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This article was published in 1984
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Diseases of Fish I
Dr. F.R. Doughty, B.V.Sc., PhD., Director, Animal Quarantine, Dept. of Agriculture, SYDNEY
Aquatic animals are acutely sensitive to slight changes in water quality and the effect of these changes are exacerbated in artificial situations such as aquaria and ponds. Factors to consider include water temperature, dissolved oxygen, pH, ammonia and nitrate/nitrite content.
FACTORS RELEVANT TO DEVELOPMENT OF FISH DISEASE
When disease occurs in fish, a pathogenic organism such as bacteria or virus is often involved. However, disease is only in rare cases caused by obligate pathogenic organisms. Most pathogens are facultative, that is they can exist quite well without fish, but where they are present in large numbers and the fish is stressed they cause serious outbreaks of disease. It is not possible to eradicate these bacteria from a hatchery as they are universally present in the water supply and surrounding land. All that can be done is to reduce their numbers by keeping the pond clean and reducing stress on the fish to a minimum.
The resistance is reduced if the fish is in a certain state of stress induced by environmental factors.
Thus the outbreak of disease in fish requires the interaction of fish, pathogen and environment.
Pollution induced stress, however seems to be able to cause changes of a non-infectious nature, such as certain tumours and skeletal deformities. Pollution substances (organic) constitute a suitable nutrient to Vibrio and Aeromonas species, facultative pathogens which may cause ulcers and septicaemia in fish.
ANATOMY AND PHYSIOLOGY OF FISH
The purpose of briefly describing the anatomy and physiology of the fish is to assist you in understanding the subtle differences between fish and mammals, and allow you to build on your existing knowledge of disease and disease process of animals.
Many basic functions are similar. The major difference from animals is the constraint and adaptation the aquatic environment place on the fish. An understanding of the characteristics of the aquatic environment will make the interpretation of disease data meaningful.
Fish are poikilotherms - the body temperature reflects the temperature of the waters in which they swim. Unlike mammals which have a mechanism to control temperature in a narrow range, fish can survive a relatively wide range of temperatures. As a result, the cardinal signs are of little value because the parameters such as heart rate, rate of digestion, growth, immune response are all temperature dependant and consequently are not good references of the state of the animals.
The internal organs of fish like those of other vertebrates are grouped into various systems according to the function they serve. The major systems include, respiratory, digestive, circulatory, nervous, reproductive and excretory.
There are special organs including (a) the swim bladder and (b) lateral line.
(a) Swim bladder, located below the backbone provides buoyancy. Most fish gain buoyancy by inflating their swim bladder with gasses produced by their blood. Water pressure increases with depth as fish swims deeper, the increased water pressure makes the swim bladder smaller and so reduces the fish's buoyancy. A fish's nervous system automatically regulates the amount of gas in the bladder so that it is properly filled for buoyancy. Some bottom dwellers don't have swim bladder.
(b) Lateral line special sense organ - "touch". Nerve endings throughout the skin react to the slightest pressure and change of temperature.
Vibrations enter the lateral line through pores and activate certain sensitive areas in the line. Nerves relay information to the brain. Changes in pattern of vibrations may warn a fish of approaching danger or indicate the location of objects outside the range of vision.
ANATOMY AND PHYSIOLOGY OF FRESHWATER CRAYFISH
The freshwater crayfish (yabby) has a highly developed circulatory system. Clear coloured blood passes from its heart via arteries into minute blood vessels, then gathers in cavities and flows back to the heart via the gills where gaseous exchange takes place - that is it breathes. The yabby has two gill chambers. It can clamp the cover of these chambers shut and trap moisture, which allows it to live out of water for some time. If the water oxygen is too low, the yabby will surface and expose its gills to the moist air above the water.
DISEASES OF FISH
Non-Infectious Diseases
(a) Anoxia (aeration problem)
(b) Gas Bubble Disease
(c) Spring Flush (Acidosis/Alkalosis)
(d) Chemical poisons (insecticides)
(e) Miscellaneous includes sunburn, nutritional, trauma and predation.
