Fungal Disease
Fungi are a group of organisms that require living or dead matter for growth and reproduction. In most cases, fungi serve a valuable ecological function by processing dead organic debris. Fungal problems appear as cotton-like tufts on the body or fins of fish. Fungal infections are rare in a well-kept aquarium and are very seldom primary causes of disease. In most instances, fungus infections are secondary or tertiary infections. Unless the primary problem is solved, even an effectively treated fungal infection is likely to return. Most fungal infections of rainbowfishes and their eggs are probably associated with the fungi genera Saprolegnia and Achlya, although other groups are undoubtedly also involved. Achlya is commonly found on wild-collected rainbowfishes, which have had skin/scale damage during the collecting process. Epizootic Ulcerative Syndrome or ‘red spot’ disease has been identified in rainbowfishes from a number of river systems in the Northern Territory. This condition is frequently fatal to juvenile fish.
Mycologists and others have given much study to aquatic fungal diseases infecting fish species, but the results have not always been in agreement. They comprise tens of thousands of species and an extensive number are associated with health problems in fish. The name saprolegniasis is used because it relates particular disorders of fish much more succinctly to fungal infections than do such names as “fungused fish”. The identity of the various organisms associated with fungal diseases in fishes has long been a problem.
Fungi are a group of organisms called heterotrophs, meaning that they obtain their energy and carbon compounds from organic nutrients. They are saprophytic (feed on decaying organic matter) and parasitic organisms that require living or dead matter for growth and reproduction. Unlike plants, they are incapable of manufacturing their own nutrients by photosynthesis. Fungi are present everywhere – in most cases, they serve a valuable ecological function by processing dead organic matter.
Almost every freshwater fish will be exposed to at least one species of aquatic fungi during its lifetime. All fungi produce spores - and it is these spores which readily spread disease. The spores of the fungus are always present and will take any opportunity to grow and develop. Fungal spores are freeswimming in water and can therefore be introduced or transferred between aquariums. Aquatic fungal infections are considered difficult to prevent and treat, and are reported to be second only to bacterial disease in importance. Fungal infections are generally restricted to chronic losses. The diseases they cause are almost always external in nature, rarely becoming systemic to include internal organs.
It is still widely believed that fungal infection of fishes is largely a secondary development. The hyphae (fungal branches) start their growth on skin or gill lesions that are usually initiated by conditions such as bacterial infections, poor water quality, injuries associated with handling and social interaction, sudden changes in temperature, infestation by parasites and nutritional deficiencies. If these factors weaken the fish or damage its tissue, fungus may infest the fish and can, without treatment, lead to the death of large numbers of fish. However, there is supporting evidence that some fungi may affect healthy fish in certain circumstances. They are commonly known to colonise plant and animal debris in freshwater.
The most commonly identified fungal pathogens of fish are Oömycetes (commonly called water moulds). Oömycetes are like fungi. They have the same filamentous, branching, indeterminate bodies and absorb food by excreting digestive enzymes and absorbing the resultant mixture (absorptive nutrition). However, Oömycetes are not considered to be ‘true fungi’ taxonomically, but have been placed in the phylum Oomycota. Within this phylum is the family Saprolegniaceae, containing among others the genera Achlya, Aphanomyces and Saprolegnia, with some species being pathogens of fish, crustaceans and plants.
The Oömycetes are true aquatic organisms, largely saprophytic and are considered ubiquitous in freshwater systems throughout the world. Although little quantitative information is available, their cosmopolitan distribution and ability to colonise a wide variety of substrates suggest a role in the decomposition of organic materials in freshwater ecosystems.
Fungal infections of fish by water moulds are widespread in both wild and captive fish populations. It is known that the members of at least six genera are natural parasites of fish, fish eggs and crustaceans, including Saprolegnia, Achlya, Aphanomyces, Pythium, Leptomitus and Allomyces.
