins 

• Patchy/blotchy, dark skin 

• Stress following handling 

• Strong reaction to ‘light’ anaesthesia 

• Gill hyperplasia 

Diagnosis 

Macroscopic examination of gills, ‘gill maggots’; easily recognisable on gill tissue at 40× magnification; immature forms may not be grossly visible.

Treatment 

Tanks: 

• trichlorfon 0.25 mg/L active ingredient; or 

• salt (NaCl), 10 g/L continuous for 3 days. 

Ponds/cages: 

• trichlorfon 0.5 mg/L active ingredient, prolonged immersion. 

Prevention 

Quarantine and prophylactic treatment prior to stocking; lowering of stocking density; improvement in water quality; use of high quality feeds. 

Cestodes and nematodes 

Cestodes and nematodes (tapeworms and roundworms respectively) are endoparasites inhabiting the intestine of many vertebrates including fish. Both parasites have similar life cycles with at least one intermediate host (e.g. copepod, insect nymphs, worms). 

The final host can be piscivorous birds or predatory fish. Some cestode and nematode larval stages (plerocercoids) are commonly found in the viscera or musculature of some fish. Long, white, ribbonlike cestodes (species unknown) have been reported protruding from the anus of silver perch held in purging systems (Fig. 60). Comparatively, nematodes are characterised by a slender and cylindrical body having tapered ends and no true segmentation. Nematodes (species unknown) have been found in the digestive tract of silver perch and a species of the genus Camallanus, has parasitised the gut of Barcoo grunter (Scortum barcoo) (Figs. 61 and 62). Normal behaviour and growth was reported for silver perch parasitised by cestodes and nematodes. 

Pathogen 

Cestodes: up to 40 cm; body long, ribbonlike; white to cream colour; scolex (head) not pronounced having adhesive grooves (bothria). Nematodes: slender, cylindrical body, covered with cuticle; up to 10 mm length; red in colour.

Signs 

• Worms protruding from vent (Fig. 63) 

• Swollen abdomen 

Diagnosis 

Large worms identified grossly; wet mounts of faecal matter may be useful in determining the presence of eggs or larvae. Microscopic examination of physiological features (scolex, shape, size, and segmentation) required to identify species. Samples should be sent to a reference laboratory for definitive identification. 

Treatment 

There are currently no treatments which are registered, or the subject of an Australian Pesticides and Veterinary Medicines Authority (APVMA) permit. Chemicals which have registration in other food producing species of animals (non-fish) may be used under an offlabel prescription from a registered Veterinary Surgeon. 

Prevention 

Regularly dry, desilt and disinfect ponds using calcium hydroxide or calcium oxide. Minimise access of animals that could be carrying parasites, including birds and turtles.

Fungal diseases 

Fungi or water moulds belong to the Class Oomycetes and are ubiquitous in freshwaters. They form a large group of saprophytic organisms that feed opportunistically on dead organic matter. Fungi are important pathogens of fish. They are generally considered to be secondary pathogens, with infections often induced by stress, physical damage and/or immunosuppression. Fungal infections are inherently difficult to treat because of the complex biology of aquatic fungi. 

Saprolegniosis 

The fungus, Saprolegnia parasitica, causes the disease saprolegniosis in silver perch. Fungal infections in silver perch are common after rough handling, removal of mucus and damage to the epidermis. A new, important disease in silver perch, winter saprolegniosis, usually affects larger (>250 g) fish (Fig. 64) and is prevalent at temperatures <16°C, usually following rapid decreases in water temperatures (April – August) associated with cold fronts in winter; 100% mortality has been recorded in some ponds on farms, and epizootics can be severe and rapid (4–6 days). Saprolegnia parasitica has been recorded on silver perch fingerlings (25 mm length) held in tanks following a temperature decrease (12 to 8°C) over two days. It is associated with superficial fungal infections of the skin and gills. 

Pathogen 

Opportunistic water mould; white mycelial growth on skin and gills (Fig. 65); often coloured by trapped organic matter (Fig. 66); clubshaped hyphae (20 µm diameter) release zoospores (sporulation); spores motile in water; hooked, hairlike projections may assist spores attachment to host skin and gills of live fish (Figs. 67 and 68); pathogenicity appears to increase with decreasing water temperatures (<16°C).

Signs 

• Abnormal swimming and behaviour (fish at pond edges, in currents, on surface, isolated) 

• Focal areas of pale skin (early stages) 

• Haemorrhaging on underside of abdomen (early stages) (Fig. 69) 

• Light fungal growth on caudal fin (early stages) 

• White ‘cotton woollike’ patches on skin or gills (advanced stages) 

• Fungal plaques attached to the soft tissue on the distal end of the opercula (early stages) 

Diagnosis 

Macroscopic examination of skin and gills; pale and/or haemorrhagic areas, white to greenbrown coloured fungal growth. 

Microscopic examination of skin and gills; fungal hyphae of variable widths (10–30 µm); sporangium often with zoospores; easily diagnosed at 100× magnification; fungal growth (mycelium) not so obvious in early stages. 

Treatment 

Treatment difficult, many fungal infections show resistance to chemotherapy; recovery related to the amount of skin and gill infected by fungus; gill infections are usually terminal. 

Tanks: 

• salt (NaCl) 2–5 g/L preventative treatment, continuous, indefinite bath (adults, juveniles, larvae); elevated temperature may assist, maintain >20°C; 

• eggs, larvae, salt 2 g/L preventative treatment, continuous, indefinite bath; 

• eggs, formalin 1,000 mg/L for 15 mins daily (used to control saprolegniosis on Murray cod eggs). 

Ponds/cages: 

• emergency harvest is often the only way to avoid a major loss from saprolegniosis. Fish need to be dip bathed in a high concentration of formalin 100 mg/L for 30 minutes, prior to being held in a very clean recirculation system with at least 2–5 g/L salt. 

• formalin 30 mg/L initially (when water temperature <25°C), then maintaining levels between 25–30 mg/L daily (see chapter Calculations, Treatments and Dose Rates), until disease controlled; aeration continuous and supplemented; or 

• copper (as copper sulfate, CuSO4) 0.1–0.2 mg/L, recommend 0.2 mg/L initially, then monitor and adjust free Cu+ ion levels daily to maintain copper concentrations between 0.1 and 0.2 mg/L until disease controlled; continuous aeration during treatment; alkalinity must be > 50 mg/L. Copper may provide some degree of prevention if used regularly up to and throughout the high risk period of winter. 

Prevention 

Salt bath (2–5 g/L) prevents or slows rate of infection in tanks. Eliminate or reduce predisposing stressors late summer/autumn when water temperatures start to fall < 18°C; ensure fish free of ectoparasites; treat parasitic diseases; reduce feeding; reduce high organic load in ponds (dead fish and organic matter, including feed are fertile substrates for colonisation of the fungus) (Fig. 70); reduce biomass especially large (>500 g) fish; maintain good water quality, especially following crashes of algal blooms; avoid handling and skin damage; on farms where winter saprolegniosis is a problem, weekly applications of formalin or copper may be beneficial after temperatures decline to 16°C in late autumn/winter; remove daily any dead fish (or eggs); dry and clean ponds between crops.