Diagnosing Fish Disease

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Diagnosing Fish Disease

It is not possible to diagnose fish disease purely on the basis of  behaviour or physical changes. These “clinical signs” that something is wrong can be useful as pointers to possible problems. However, many of these clinical signs can be caused by more than one disease condition, which is why further examination and tests are essential to make a definite diagnosis.  Without any examination or tests, it is simply guesswork as to what is wrong! 

The table below indicates some of the most common disease signs, and details further steps to take to help confirm a diagnosis.

Clinical Signs

Possible Causes

The sudden death of many fish over a short period of time with few preceding symptoms. Surviving fish hang around the water surface / stay on the bottom and may lose equilibrium. They are lethargic and don’t eat.

1) Acute poisoning. Water should be tested for core parameters and fish examined for signs of severe parasite  or bacterial infection. If possible, post mortem recently dead fish.  If these are negative carry out a 75% or more water change on the presumption of some (as yet, unknown) toxin.  Send water sample away for toxicology testing.

2) Low dissolved oxygen levels. Measure levels in pond/ filter early in morning when DO levels at at their lowest

3) Peracute bacterial disease. Diagnosis would require post mortem and aseptic bacterial sampling of organs

Fish rub against solid objects and “flash” when they turn to rub their sides giving a flash of silver colour as their undercarriage is exposed. They may also leap out the water.

This indicates some form of irritation of either the skin or gills. 

1) This could be caused by raised levels of ammonia, nitrite or high/low pH. Widely fluctuating diurnal shifts in pH can also cause irritation. Check all water parameters. 

2) Irritation can also be caused by ectoparasites such as flukes, Trichodina, white-spot etc on the skin or gills. Do a skin scrape of 2-3 fish. Examine the gills. 

Fish swim normally but appear to be breathing heavily. (This can be judged by operculum movements).

Fish appear to be gasping, they may crowd water inlets

1) Hypoxia. Dissolved oxygen levels may be low, test levels early in morning. 

2) Poor water quality causing gill hyperplasia, excess mucus production or nitrite toxicity. Check ammonia, nitrite levels and pH.  Check that pond and filters are clean

3) It could indicate the early stages of gill disease. If the condition persists it is important to examine 2-3 fish for gill parasites or gill disease.

4) Gill parasites. Take gill and skin scrapes

5) Anaemia. Gills look pale

Individual fish stay apart from others. Breathing is normal.  They may refuse food. They may have “cloudy” or grey areas on the skin. There are no other physical signs.

Lethargy and inappetence are common symptoms of many diseases, so further investigation is essential

1) Fish may be suffering from a parasite problem. Take a skin and gill scrape. Examine the gills. 

2) There could be internal disease problems 

3) Early stage of systemic bacterial infection. Isolate and observe

4) Test core water parameters

Several fish are lethargic. Fish are not eating or eating very little.

Fish are skittish and shy. Some fish may have split or ragged fins

Lethargy and inappetence are common symptoms of many diseases, so further investigation is essential

1) Poor watery quality. Check ammonia, nitrite, pH. 

2) High levels of organic pollution.

3) Ectoparasite infestation. Check 2-3 fish for parasites or gill problems

4) Early stage of bacterial disease. Examine fish for lesions, inflammation and ulceration.

Overproduction of gill and/or skin mucus. Grey slime/ trailing mucus. May be accompanied by rubbing/flashing, heavy ‘breathing’ and/or lethargy.

1) Ectoparasites. Take skin and gill scrape

2) Poor water quality. Test core parameters

Skin lesions or ulcers

Reddening or inflammation on the body or fins. Raised scales. Localised swelling. Open wounds or lesions that do not heal within a day or two

1) Physical damage. This will normally present as a  ‘clean’ lesion with little inflammation. Usually heals within days. Always a risk of secondary infection.

