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This article was published in 1984
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Diseases of Fish II
Dr. F.R. Doughty, Director, Animal Quarantine, Department of Agriculture, SYDNEY
The severity and sensitivity of the fish to stress depend on the nature of the enterprise. For example, a highly intensive fish farm has little room or tolerance before an additional stress situation will cause a significant loss. In contrast, fish in dams will not be as sensitive to an increased water temperature as they are farmed under extensive systems because they are not as highly stocked.
1. NON-INFECTIOUS DISEASE
1.1 Anoxia (Aeration problem)
Characterised by gasping, fish surfacing, crowding near inlet and probably death. Caused by lack of oxygen due to:
(a)fish density too high
(b) pump failure/faulty - not enough water
(c) hot weather - less dissolved oxygen
(d) pollution of water by algae, plants, detritus resulting in high biological oxygen demand
(d) chemical poisoning - insecticide, agricultural spraying -particularly aerial spraying.
Clinical signs and pathology. Sudden "fish kill" or gradual loss with fish doing everything to get oxygen - even trying to breathe air. No significant lesion evident at post-mortem examination.
Treatment. Remove cause quickly. This condition is a major problem to the inexperienced fish farmer because of our high water temperature and consequently decreased oxygen carrying capacity. Problem exacerbated by our dry summer and decreased water flow.
1.2 Gas Bubble Disease
This disease was first seen in aquaria when apparently healthy fish suddenly began to swim erratically, lose equilibrium and die within a few hours.
The lesions observed in affected fish consist of bulging eyes and gas bubbles in the gills and blood vessels. Post-mortem examination will reveal small bubbles in the heart and between the fins. This condition affects a wide range of species - fish, crabs, lobsters and molluscs. The cause is a malfunction of the gas exchange system of the fish. The condition is similar to 'Diver's Bends' - the super-saturated gas changes from the liquid to the air phase and results in malfunction of the heart.
1.3 Spring Flush (Acidosis/Alkalosis)
Characterised by the early rains of the season resulting in rapid water quality change, particularly pH.
Clinical signs and pathology. Rapid change of movement, jumping out of water, general disturbance, usually not lethal but frightening. No lesions if should die
Treatment. Control water intakes or allow gradual acclimatisation.
1.4 Poisons
A large number of poisons can occur usually very difficult to identify such as pesticides, etc.
1.5 Miscellaneous
Sunburn - erosion of skin.
Temperature shock - moving fish from one water supply to another.
Nutritional - similar to animals - difficult to diagnose usually found when own food prepared.
Trauma - rough management or equipment.
Fish fighting
Predation - big problem usually get report of 'fish dying'. Birds, water rats are major causes. Platypuses are a problem due to damage to ponds. Unique to Australia.
2. INFECTIOUS DISEASE IN AUSTRALIA
2.1 Viral diseases
None in Australia.
2.2 Bacterial diseases
(i) Bacterial Haemorrhagic Septicaemia
Characterised by septicaemia due to Aeromonas hydrophila. Probably the most important bacterial disease in Australian fish farms.
Clinical signs and pathology. Ulcers, abdominal distention, usually more than one affected. Internal lesions include excess blood tinged abdominal fluid, visceral haemorrhages.
Diagnosis. Isolate bacteria from kidney.
Treatment. Antibiotics and management (disinfection of ponds, reduce overcrowding).
(ii) Columnaris disease ('Cotton Wool Disease')
Characterised by grey-white ulceration of skin, fins, gills due to Flexibacter columnaris.
Clinical signs and pathology. As above.
Diagnosis. Lesions and isolation of bacteria.
Treatment. Keep water temperature below 15°C and avoid stress.
2.3 Fungal diseases
(i) Saprolegnia (or 'Fungus')
Characterised by white, cottonwool type growth on skin, fin, tail by Saprolegnia parasitica causing ulceration.
Clinical signs and pathology. As above.
Diagnosis. Smear and presence of fungus.
Treatment. Malachite green formalin flush and baths.
(ii) Ichthyophonus
Condition characterised by systemic granulomatosis and found in both fresh and marine fish. Caused by phycomycete Ichthyopbonus hoferi.
Clinical signs and pathology. Raised granulomatous lesions develop in the skin giving it a 'sandpaper' effect which can develop into ulcers. Internally, the heart and liver may have many yellow granulomatous lesions similar to tuberculosis. Histological examination reveals a severe granulomatous reaction with the developing stage of I. hoferi.
Transmission. Ingestion of infected material or infected copepods (crustacea).
Diagnosis. Characteristic histological lesion and mass of lesions.
Control and treatment. Prevent feeding infected material. No treatment.
2.4 Protozoan
There are a large number of protozoan diseases.
(i) Costia necatrix (Ichthyobodo) A condition characterised by epidermal necrosis and excess mucous production.
Clinical signs and pathology. Skin irritation and 'flashing', rub against rocks and may have secondary infection. Found on gills as well.
Transmission. By a free-swimming stage.
Diagnosis. Skin scraping.
Control and treatment. Prevent overcrowding. Treatment formalin bath, malachite green.
(ii) Ichthyophthirius multifilis ('Ich', White Spot) Condition characterised by presence of round white spots on skin, gills and fins.
Clinical signs and pathology. Fish flashes suggesting skin irritation, rubbing against objects, distinctive round spots on skin. No internal organs affected. Causes considerable losses.
Transmission. Skin scraping - characteristic horseshoe shaped structure. [sic]
Control and Treatment. Prevent introduction of infected fish.
Treatment. Malachite green/formalin bath.
2.5 Metazoan
(i) Flukes - Gyrodactylus and Dactylogyrus
Monogenetic trematodes that infect skin, gills, fins of a wide range of fish.
Treatment. Salt bath, Trichlorofon.
2.6 Crustaceans
(i) Lerneae (Anchor Worm). A condition characterised by the presence of parasites attached to the skin, resulting in ulceration and loss of condition.
Clinical signs and pathology. Multiple red spots where parasite has been attached or still attached. Fish shows skin irritation. All lesions external. Ulcers and focal haemorrhage leading to secondary infection.
Diagnosis. Identification of parasite.
Treatment. Trichlorofon. This is a disease that has become a problem in Murray Cod hatcheries.
(ii) Argulus (Fish Louse)
A condition characterised by skin damage and irritation due to presence of large numbers of Argulus on head, gills, fins and tail of fish.
Clinical symptoms. Excess mucous production, ulceration and secondary bacterial infection.
Diagnosis. Identification of parasite.
Treatment. Trichlorofon.