The most widespread species parasitising fish include members of the genera Saprolegnia and Achlya. Saprolegnia is ubiquitous in freshwater ecosystems and is the main genus of water moulds responsible for significant mycoses of freshwater fish and eggs. Saprolegnia parasitica is common on fish eggs and on fish skin. Saprolegnia ferax and Saprolegnia parasitica most frequently cause death of fish both in aquariums, and their natural environment. Saprolegnia torulosa has been encountered on the eggs of freshwater fish species. Saprolegnia infects the eggs by adhesion and penetration of the egg membrane, and can spread from dead eggs to live eggs. In warmer climates the role of Saprolegnia is largely taken over by species of Achlya. Achlya is commonly found on wild-collected rainbowfishes, which have had skin or scale damage during the collecting process.
Saprolegniasis affects all stages in the life cycle from eggs through to adult fish. The fungus produces long filamentous strands called hyphae, which grow on the surface of fish, eggs and organic material. The fungus looks like greyishwhite wool-like growths of different sizes. New growths may be difficult to distinguish; the older ones are usually greyish-green. Microscopically, individual hyphae are evident that are nonseptate and about 20 microns in diameter. Older segments of hyphae often terminate in zoosporangia containing zoospores.
Reproductive motile spores are released from the ends of the hyphae into the water and these quickly find other sites to colonise. The rate of development depends on water temperature and the condition of the fish. Up to 40 or 50% of the body surface may be covered and the gills, nasal openings and eyes may be infected. The tissue degeneration resulting from the invasion of the fungus disrupts the osmotic balance of the fish. Diseased fish become increasingly lethargic and lose equilibrium shortly before death. Mortalities can range from 10 to 50%. As the fungus radiates away from the focus of the infection, the hyphae penetrate and destroy the layers of skin, and in some cases extend into the muscle. Very severe cases have been reported where the fungus blocked the pharynx of first feeding fry and grew out over the gill lamellae preventing feeding or normal respiratory functioning.
Members of the genus Pythium are soilor water-dwelling organisms. More than 200 species of this genus have been described. They usually live as saprophytes, but several species have been reported to cause disease in plants, fish and crustaceans. Pythium rostratum invades the eggs of many freshwater fish species. The genus Aphanomyces contains two specialist aquatic animal pathogens, Aphanomyces astaci and Aphanomyces invadens.
The latter species is associated with the tropical and subtropical fish disease Epizootic Ulcerative Syndrome (Red Spot Disease). In Australia, red-spot disease has been reported in the Northern Territory, New South Wales, Queensland and Western Australia. It begins as a small area of reddening over a single scale, which subsequently spreads to involve a number of adjacent scales; this is the characteristic ‘red spot’. As the condition progresses, the ‘red-spot’ expands and deepens, giving a deep ulcer, which sometimes extends into the abdominal cavity.
Some fish, especially Lates calcarifer develop unilateral or bilateral cloudiness of the cornea; these changes in the eye may or may not be accompanied by lesions in the skin. Some cases of red-spot disease heal spontaneously, but many affected fish, especially juveniles, die. Aphanomyces invadens has been identified in rainbowfishes from a number of river systems in the Northern Territory and Queensland.
While precise identification of fungi species from lesions on fish requires a considerable level of familiarity with taxonomy of the aquatic phycomycetes, the detection of a significant level of fungal infection does not require rigorous classification. Once recognised, the growth of aquatic phycomycetes on lesions is difficult to confuse with any other aquatic disease.
Treatment Avoidance of exposure to the disease is the primary method of prevention. The most effective strategy for controlling and preventing fungal infections is good fish keeping practices. It is imperative that aquariums are maintained under conditions conducive to good health. Well-nourished fish reared in highly favourable environmental conditions will be resistant to most pathogens.