2) Local tissue irritation caused by ectoparasite infection. Take skin and gill scrape

3) Bacterial disease

4) Water quality problem. Test core parameters

5) High levels of organic pollution. Check pond and filters for decomposing solids

Focal red or white lesions on the body

Larger ectoparasites such as lice, leeches or anchor worms. These parasites are normally visible with a close examination

Fin rot

Frayed, split or ragged fins. Possibly with whitish edge and some reddening of the fins

1) Bacterial infection

2) Ectoparasite infection. Take skin and gill scrape

3) Poor environmental conditions. Test core parameters and check that the pond and filters are clean

4) Physical damage

5) Over crowding

6) Saprolegnia (fungus) infection

7) Columnaris infection (mouth fungus/cotton wool disease)

Fish has swollen abdomen (dropsy). Possibly raised scales and some reddening on the body or fins. They may possibly exhibit pop-eye (exophthalmos)

1) Viral infection.

2) Systemic bacterial infection. leading to ‘dropsy’ (build up of fluid in the abdomen. Can be differentiated from tumour by shape, symmetry and feel (feels soft and fluidy)

3) Neoplasm (tumour). Can usually be determined by size, shape (usually asymmetric) and palpation (feeling – usually hard)

4) Heart or kidney disease. Can only be diagnosed by post mortem

5) Internal parasites (endoparasites)

6) Genetic disorder

7)  Intestinal blockage. Only established at post mortem

Fish have respiratory problems shown by gasping or laboured gill movements. Gill examination shows areas of necrosis (dead tissue), mottled appearance, heavy mucus.

1) Bacterial gill disease

2) Viral infection

3) Branchiomyces (gill rot)

4) Gill parasites. Take biopsy/ scrape for microscopic examination

Tiny white spots over the skin and fins, as though fish has been sprinkled with salt. Usually with thickened mucus

Ichthyophthirius (white spot). Definitive diagnosis by skin scrape

White or or discoloured ‘cotton-wool’ patches on the skin or fins. Often accompanied by inflammation and skin erosion

1) Saprolegnia (fungus) infection

2) Columnaris infection (mouth fungus/cotton wool disease). Take sample for microscopic examination which would show typical long ‘gliding’ bacteria (Flexibacter sp).

Swellings on the skin

1) Parasitic cysts. Take ‘deep’ skin scrape

2) Physical damage. Examine fish and observe

3) Bacterial infection. Usually accompanied by inflammation and raised scales. Examine fish for inflammation and/ or raised scales.

4) Internal tumours

Growths on skin / fins

Tumours or viral infections such as;

1) ‘carp pox’,  which resembles melted wax poured over the fins or skin

2) papillomas, ‘warty’ types of growths, can be quite large, smooth or cauliflower-shaped, white, red or pink

3)  lymphocystis, various sized, white to pink masses on the skin and/or gills. Diagnosis is by microscopic examination of ‘wet mount’ taken from the lesion.

* There are no treatments for these growths. They are usually benign and often self-resolving. There is always the risk of secondary bacterial infection

Corneal Opacity (cloudy eye)

1) Physical damage

2) Nutritional deficiencies

3) Poor water quality

4) Bacterial infection

Skeletal deformities / kinked spine

1) Electrical shock. Faulty electrics or lightening strike

2) Use of organophosphates

3) Nutritional imbalance

4) Genetic abnormalities

5) Muscular damage / physical damage

6) Toxins

Unless there is an obvious known problem, it is difficult to establish the cause. A radiograph (sort of x-ray) would assist for skeletal problems

Loss of equilibrium with fish unable to stay upright. Fish may be able to maintain position when swimming, but turn over when they stop swimming.

1) Swim-bladder disorder / disease  caused by viral or bacterial infection, constipation (mainly fancy goldfish) or other organ disease.

2) Internal organ disease, i.e. kidney / intestinal disease or blockage.  Usually only established at post mortem

Lethargy, weight loss, chronic mortality. There may be worms protruding from anus

1) Internal parasites (endoparasites). Requires blood sample, wet mounts from dissected tissues and  wet mounts of faecal sample