Fish diseases are divided into two large groups: contagious (infectious and invasive) and non-contagious. Contagious diseases can be infectious, caused by viruses, bacteria, fungi, and algae, and invasive, caused by animal parasites such as protozoa, helminths, crustaceans, etc. Non-contagious diseases are so-called pathogen-free diseases. Non-contagious diseases include those resulting from sudden changes in environmental conditions (oxygen deficiency, water pollution, sudden cooling), metabolic disorders due to improper feeding (alimentary diseases), avitaminosis, trauma, etc.
Contagious Diseases.
Contagious diseases are those transmitted from a sick animal to a healthy one. Many of the pathogens of contagious diseases affect specific species of fish. Infectious diseases are caused by microorganisms. The latter represent an extensive group of microscopically small creatures, which include: bacteria, spirochetes, rickettsia, actinomycetes, viruses, fungi, and algae. They have the ability to parasitize and reproduce in the body of fish. It is difficult to combat contagious diseases caused by plant-origin (infectious) and animal-origin (invasive) parasites.
Invasive (parasitic) diseases caused by animal-origin parasites include: protozoosis (caused by flagellates, sporozoans, infusoria), helminthiasis (caused by parasitic worms), crustaceosis (caused by parasitic crustaceans). Often, pathogens of contagious diseases enter the aquarium along with food harvested from a body of water where fish live. Vectors of worms, infusoria, fungi, and other pathogenic protozoan organisms are many species of planktonic and benthic animals used as food.
Ways of Disease Spread.
Direct contact. Quite often, new fish that may be sick or parasite carriers are introduced into general aquariums. Most ectoparasites and their larvae are equipped with locomotion organs—cilia, flagella, segmented legs—thanks to which they freely move from a sick fish to a healthy one. These include infusoria, flagellates, monogeneans, crustaceans, and leeches.
Intermediate hosts. When introducing mollusks such as *Lymnaea* (pond snails) into the aquarium, which inhabit natural water bodies and are infested with one of the larval stages of trematodes of the genera *Sanguinicola* or *Diplostomulum*, fish can contract sanguinicoliasis or diplostomiasis.
Water can also be a direct vector of fish pathogens. Undisinfected substrate from natural water bodies and aquariums affected by fish diseases, as well as aquatic vegetation, can be the source of many parasitic fish diseases. Shared nets, feeders, thermometers, scrapers, air stones, rubber hoses, etc., are mechanical vectors of parasitic fish disease agents from one aquarium to another.
Pathogens of parasitic diseases are quite often present on fish, but the fish does not always get sick. This is explained by high body resistance. However, as soon as the fish enters unfavorable environmental conditions, the organism weakens and its defense mechanisms cannot restrain the pathogenic impact of the disease agent. Thus, the pathogen enters optimal conditions for its development and reproduction, and the fish—acting as a parasite carrier—gets sick with clinical signs of the disease.
Infectious Diseases.
Infectious diseases are caused by microbial pathogens: bacteria, viruses, parasitic fungi. If a disease is caused by a single pathogen, the infection is called simple. Sometimes simultaneous infection with two diseases caused by different pathogens is observed. Such infections are called mixed. The presence of a mixed infection complicates diagnosis and therapeutic-prophylactic measures. Infections can be primary and secondary. Primary infection occurs when parasites attack healthy fish tissues, causing infection and disease. Some bacteria and most fungi attack tissues that have already been damaged by other pathogens or trauma—this is a secondary infection. They are also called opportunistic organisms.
Protozoa are single-celled animals, mostly microscopic, although some are visible to the naked eye. They lead a parasitic or free-living lifestyle. Depending on the pathogens, they are subdivided into:
• Algae-related infections (algeosis) – single-celled algae.
• Bacteriosis – bacteria.
• Virosis – viruses.
• Mycosis – parasitic fungi.
• Rickettsiosis – bacteria of the family Rickettsiaceae.
Bacteria are predominantly unicellular organisms, some forming spores. Many are motile and have flagella. In shape, they can be spherical, rod-shaped, spiral, etc. They are capable of growing both in the presence of oxygen (aerobes) and in its absence (anaerobes). They attack fish kept in poor conditions.
Viruses are intracellular parasites. They multiply only in living cells. Viruses spread to neighboring cells and are carried by the blood. They remain in the body, making the fish a virus carrier while it looks completely healthy. Various tumors caused by viruses and parasites are called hyperplasias. There are currently no ways to cure virus-infected fish. Antibiotics do not destroy viruses.
The presence of an infection source is mandatory for an infectious disease to occur. In aquarium practice, such a source includes sick fish and microbe carriers that release the infectious agent into the environment. As a result, the substrate, water, plants, fishing gear, and fish-farming equipment become infected. Substrate and water, in which certain pathogenic microbes can not only survive but also multiply, become reservoirs of infection. The latter can also include all types of live food, mollusks, ichthyophages, aquatic vegetation, etc.
