Diseases caused by ciliate infusoria.
Invasion refers to the introduction and subsequent development of animal parasites in an organism. For most pathogens of invasion diseases, the conditions of parasitism on aquarium fish (temperature and hydrochemical regimes, ecological factors) are optimal, and their development in indoor water bodies continues intensively year-round. Fighting such a huge number of parasites would be unthinkable were it not for one circumstance: only certain species of parasites pose a danger to a specific animal species, while against the rest they have an innate or acquired immunity (resistance).
Innate or specific immunity is the resistance of a particular animal species to certain types of parasites, which is inherited. For example, the goldfish compared to other aquarium fishes is least resistant to cryptobiosis, but most resistant to ichthyophthiriosis. Acquired immunity arises only in relation to a disease that the animal has suffered from previously and which, as a rule, is not inherited. An example is ichthyophthiriosis, after which fish develop relative immunity; upon re-infection, the disease is much milder than the first time.
However, there is also the so-called non-sterile immunity — a peculiar resistance where the parasite does not die in the host organism, but does not reproduce either. This situation can persist for quite a long time, until the parasite dies of old age. If during this time the body’s defenses are weakened by any other harmful factor (stress, secondary infection), active reproduction of the parasite and the onset of the disease are likely.
Parasitic diseases. This group of diseases is caused by various parasitic protozoa, infusoria, fluke worms, and parasitic crustaceans. They settle on the surface of the fish’s body (the fish’s integument becomes covered with a mealy gray or golden coating), on the gills, and some parasitize inside the fish’s body. The greatest damage to aquarium keeping is caused by the infusoria Ichthyophthirius multifiliis. The parasite passes part of its cycle under the host’s epidermis, and for reproduction it ruptures the epidermis and enters the water, resulting in massive infection of the fish. On a sick fish, rashes resembling semolina or salt crystals are visible.
Ichthyophthirius. Protozoan parasites Ichthyophthirius are responsible for causing the disease that is probably the most well-known disease of aquarium fish. This is ichthyophthiriosis. There is only one species of Ichthyophthirius, namely Ichthyophthirius multifiliis, although the existence of several varieties has been established. These organisms have a multi-stage life cycle that includes both parasitic and free-living forms.
Considered an ectoparasite, at the infectious stage of development it lives not so much on the skin as in the skin of the host. Thus, it can be classified as an endoparasite, although it does not affect subcutaneous tissues. It also attacks the gills, cornea of the eyes, and the oral cavity. These protozoa tolerate a wide temperature range and can therefore easily be transmitted from cold-water fish to tropical fish, and vice versa.
Trichodinella, Trichodina, synonym – Cyclochaeta. Belong to the family Urceolariidae. Protozoa, peritrichous infusoria. Disease Trichodiniasis. Pathogen — several species of peritrichous infusoria parasitize aquarium fish, most of which are Trichodina — a round or dome-shaped ciliate infusoria. The infusoria is equipped with sharp processes arranged in a circle, with the help of which it attaches to the skin and gill filaments of the fish, and cilia all over the body, thanks to which it moves. They can live on the body of fish as harmless symbionts. Under certain circumstances, they become parasites causing excessive skin mucus production. A massive infection must be preceded by poor fish condition, unfavorable maintenance conditions, or bacterial infections.
Trichodina domerguei forma acuta, Т. reticulata, Т. pediculus, T. nigra are localized mainly on the surface of the fish’s skin. At the same time, they can also be found on the gills. Т. modesta and Т. mutabilis predominantly parasitize the gills. Often several species of infusoria can be found on the same fish. Some infusoria from the genus Trichodina and other related genera parasitize fish, but when it comes to treatment, exact identification of the parasite species is not mandatory. The listed pathogens of fish trichodiniasis are ectoparasites. Its reproduction occurs at the site of localization by simple transverse division into two daughter cells. Some individuals spend most of their life on the host’s body, but are able to survive without fish for up to two days. In the parasitic stage, they feed on the skin and gill tissues of their host, and if there are many of them, they can cause serious damage.
Severe parasite infestations can cause respiratory problems, forcing the tissue of the gill arches to produce excessive mucus. It enters the aquarium along with food, plants, and soil from a water body where fish live, as well as with fish, plants, water, and equipment from an infected aquarium. Therefore, transmission can occur both through water and by direct contact with an infected fish.
Chilodonella, family Clamidodontidae, order Holotricha. This is a genus of ciliate infusoria, one of many genera of protozoan parasites that can cause excessive skin mucus production in fish. These are obligatory parasites, meaning they can only survive on a host. Chilodonella cyprini and Chilodonella hexastichus. Causes the disease Chilodonelliasis. They reproduce by binary fission. Lives on cold-water fish. At a temperature of 25°C, reproduction stops.
Damages the skin and gills. They feed mainly on dead skin cells. In small numbers, these parasites cause almost no harm, but if the fish’s immunity is weakened due to unfavorable environmental factors or if there is heavy organic pollution in the aquarium, the parasites can multiply rapidly and attack healthy tissues. Strong fish do not die from it, while injured ones are massively affected and die. Under unfavorable aquarium conditions, cysts remain viable for a long time without fish.
Apiosoma piscicolum (formerly Glossatella piscicola). Family Scyphidiidae. This is a genus of stalked ciliate infusoria that usually live freely, but sometimes settle on the skin, 
fins, and gills of freshwater fish. The body of the parasite resembles a goblet on a long stalk. The upper, wide edge consists of three rows of cilia fused into a membrane. The rest of the parasite’s body surface is smooth. The lower, narrow part of the body forms a sole, which the apiosoma inserts into the surface epithelium of the fish. Apiosomas reproduce directly on the fish by direct binary fission. The optimal water temperature for the parasite is 22°C. Its reproduction is also stimulated by organic pollution of the aquarium. Disease Apiosomiasis, Glossatellosis.
These infusoria prefer oxygen-rich, organically polluted cold water. Even with severe infestation, they do not harm fish. Many are species-specific and occur on only one fish species. It is believed that these are not true parasites, but symbionts, since they do not harm fish or feed on their body tissues. The main pathogenic effect on the fish consists of the deformation of epithelial cells and the disruption of their function when the parasite’s sole penetrates the fish tissue.
Large accumulations of these parasites form naked-eye-visible “growths” that should concern the aquarist. Apiosoma parasites sometimes accumulate in large numbers in areas of damaged skin. Sometimes, although very rarely, it happens that a large number of these parasites settling on gill tissues cause impaired gill function and a certain degree of physiological hypoxia.
Epistylis. This is a genus of stalked ciliate infusoria. Disease Epistyliasis. Sometimes they attach to fish, colonize tissues affected by bacteria, and feed on these tissues. Many are species-specific and found only on a single fish species. They are considered symbionts rather than true parasites, but there is evidence that their stalk can penetrate deep under the skin and further exacerbate the damage caused by a bacterial infection (which is considered the root cause of any clinical problem). Even with heavy infestation, they do not harm fish.
Tetrahymena. Single-celled ciliate infusoria. Most species are free-living, although some are known to occasionally cause diseases in fish. This is especially true for T. corlissi, representatives of which are sometimes found in tropical aquariums where they can cause guppy disease in fish. Introduced with live food. Disease Tetrahymeniasis (Guppy disease). Affects the skin and gills of only weakened, previously injured fish. Appears only in case of fungal or bacterial infection of the mucous membrane. Can lead to death.
