There are over 2000 species of ornamental fish kept as pets. However, whilst the diversity of species kept is large, the most commonly seen ornamental fish in veterinary practice are either goldfish (Carassius carassius) or koi (Cyprinus carpio). Similar to the mammalian species veterinarians treat, understanding the environment the animal comes from, or is kept in, significantly reduces the number of likely diseases. The environment is so important in ornamental fish that all clinical cases require an examination of the environment, including measurement of water quality parameters. Incorrect or variations in water quality (e.g., overstocking, overfeeding, temperature variation, variation in pH) are the major causes of infectious disease, and it might be better that we refer to infectious diseases of ornamental fish as "dis-ease".
Dis-ease is an appropriate term since many causes of infectious disease are ubiquitous in the environment (e.g., aeromonads and Saprolegnia). Infectious disease becomes manifest in the fish due to stressors, most commonly water quality or sometimes management procedures such as transportation. In koi, large aeromonad ulcers can occur within hours and White Spot/Ich (Ichthyophthirius multifiliis) very commonly occurs after transportation or temperature changes despite no previous signs. Even relatively species-specific diseases such as Malawi bloat (African cichlids) appear to lie dormant until the fish is stressed and then can cause death within 72 hours. Thus, it is not uncommon for an apparently healthy population of fish to suffer an acute outbreak of infectious disease. However, in all cases the workup of the case is pretty much the same. You should always take samples from a gill snip, skin scrape and fresh faeces (fish often defecate when sedated) which when coupled with water quality test will provide you with a working diagnosis in over 80% of cases.
As with any other species, infectious diseases of ornamental fish are investigated by taking an appropriate history:
Pond or tank, indoor or outdoor, freshwater or marine?
Volume and stocking density?
Recent introductions or not?
Symptoms seen and timespan?
Water quality as measured by the owner (temperature, pH, NH3, NO2, NO3 as a minimum plus if possible GH, KH and DO2 and any other readings they might have)?
Any other relevant environmental factors?
Once the history is taken, you can decide whether or not to see the fish in situ or bring an individual or representative number to the surgery to be examined. Overall, it is better to see the fish in the pond/tank than a visit to the surgery for infectious conditions, as this minimises any stress caused and also allows you to see the environment the fish are kept in, which may lead to further questions.
Any clinical examination should include a visualisation of the fish looking for increased opercular rates (indicative of a respiratory problem), obvious lumps, bumps (possible granulomas - fish Tb) and ulcers (aeromonads/pseudomonads), any fin or tail erosion (possible Flexibacter infection), is the colour dull (excess mucous or slime possibly due to parasites), are the white areas erythematous (possible septicaemia) and any other lesions apparent (cotton wool indicative of fungal infection). It is also worthwhile at this stage taking your own water quality measurements, as these can frequently be different to the owners for a variety of reasons. If the fish is seen at the hospital/surgery, then you should ensure the client brings along a water sample.
Once you have examined the fish, then you need to take a skin scrape (looking for parasites such as Ichthyophthirius, Dactylogyrus, Trichodina, and Ichthyobodo) and a gill snip (again looking for parasites or thickening/clubbing of the gills possibly indicative of Flexibacter or KHV). If something like Hexamita is considered, then taking a faecal sample will be useful. All these samples can be examined under the microscope as fresh wet mounts and will provide good additional information as to what is happening.
In virtually all cases, you should now have a clear understanding of the problem and the possible cause. If a virus is suspected, then you will likely have to sacrifice a fish (preferably at least 2–3) to obtain samples for culture. In the case of koi herpes virus also called Cyprinid herpesvirus 3 (CyHV-3), then pond-side PCR testing is possible, although as this is a listed disease in many countries further laboratory testing is desirable.