Infectious Diseases
(a) Viral
The following major viral diseases of fish are exotic to Australia:
(i) Infectious pancreatic necrosis (IPN)
(ii) Viral haemorrhagic septicaemia (VHS)
(iii) Infectious haematopoietic necrosis (IHN)
(iv) Channel catfish virus disease (CCVD)
One viral disease lymphocystis does occur in Australia. This viral infection results in the formation of multiple lymphocystic lesions over the body.
(b) Bacterial diseases are responsible for heavy mortality in both wild and cultured fish. The actual role of these micro-organisms may vary from that of primary pathogen to that of an opportunist invader of a host rendered moribund by some other disease process.
Water, especially where organic loads are high, is an environment in which many genera of bacteria can thrive and it has been shown by several workers that the normal bacterial flora of fish is a direct reflection of the bacterial flora of the water in which they swim.
(i) Bacterial haemorrhagic septicaemia characterised by septicaemia due to Aeromonas hydrophila. The most important bacterial disease in Australia fish farms.
(ii) Columnaris Disease (Flexibacter columnaris)
(iii) Vibriosis
(iv) Tuberculosis
(v) Furunculosis. This disease caused by Aeromonas salmonicida, is exotic to Australia.
(c) Fungal Diseases. The most significant fungal diseases of fish are the mycoses of the integument (usually skin, fins and gills) associated with members of the order Saprolegniales.
(i) Saprolegnia (or fungus)
(ii) Ichthyophonus (I. hoferi)
(d) Protozoan Diseases. There are numerous protozoan diseases:
(i) Costia necatrix (Ichthyobodo)
(ii) Ichthyophthirius multifilis ("Ich" or White Spot)
(iii) Myxosoma cerebralis (Whirling Disease) - this is exotic to Australia.
(e) Parasitic
(i) Flukes (Gyrodactylus and Dactylogyrus)
(ii) Crustaceans
- Lerneae (Anchor Worm)
- Argulus (Fish louse)
NEOPLASIA
Fish are subject to tumours similar to those found in mammals, including man. Interest in fish tumours as an aspect of comparative pathology has been stimulated by the current awareness of the potential hazards of pollutants in the aquatic environment.
Lamellar telangiectasis (or aneurysm) is a characteristic pathological change of the gills, associated with physical or chemical trauma.
EXAMINATION OF FISH
1. Observe the fish in the facility without disturbing them. Note where they are both in the pond and in relation to the rest of the group. Check on their behaviour, are they bright and alert or depressed, are they swimming normally or whirling, flashing, etc. Get the farmer to feed them to check on appetite and feeding behaviour. Look for fast respiration and faecal casts (i.e. long white strands trailing from the anus) etc.
2. Net out some affected fish for closer examination. View them in water. If possible look along the body, head to tail looking for abnormal colouration, areas of loss of 'shine' and mucus production. These areas will be grey-white. Look for white spots this may be Ichthyophthirius infection, i.e. Ich, or Itch. Lesions or wounds may be secondarily infected with fungi and this is best seen with fish in water where the hyphae stand out with the support of the water. Check on the general condition of the fish. Are the scales raised, the fins frayed or the abdomen distended?
3. Take the fish from the water to an area suitable for post-mortem, i.e., with good light and able to be easily cleaned and disinfected afterwards. Give it a thorough external examination. Look in the mouth, under the gill covers (the operculum), check the eyes and vent. You will be looking for pop-eye haemorrhages, lesions, parasites, etc. The skull may be sunken behind the eyes or the jaw deformed, this would be a sign of Whirling Disease.
POST-MORTEM EXAMINATION
Samples should be collected from each affected tank or lot. Select any suspect fish, i.e. symptomatic or moribund specimens, as well as non-symptomatic individuals. Fish must be alive when collected. Rapid rate of autolysis of fish tissues compared to that of mammals means that they must be handled rapidly to prevent degenerative changes within the specimen making ultimate diagnosis either unreliable or impossible. Fish may be killed by pithing, decapitation or overdose of anaesthetic. (Quinaldine at 1:20,000 proves lethal within 5-10 minutes).
THE PROCEDURE
With a scalpel make an incision into the abdominal cavity in the mid-line just anterior to the vent. Take care not to penetrate the gut as this will result in bacterial contamination of the abdominal contents. Use scissors to extend the cut along the mid-line through the heart cavity and into the mouth. Divide the lower jaw at the mid-line. Check for fluid in the abdominal and heart cavities.
