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There are two broad categories of disease that affect fish, infectious and non-infectious. Infectious diseases are caused by pathogenic organisms present in the environment or carried by other fish. Infectious diseases are broadly categorised as parasitic, bacterial or fungal. They are contagious, and some type of treatment will be necessary to control the disease outbreak. In contrast, environmental problems, nutritional deficiencies, or genetic anomalies cause non-infectious diseases, they are not contagious and cannot be cured by medications.
It is impracticable to cover all the diseases that rainbowfishes are capable of catching in an aquarium. There are many good books readily available on fish diseases and their treatment. However, I will cover some of the common diseases that you, or rather your rainbowfishes, are likely to encounter. Fish disease outbreaks are often complexed, involving both infectious and non-infectious processes. Therefore, appropriate therapy often involves medication and changes in husbandry practises. Often assistance from a qualified veterinarian or aquarium specialist will be required to help you treat disease outbreaks and develop a management program.
Fish health has been a relatively small discipline of veterinary attention in the past because of many factors, the most important of which is the perceived value of aquarium fish. As more people invest in expensive species, such as koi and various reef species, the demand to provide a higher level of care for these animals is increasing. This trend is also evident in the commercial food and bait fish industry, where aquaculture producers are expecting improved standards of care for populations of fish that are worth millions of dollars. With increasing numbers of aquarium and aquaculture operations, veterinarians will be expected to have the abilities and knowledge to diagnose and treat aquatic species and provide a standard of care commensurate with other commonly treated animal species.
Compared to mammalian diseases relatively little (and in most cases very little) is known about individual fish diseases. The biology of most fish diseases (how they can spread, how infective they are etc.), especially those of rainbowfishes, is little understood, and thus assessments of and solutions to the problems they can cause is, at this point of time, based on incomplete information.
Parasitic Infections
Parasites can cause disease in their own right. Most of the commonly encountered fish parasites are protozoans. They all either puncture the body and draw off fluids, browse off layers of the skin, or feed on the mucus on the skin, gills or gut. They may ingest infected materials and carry them to their next host. The wounds they cause also open up a perfect entry point for some bacteria and viruses. Several parasites actually physically carry pathogens.
With practice, these can be among the easiest to identify, and are usually among the easiest to control. Protozoans are single-celled organisms, many of which are free-living in the aquatic environment. Typically, no intermediate host is required for the parasite to reproduce (direct life cycle). Consequently, they can build up to very high numbers when fish are crowded causing weight loss, debilitation, and mortality. Most protozoans do not seem to bother the host fish until numbers become excessive. Uncontrollable or recurrent infestations with protozoans are indicative of a husbandry problem. Many of the parasites proliferate in organic debris accumulated in the bottom of a tank. They are easily transmitted from tank to tank by nets, hoses, or on the fishkeepers' wet hands. Symptoms typical of protozoan parasites include skin and gill irritation displayed by flashing, rubbing, and rapid breathing.
The major ectoparasitic pathogens and the diseases affecting rainbowfishes in captivity are Ichthyophthirius multifiliis (Ichthyophthiriasis), Piscinoodinium pillulare (Piscinoodiniasis) and Trichodina sp. (Trichodiniasis). These diseases probably account for almost 80% of all parasitic infections reported. Because of the lack of information on protozoan parasites of rainbowfishes, most cases are not identified or more often, are simply misdiagnosed. There is almost nothing known about freshwater parasites that affect rainbowfishes in their natural environment. Langdon et al. (1985) reported mortality of Melanotaenia tatei due to the ciliate protozoan Chilodonella hexasticha in the Finke River, central Australia. Rainbowfishes in captivity are also subjected to common fish parasites from ornamental species imported into Australia. However, there is such poor data on Australian fish parasites that there are doubts about what is endemic and what is translocated or introduced.
Several larval trematodes infecting fish causing what is commonly known as "black spot" because of the characteristic, small (about 1~2 mm in diameter) dark brown or black spots which develop in the muscle and on the body, fins, gills and eyes of infected fish. They are easily visible to the naked eye. When the parasite infects the fish it forms a cyst (metacercaria) within the host tissue. The cyst then becomes surrounded by pigment cells, giving it the characteristic dark colour. "Black spot" infection is often found infecting wild-caught rainbowfishes but occur in several species of freshwater fish. Galaxiids and Retropinna semoni appear to be particularly susceptible to infection. There are several species of trematodes which have larval stages which cause black spots; these species have yet to be identified. These trematodes usually will not harm the fish and will not progress unless the fish is consumed by an appropriate primary host animal. The adult trematode is generally found infecting fish-eating birds. There is no practical treatment or control of this parasite available at this time. If the metacercaria are not too numerous, they can be removed safely with a clean scalpel.
