Parasitic Infections
A parasite living on the surface of its host is referred to as an ectoparasite but if it lives inside its host it is termed an endoparasite. Some authors also refer to a third group of parasites, mesoparasites, which live in the external openings of an animal’s body e.g., the buccal cavity or the cloacae. Parasites cannot complete their life cycle without spending at least some part of it in a parasitic relationship with another organism. Some animals can also become parasitic “accidentally” when they enter the body of another organism via a wound or other opening e.g., the mouth if they are eaten. Parasites can be permanent, temporary or intermittent, depending on whether they spend all, part or repeated short periods in contact with their host.
Fish are often not the only host in a parasites life. A parasites life cycle often starts with the first larval stage entering the body of an aquatic invertebrate such as small aquatic crustaceans. From the crustacean a new larval form is transmitted to a fish, either through ingestion of the crustacean or directly through the water. Once inside the fish, another larval stage of the parasite encysts itself. Then the fish gets eaten by another fish or by a bird, the final host, inside of which the adult parasite develops. The eggs of the adult parasite are then passed on in the predators faeces, hopefully over water in the case of a bird, and from there infection of a crustacean can start the cycle anew. So, once inside a fish, it is in the parasites interest to somehow alter the behaviour or appearance of this fish in a way that increases its chance of being captured and eaten by the next host in the cycle.
Most studies have looked for evidence of this so-called “manipulation hypothesis”. They found correlations between the level of infection and the frequency of conspicuous behaviours such as flashing, contorting, shimmying, and coming to the surface. The heavier the infestation, the more common the odd behaviours were.
Much of the research has centred around the effects parasites have on their hosts and on the development of methods to treat or immunise fish against infection. As such, for many common parasites of fish, there is a general paucity of information regarding the ecology of parasites in their natural environments. In a confined setting such as a home aquarium or pond, parasites can impair growth and reproduction and cause substantial morbidity and mortality.
Parasites of the gills can cause irritation, leading to hyperplasia and increased mucus production, which may result in decreased respiration and ion-exchanging capabilities. On the skin, parasites can cause physical damage that predispose the affected fish to osmotic imbalances and serve as a portal of entry for viruses, fungi, and bacteria. Such contact may lead to infection which, depending on the species and the virulence of the strains encountered, may have lifethreatening consequences. In the intestine, parasites compete for nutrients and cause ulcerations, inflammation, and emaciation. Several parasites physically carry pathogens and may transfer them to their next host.
Clinical signs of a parasitic infection are related to parasite feeding activity. Pruritus may be exhibited by fish as flashing. Flashing is when the fish rub themselves against the sides or bottom of the aquarium and quickly turns laterally to display the lighter-coloured ventrum. Almost all cases of flashing indicate that they have some sort of parasite. Fish will often injure themselves when they scratch (flash) against sharp rocks or other objects, and will often lose scales in the process. Other clinical signs may include excess skin and gill mucus with epithelial growth, clinical hypoxia (pale gills, rapid opercular movement, gasping for air at the surface). Fish may exhibit listlessness and lack of appetite. At advanced stages of infection, they adopt a vertical hanging position near the water surface with continuous lethargic swimming motions. A “tattered” appearance of the scales and fins may be apparent, with strands of mucous attached. When this occurs, the outer defence against infectious invaders is removed and essential salts and body fluids are lost.
Adequate control of pathogens in an aquarium system and reduction of disease in these systems requires an understanding of where pathogens may be found, how they can be transmitted to fish, and how their numbers may be reduced. Sometimes you may need to medicate a fish that has a bacterial or fungal infection for parasites because the bacterial or fungal infection will often be a secondary infection. A fish seen gulping air at the surface or sitting in the water return stream from a power filter may have gill damage caused by a parasite and may not be a water quality problem. Gill flukes, for instance, are notorious for damaging the gills of fish. You must get rid of the cause before you can get rid of the infection. In addition, understanding the proper use of chemicals to reduce or eliminate pathogens is an essential part of good aquarium keeping.
The only sure way to diagnose whether your fish have parasites and, most importantly, what kind of parasite is present is to have a mucus scraping done by a veterinarian. It’s very important to know what kind of parasite your fish have. Many kinds of microscopic parasites can be treated with common salt (sodium chloride) while others such as flukes, fish lice and anchor worm require a different mode of treatment. Without professional assistance, any diagnosis is purely an educated guess based on yours or someone else’s experience. This usually means that any medications or procedures used in an attempt to cure the disease are chosen on the basis of the type of behaviour and physical appearance of the fish, as opposite to decisions made on the basis of diagnostic tests.