Now cut away the left abdominal wall by making an elliptical cut from the vent area forward following the path where the abdominal wall meets the roof of the abdomen. This cut will finish behind the operculum. This latter is cut away on both sides.
Examine the abdominal organs and the gills. note any abnormalities in colouration, size or shape, look for haemorrhages, reddening etc. Make a point of looking for parasites on the gills and note their condition, i.e., are they pale, with haemorrhages or necrotic areas, are they frayed at the tips, etc.
Three or more fish should be examined from a group of sick fish. Select both moribund and less severely affected individuals so you get a range of lesion stages.
SPECIMEN COLLECTION
This should include both chilled fresh and formalin fixed tissue. The fresh material must be kept chilled in transit to the laboratory wherever possible sample live or moribund fish. This is because many external parasites will quickly leave the dead host. Also because fish enzymes continue to work over a wide range of temperatures, fish tissue autolyses extremely rapidly making it valueless for histology and producing favourable conditions for the penetration and growth of contaminating bacteria.
BACTERIOLOGY AND VIROLOGY
Wet tissue taken in an aseptic manner should be kept chilled (i.e. 4°C) until it reaches the laboratory. Disposable scalpels are useful for taking these specimens. However, flame sterilisation or cleaning and disinfection followed by thorough washing in boiled water would be a satisfactory method. If too many contaminating bacteria are present these will overgrow the bacteria or viruses causing the disease and your laboratory result will be negative.
Where fish are small send them whole and 'unopened' and, of course, chilled.
Small fish can be sent to the laboratory alive where transport permits. To pack them put the fish in a plastic bag within another outside (to reduce possibility of leakage). Use a small amount of water with 10 times the water volume as air or oxygen above, Twist the bag tops, fold over, and clamp with 2 or 3 rubber bands. Put in a polystyrene box or strong cardboard box. Cool pads or ice in a sealed bag may be used especially in summer.
TREATMENT OF FISH DISEASES
After making your diagnosis don't rush straight into treatment. First consider the following:
1. Changes in management
In very many cases fish disease is the result of poor management. If the management faults are not corrected then the disease will return or persist. In most cases a change in management is all that is required.
2. Cost of treatment
Assess the value of the fish to be treated and relate this to the drug and labour costs. It may be necessary to modify the form of treatment due to cost restraints. There may also be problems with discharge of chemicals into supply stream.
There are five basic methods of treatment:
1. Dip treatment. Solution containing medicament in a separate container and fish caught in net and dipped into solution for requisite time.
2. Bath treatment. Ponds, aquaria can be utilised as a bath by adding therapeutic agent for required time and water gradually added to remove agent. Aeration necessary. Water level may be dropped to half depth prior to treatment. Separate bath treatments are best for ornamental fish. Prevents loss of plant and staining of glass, i.e. methylene blue/malachite green.
3. Flush treatment. Chemical added to inlet of pond and allowed to flow through pond. Can be dangerous, difficult to control concentration.
4. Incorporation in feed. Chemical or antibiotic added to feed - difficult to get sick fish to eat and dosage control inaccurate although easiest method.
5. Individual treatment. For valuable fish only, such as brood stock and ornamentals - intramuscular injection or intraperitoneal.
Therapeutic Agents
(a) External parasites - formalin both 160-250 mg/litre (40% formalin). 15 mg/litre indefinitely. Water hardness important - soft, acid water more toxic.
(b) External fungi - Saprolegnia - malachite green.
(c) External protozoan - Ich - malachite/formalin - 1-2 mg/litre malachite green; 167-250mg/litre formalin for 1 hour.
(d) Fluke - Trichlofon (Neguvon) 0.25mg active drug/litre for 48 hours.
(e) Antibiotic - incorporation in feed - Terramycin fed rate of 5g active drug/100 kg fish per day for 2 weeks. Need veterinary surgeon prescription. Bath - 15-50 mg/litre for 2-3 days.
Summary of diseases of crayfish
| Viral disease | None reported in Australia |
| Bacterial disease | Pseudomonas fluorescens moderate pathogen Citrobacter |
| Fungal disease | None reported in Australia, Aphomyces astaci reported in Europe, also Dictyuchus and Fusarium |
| Protozoan disease | Thelohania sp. |
| Metazoan disease | Few - not important |