The larvae (glochidia) of freshwater mussels are parasitic on fish. They are released into the water by adult mussels and, when a fish passes close enough to disturb them, the glochidia attach themselves to the skin or gills of the fish by means of their barbed valves. Irritated host tissue then grows and forms a cyst over each glochidium. Development from glochidium to small mussel takes about 10 weeks, at which time the mussel bores through the cyst, leaves its host and settles to the substrate. The presence of a glochidia infestation is indicated by numerous white or greyish "bladders" on the gills, skin and fins of the fish. Fish may be severely stress by the attachment of large numbers of glochidia, particularly when the infestation affects the gills and may greatly impair respiration. Glochidia are able to affect most native species but are not known to affect introduced species.
Leeches are occasionally seen on wild or pond-raised rainbowfishes. Leeches resemble trematodes but are much larger and have anterior and posterior suckers. They have a direct life cycle with immature and mature worms being parasitic on host's blood. Pathogenesis varies with number and size of worms and duration of feeding. Heavily infested fish often have chronic anaemia. Fish may develop secondary bacterial and fungal infections at the attachment site. Dips in 3% (30 g/L) saltwater are effective in controlling leeches. Ponds with heavy leech infestation require drainage, treatment with chlorinated lime, followed by several weeks of drying. This will destroy the adults and their cocoons containing eggs.
Bacterial Infections
Bacterial disease is the most common infectious problem for aquarium fishes. Most cases require scientific identification of the bacterial types involved and selection of a specific antibacterial agent under guidance of a veterinarian. The most common bacterial infections in aquariums are caused by organisms such as Aeromonas, Pseudomonas, Mycobacterium and Flavobacterium. They can cause diverse pathological conditions that include both acute systemic and/or chronic diseases. The effects on the fish are as varied as the signs.
External signs of bacterial infection are variable and include shallow reddened ulcers with irregular edges; loss of tail and finnage; missing or raised scales; haemorrhagic areas on the body, in the fins, and on the mouth; protruding eyes (exophthalmia); dropsy; and a protruding and inflamed vent. Dropsy is a distention of the abdomen, giving the fish a "pot belly" appearance. This is a strong indicator of disease problems which may include swelling of internal organs (liver, spleen or kidney), the build up of body fluids, parasitic problems, or other unknown causes. At this stage, the infection has usually become systemic. External lesions expose the body surface to secondary invaders as well as serve as sites for the loss of salts and body fluids. Fish stop feeding and abnormal swimming may become pronounced. When diagnosed early, treatments available from an aquarium fish specialist may help.
Fungal Infections
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 been damaged 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.
Worm Infections
In addition to the usual common disease pathogens, such as external parasites, bacteria, fungus, etc, rainbowfishes may also host internal infestation of parasites. One group of parasites that may cause problems in rainbowfishes is worms. Worms include harmless free-living types, roundworms, tapeworms, and trematodes. Rainbowfishes with internal worms may appear completely healthy, exhibiting no symptoms of infestation. Intestinal parasites are readily removed with various drug treatments. However, others have larval stages that live in lymph ducts and blood vessels, and they are difficult to treat without dangerous side effects. Most worms do not pose a serious health risk to rainbowfishes because they often have complicated life cycles in which the fish may serve as only one of possibly several intermediate hosts.
Picornavirus
Turquoise rainbowfishes (Melanotaenia lacustris) that exhibited whirling symptoms and obvious impairment of the central nervous system were submitted by a breeder for diagnostic evaluation. All were negative for bacterial and parasitic pathogens, Electron microscopy revealed numerous picornavirus particles in the brain. Attempts to grow the virus in cell culture were unsuccessful. No similar cases have been reported since. Picornaviruses have been observed as incidental findings in various freshwater fishes. The significance of their presence is unknown. A similar virus has been found in yabbies in Western Australia with associated mortalities, but the conditions under which disease outbreaks occur are not understood.
© Copyright Adrian R. Tappin
Created July, 2005
Updated December, 2008
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