Experienced aquarium hobbyists however, can and do make accurate, presumptive diagnosis’s based on examination and assessment of the clinical signs, and then apply appropriate control measures. Parasitic infections can be among the easiest to identify, and are usually the easiest to control. Protozoan parasites are the most common type of parasites that infect aquarium fishes. Protozoan parasites are singlecelled organisms, many of which are free-living in the aquatic environment. Their reproduction cycle is temperature dependent, which affects treatment. 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 generally indicative of a fishkeeping problem. Many of the parasites proliferate in organic debris accumulated in the bottom of the aquarium or pond. They are easily transmitted from tank to tank by nets, hoses, or on the fishkeepers’ wet hands.
The major ectoparasitic pathogens and the diseases affecting rainbowfishes in captivity are Ichthyophthirius multifiliis (Ichthyophthiriasis), Piscinoodinium pillulare (Piscinoodiniasis) and Trichodina spp. (Trichodiniasis). These diseases probably account for almost 80% of all parasitic infections reported. There is almost nothing known about freshwater parasites that affect rainbowfishes in their natural environment. Langdon et al. (1985) reported mortality of Melanotaenia splendida tatei due to the ciliate protozoan Chilodonella hexasticha in the Finke River in central Australia. However, because of the lack of information on protozoan parasites of rainbowfishes, most cases are not identified or more often, are simply misdiagnosed. 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.
Ciliates such as Tetrahymena, Ichthyophthirius and Chilodonella can cause gill and skin lesions and may give rise to more serious disease if they invade internal organs. Infections often appear as small, white patches on the skin, especially around the eye. Because these organisms can survive off the host, the environment must be cleaned and disinfected in addition to treating the fish. Fish can develop severe osmotic imbalance due to parasitic damage to the skin and gills.
Dinoflagellates such as Piscinoodinium are commonly found on the gills but may also affect the skin, fins, and gastrointestinal tract. In severe infections, the skin may become velvety gold in appearance (velvet disease). Mortality is attributed to severe osmotic imbalance. Trichodina spp. are usually indicators of poor water quality or overcrowding. They can survive off the host for 1 or 2 days and can be transported with plants and other aquarium objects. Although they are not problematic in low numbers, heavy infestations can cause epithelial damage resulting in anorexia, loss of body condition, and low-level mortality.
Tetrahymena is an opportunistic parasite that is a normal part of the microfauna of an established freshwater aquarium. It is a ciliated protozoan which can and does live harmlessly in the background. But it can become pathogenic if the opportunity arises. Normally it feeds on bacteria and organic matter and its numbers will increase dramatically in aquaria with high organic loads. The indications of the disease are lethargy, clammed fins, white spots which are almost identical to those seen in whitespot disease (Ichthyophthiriasis), frayed and tatted fins, raised scales (similar to dropsy) and death. In large numbers it can cause skin and gill irritation that can cause secondary bacterial or fungal infections. Dead fish must be removed promptly because Tetrahymena parasites will continue to multiply in a dead host allowing their numbers to increase greatly and making treatment that much more difficult. Proper aquarium maintenance is the best way to combat this protozoan.
Several larval trematodes infecting fish can cause 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 the blood of the host. Pathogenesis varies with the 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.
Monogenean flatworms such as Dactylogyrus and Gyrodactylus are also fairly common. These parasites may be found on the gills and skin, but Dactylogyrus are generally found on gill tissue. The life cycle is direct; the eggs of dactylogyrids hatch into free-swimming larvae that locate a fish and begin maturation. Gyrodactylids, on the other hand, bear live young that spread to other fish through direct contact. Large numbers of these organisms can be lethal to fish, as both the attachment of the parasite by hooks and/or suckers and its feeding activity cause physical damage to the skin and gills.
Digenean trematodes have indirect and often complex life cycles involving two or more intermediate hosts. They are generally found in the gastrointestinal tract or musculature of the host. As they cannot complete their life cycle without intermediate hosts, these parasites are often an incidental finding in aquarium fish. However, they may cause severe internal damage in large numbers. Intermediate hosts (e.g., mollusks) should be removed from the environment to prevent fish-to-fish transmission. Final hosts (e.g., birds) should be deterred from outdoor ponds.
Lernaea, Ergasilus, Argulus, and Water Mites appear as tiny crab-like creatures. In low numbers, they may cause local inflammation and ulceration that can lead to secondary infections. Most of these parasites are rarely a problem in aquaria but can be common on wild-caught or pond raised rainbowfishes. Some of these organisms can be seen with the naked eye and removed manually.