Your veterinarian can make a diagnosis of fungal disease based on microscopic examination. Diagnosis is quick, accurate and generally inexpensive. A biopsy sample is examined using a microscope and the presence of thick (10–25 µm), nonseptate, branching hyphae (fungal stalks) confirms fungal infection. A number of other pathogens and saprophytes (opportunistic pathogens) including algae, crustaceans, helminths (worms) and protozoa commonly colonise the fungal tuft.
After evaluating the environment and history of your fish, your veterinarian may or may not decide to implement a treatment protocol. Several chemotherapeutic (drug treatment) options are available. Some lesions are treated topically with a disinfectant like povidone iodine after the water mould and necrotic tissue have been surgically removed. If the infection is not severe, many fish will heal with supportive care (good nutrition and clean water). In severe cases, death is frequently, due to impaired osmoregulation and the fish's inability to maintain fluid balance.
For many years infections have been largely controlled with malachite green. Unfortunately, the potential teratogenic or mutagenic properties of malachite green have resulted in its limited or curtailed use in many countries. The search for alternative chemical treatments or other means of controlling fungal infections has resulted in many investigations being conducted. However, to date, only a limited number of chemical compounds show any potential as fungicides and none are considered as effective as malachite green.
Malachite green is extremely toxic to Saprolegnia parasitica, as this species is unable to infest new seeds when treated with as little as 1 mg/L for one minute. This undoubtedly explains why malachite green is very effective in controlling certain fungus infections, many of which are due to this particular pathogen. Malachite green treatments for 24 hours at 5 mg/L are sufficient to control most fungal infections. However, when fish are exposed for longer periods, even to much lesser concentrations, malachite green can be highly toxic to some fish. Malachite green has also been recommended for use concomitantly with another antifungal agent, formalin. However, the chemical evidently is not without its own harmful elements. Some strains of water moulds implicated in fungal infections supposedly are more resistant to malachite green than are others.
Another common dye, methylene blue, is also effective against fungal infections of fishes and may be used as an alternative to malachite green. It is particularly effective against Saprolegnia by applying 3 mg/L in a long duration bath. Methylene blue can be used for the treatment of fungal infections on all ages of freshwater fishes at 2 to 3 mg/L in a permanent bath. It is safe for use with fish eggs and fry. Methylene blue has a wide safety margin and is non-toxic when used as recommended. Fish tolerate relatively high dosages without side effects. However, it should not be used in recirculation systems that utilise biological filtration, as it will interfere with the normal biological processes of nitrifying bacteria. It can also interfere with normal plant growth. Also, be aware that this material will stain almost everything with prolonged contact.
Formalin (a solution of 37% formaldehyde) treatments are an alternative to malachite green but are not as effective, and it may have detrimental effects on the fishes it is intended to cure. The dose has to be adjusted according to water pH. Low doses should be used at low pH and higher doses at higher pH values. Oxygen depletion of the water is rapid at high temperatures. However, formalin is for specialist use only. It is inadvisable to use this compound where other treatments are available.
A successful treatment formulated by Dr. Gerard Bassleer, a wellknown fish disease expert, in 1983 consists of the following combined ingredients:
Formaldehyde (37%) = 1 litre [or 100 ml] Malachite Green (oxalate) = 3.7 gm [or 0.37 gm] Methylene Blue = 3.7 gm [or 0.37 gm]
Dosage: 1.0–1.2 ml/100 litres water (25 drops/100 L).
Change the water (50%) after one day treatment and add another dose. This medication can be more toxic in soft acid water, and also at higher temperatures
The use of ordinary salt (or sea water) was among the first of the methods proposed to combat fungal disease. Often, the application of salt either directly onto the diseased part of individual fish, or as a solution in which to bathe the fish. Salt can be used at 10g/L for 20 minutes for young fish and 25g/L for 10 minutes for older fish. A continuous well-aerated salt bath of 2–5g/L may assist in the recovery from fungal infections. However, there appear to be significant differences among species and possibly families as well in the tolerance of the larval and fry stages to salt treatment.