The course of infectious diseases is quite typical. Common to all infectious diseases is the presence of an incubation (latent) period—the time interval between the introduction and multiplication of the pathogen in the body and the appearance of clinical signs of the disease. Its duration varies across different infections and animals, depending on the number and entry site of the microbes, their pathogenicity, the condition of the fish, etc. Depending on the nature and duration of the disease process, acute and chronic courses of the disease are distinguished.
Environmental conditions where the pathogen and susceptible organism reside have a decisive influence on the course of the infection. For aquarium fish, predisposing factors for disease outbreaks are feeding and maintenance conditions.
Invasive Diseases.
“Invasion” refers to the introduction and subsequent development of animal-origin parasites within an organism. For most invasive disease pathogens, the conditions of parasitism on aquarium fish (temperature, hydrochemical regimes, ecological factors) are optimal, and their development in indoor water bodies continues intensively year-round. Fighting such a vast 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 possess innate or acquired immunity.
Innate or species immunity is the resistance of a particular animal species to specific types of parasites, inherited genetically. Acquired immunity arises only in relation to a disease the animal has previously suffered from and is generally not inherited. An example is ichthyophthiriasis, after which fish develop relative immunity—upon reinfection, the disease proceeds much easier than the first time.
However, there is also so-called non-sterile immunity—a peculiar type of resistance where the parasite does not die within the host organism, but neither does it reproduce. This state 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 another harmful factor (stress, secondary infection, etc.), active reproduction of the parasite and the onset of disease become likely.
Invasive diseases of aquarium fish are subdivided into: • protozoan diseases caused by protozoa. • helminthiases – diseases caused by parasitic worms. • crustacean diseases (crustaceosis) belonging to the phylum Arthropoda. All parasites can be divided into two large groups: unicellular (or protozoa) and multicellular. Protozoa are independent single-celled organisms consisting of two inextricably linked parts—the nucleus and cytoplasm. Pathogens of invasive diseases localize in various organs and tissues of fish. Parasites living on the body surface and gills are called ectoparasites or external parasites. Organisms parasitizing internal organs and tissues—intestines, liver, kidneys, spleen, eyeball, muscles, subcutaneous tissue, etc.—are called endoparasites. All of them exert diverse effects on fish: mechanical, toxic, slowing down growth and development, reducing body condition, and sometimes causing fish mortality.
Non-Contagious Diseases.
Non-contagious diseases are those that lack an explicit pathogen. Non-contagious diseases are not transmitted from one fish to another; however, they can cause mass mortality. This occurs due to unfavorable conditions for all aquarium inhabitants, such as oxygen depletion events. These are preceded by poor fish care and the use of poor-quality feed. Non-contagious diseases facilitate the entry of various pathogenic organisms into the fish’s body.
Non-contagious diseases not only negatively affect fish health by causing various pathological processes in the body but also contribute to the emergence of contagious diseases. Fish resistance against infectious and invasive pathogens decreases because a weakened fish organism is insufficiently protected against various parasites. Non-contagious diseases can be acute, temporary, and generally disappear after the cause is eliminated, or chronic, sometimes irreversible, leading to caudal peduncle curvature, dystrophy, and even death.
Non-contagious diseases arise as a result of environmental factors. Most non-contagious diseases are associated with metabolic disorders. Since immunity is weakened in the process, they may be accompanied by secondary contagious diseases. The causes of acute non-contagious diseases are typically: • oxygen deficiency; • temperature regime violations; • poor-quality feed poisoning; • toxic substance poisoning; • sharp shifts in water pH levels. The causes of chronic non-contagious diseases include: • prolonged exposure to low or high temperatures; • prolonged exposure to excessively hard or soft water; • monotonous feeding; • mechanical damage during transport; • deficiency of biologically active substances, vitamins, and trace elements.
Practice shows that the most frequent cause of non-contagious diseases and aquarium fish mortality is the inability to maintain necessary temperature and hydrochemical regimes. The second important factor in keeping aquarium fish is feeding. These diseases are usually caused by monotonous, insufficient, overly fatty diets with high carbohydrate content, protein deficiency, vitamin deficiency, and mineral deficiencies. A specific place among non-contagious diseases is occupied by fish poisoning from pesticides entering aquariums with live food or insect control products. Transportation and unjustified frequent transfers from one aquarium to another cause trauma to the skin, gill apparatus, fins, and sometimes the skeleton and internal organs of fish. Ichthyophages (hydra, great diving beetle, tadpole shrimp, etc.), entering the aquarium with poorly sorted food, cause mechanical damage. It has been established that eliminating the causes of non-contagious diseases helps avoid at least 70% of all fish diseases encountered by aquarists.
In case of a non-contagious disease, sick fish should be treated. If a contagious disease is suspected, all fish and plants in the aquarium must be treated. The aquarium, plants, inventory, and equipment should be disinfected.