Treatment will clearly be dependent on the infectious disease seen. If a water quality problem is seen, then this must be corrected prior to treatment; otherwise, there is a strong possibility of treatment failure. Where antibiotics are considered, it is best to carry out culture and sensitivity. For example, aeromonads are inherently resistant to penicillins and cephalosporins and many disease-causing bacteria in fish have a broad resistance profile due to inappropriate use in the past. Whilst waiting for further laboratory results, first-aid treatments might include the use of salt in freshwater systems or the use of water disinfectants which can help control the disease prior to providing an actual treatment. Varying the temperature (usually increasing) can be used in some diseases, most notably Ich. As with other species, there are few if any antiviral treatments available, although there are reports of antivirals being used with mixed results.
The above general considerations regarding infectious diseases in ornamental fish will allow the clinician to reach a diagnosis regardless of cause, but what are the commonest infectious diseases? As already said, the commonest two species seen in veterinary practice are koi and goldfish. Between these two species, the most commonly seen infectious diseases are one of the Cyprinid herpesviruses, aeromonad ulceration or septicaemia, fish Tb, Ich and Saprolegnia.
Cyprinid herpesvirus 1 - Cause of carp pox which produces lesions that look like melted candle wax in koi. Most prevalent in spring (cooler temperature) and usually self-limiting. Whilst the lesions may be unsightly and in extreme cases cause problems by interfering with the nares or eyes, there are no treatments available. Increasing temperature might help and using salt to control any secondary infections may be of benefit. Control is limited since it can be introduced unknowingly to a population. Over time within a fixed population, the prevalence will decline.
Cyprinid herpesvirus 2 - or goldfish hematopoietic necrosis herpesvirus. Unlikely to be seen in a hobbyist setup although occasionally found at producers' farms.
Cyprinid herpesvirus 3 - or KHV. Only affects Cyprinus carpio, occurs between 18 and 25°C. Morbidity up to 100% and mortality up to 80% (this can vary significantly). The incubation period is 14 days, but if fish introduced in to an infected environment then symptoms may occur within 3 days.
Usually there is severe gill necrosis, and internal pathology is nonspecific. Skin pathology is nonspecific, although mucous "plaques" might be seen, and there is often anorexia present for up to 10 days prior to clinical symptoms. There is no treatment, although a vaccine is available in some countries. Diagnosis can be made by pond-side test.
Aeromonad ulceration - Common in both koi and goldfish. It is caused by a variety of aeromonads (e.g., salmonicida, hydrophila, caviae and sobria). There is some debate as to the initiating cause or bacteria and whether other bacteria such as A. hydrophila are secondary invaders. Treatment consists of debriding the ulcer and surrounding scales, flushing the ulcer with iodophors or other topical disinfectants, possibly treating with a topical antibiotic or systemic antibacterials. Depending on the size of the ulcer, it may need sealed to prevent osmotic damage to the fish. An interesting point to bear in mind when treating ulcers is that fish skin is only one cell thick, and it is an effect of bacterial toxins which prevents the skin from sealing the erosion. If you can remove the bacteria, then the fish will heal naturally and not require any antibiotic treatment. Temperature is also useful in decreasing scarring with water temperatures above 25°C being useful.
Fish Tb - Caused by a variety of mycobacteria (marinum is the most cited, although this may not be correct) and is a zoonotic disease. The prevalence is increasing with greater use of earthen ponds by producers, particularly in goldfish. Up to 30% of goldfish produced may be affected prior to sale to the hobbyist. It is a zoonotic disease, and it is advisable that any affected fish be euthanased.
White spot/Ich - Probably the most common fish parasite in the world and virtually ubiquitous. It can be recognised by seeing grains of salt scattered across the body of the fish. Skin scrapes will confirm. Treatment is usually by use of a proprietary remedy, although the client must complete the course correctly. How long treatment is necessary depends on water temperature.
Saprolegnia - A fungus causing "cotton wool disease". Often seen on dead and decaying fish and is an indicator of poor water quality. The fungal hyphae can be seen on skin scrape, and treatment is improving the water quality.