Fish diseases constitute one of the most important problems and challenges confronting fish culturists. Fish diseases do not occur as a single caused event but are the end result of interactions of the disease, the fish and the environment. Fish in intensive culture are continuously affected by environmental fluctuations and management practices such as handling, crowding, transporting, drug treatments, undernourishment, fluctuating temperatures, and poor water quality. All of these factors can impose considerable stress on the homeostatic mechanisms of fish rendering them susceptible to a wide variety of pathogens. Parasites, viruses, and bacteria are all causes for concern to aquaculturists. Many of these pathogens are easily treatable and are not transferrable to humans. On the other hand, there are fish diseases that are not treatable and cause widespread mortality both in the hatchery and wild fish. Disease also reduces hatchery efficiency and production, which in turn, increases costs and reduces profit.

Another major issue pertaining to fish health is the concern over the introduction of infectious diseases, particularly those that are not endemic to the area, through fish culture facilities to wild fish populations. The impacts of infectious diseases are generally misunderstood or seriously underestimated, and it is not uncommon to find resistance to their assessment and control. Many of the fish health programs, guidelines and regulations are simply regarded as another bureaucratic impediment and cost placed in front of the aquaculture industry. This attitude is due, in part, to the "old school of thought" that significant epizootics rarely if ever occur in wild populations and to the abiding faith that once fish are released into the wild they will recover. Wild fish stocks can become infected and suffer high mortalities through the release of pathogens in untreated hatchery effluent or through the stocking or escapement of diseased fish into aquatic systems.

In Michigan, a pathogen-contaminated hatchery discharging into a nearby river infected the wild fish population in that river. All fish in the hatchery and river were eradicated to prevent further spreading of the pathogen throughout the watershed. Closer to home, the state of New York last year ordered that 43,000 pounds of fish be destroyed from four state hatcheries because they were infected with whirling disease. This same disease is also now plaguing state fish hatcheries in Connecticut.

The Division of Fisheries and Wildlife considers disease a serious potential threat to the health of our public aquatic resources. Some of the measures employed by the Division at its own facilities relative to fish health include annual diagnostic inspections; standard disinfection procedures on tanks, raceway and fish hauling trucks; inter-hatchery fish movement restrictions; and requiring a fish health certificate from any facility which the Division obtains fish eggs or live fish from. The Division does not require private aquaculture facilities to have annual fish health inspections at this time. However, any private facility wanting to import fish eggs or live fish into Commonwealth must obtain an importation permit from the Division of Fisheries and Wildlife (except aquarium trade fish species as defined in our regulations - 321 CMR 9.01(2)). The applicant must provide us with a copy of a fish health certificate from the source of the importation as part of the permit application process. Although the Division does not require private facilities to have annual fish health inspections, it is in their own best interest to do so.

The following section is a description of some of the more common and hazardous pathogens encountered in freshwater aquaculture. It is by no means a complete listing of all parasites, bacteria, and viruses that may impact the health of freshwater fishes.

Parasites

Parasites can be defined as animals that live on or in another animal at the expense of that animal. Parasites are the most diverse and common pathogens the aquaculturist will likely encounter. External parasites can be found on the skin or gills of their hosts while internal parasites occupy areas such as the flesh or internal organs. Many of the common parasites to freshwater fish have complex life cycles involving more than one host, usually involving changes in life stage from one host to the next. Parasites are capable of causing damage to all hosts involved, mostly by injuring tissues or organs while burrowing or consuming food. Despite the ubiquitous distribution of many of these parasites, infestation of a host is usually limited unless the host is subjected to increased stress. Increased stress may take the form of crowding, insufficient oxygen levels, or poor water quality. The damage done to the host will usually be directly proportional to the level of infestation.

Most fish parasites would not develop in humans even if eaten raw. None are harmful to humans if the fish are thoroughly cooked. All reports of people being infected with fish parasites were as a result of ingestion of raw fish or insufficiently cooked fish. Some of the more common parasites found in freshwater fish are listed as follows:

Ichthyophthirius multifilis ("ich") - Ciliated protozoan:

"Ich" is a large ciliated protozoan that is capable of infecting most species of fish. Ich can be seen by the naked eye and when full grown appears as a white spot on the fish. These parasites are commonly found in small numbers on wild warmwater fish and can also be a serious parasite of salmonids. Fish will flash or scrape on objects due to the irritation caused by the parasite. When fish are stressed, heavy infestations can occur. Heavy infestations of the gill can interfere with respiratory exchange and result in mortality, especially in fingerlings. There are no reported cases of people becoming infected with this parasite by eating fish. The parasite is difficult to control in the wild. Formalin can be used in raceways at 250 ppm, up to 1 hour. In earthen ponds: 15-25 ppm indefinitely.

Trichodina spp., Ambiphyra spp., Epistylis spp., Chilodonella spp., Trichophyra spp.- Ciliated protozoa:

These ciliated protozoans can inhabit the gills and body surfaces of most species of fish. They occur in small numbers in almost all fish populations. They can cause problems when the host is subjected to undue stress, allowing heavy infestations to develop. Heavy infections are more common in aquacultural situations where fish are crowded and water quality is poor. Moderately to heavily infected fish may go off feed. The parasites can produce bloody lesions on scaled fish and erosion of fins and spines in all species. Gills can become swollen, hemorrhagic, and produce heavy mucus. Mortality can result with heavy infections. There are no reported cases of people becoming infected with these parasite by eating fish. These parasites are difficult to control in the wild. Formalin can be used in raceways at 250 ppm, up to 1 hour. In earthen ponds: 15-25 ppm indefinitely.

Ichthyobodo (Costia) spp.-Protozoan flagellate:

These protozoans can inhabit the gills and body surfaces of fish. Like other protozoans, it is present in small numbers in almost all populations of fish. These can cause problems when the host is subjected to undue stress, allowing heavy infestations to develop. Heavy infections are more common in aquacultural situations where fish are crowded and water conditions are poor. Infected fish will commonly flash or scrape against objects. Infected fish may also stop eating and may gasp at the water surface. Infected fish that have been off feed for a period of time may show signs of starvation. A characteristic sign of ichthyobodiasis is excess mucus production, which has been referred to as blue slime. Sometimes complete removal of the epithelium may be seen and the pigment may be missing from the skin. There are no reported cases of people becoming infected with these parasites by eating fish. These parasites are difficult to control in the wild. Formalin can be used in raceways at 250 ppm, up to 1 hour. In earthen ponds: 15-25 ppm indefinitely.

Gyrodactylus spp.-Monogenetic trematode:

These worms are ubiquitous parasites of the skin, fins, and gills of many freshwater fishes, particularly salmonids, ictalurids, and cyprinids. They exist in small numbers in almost all populations of fish and only become a problem when fish are subjected to stressful situations. Heavy infections can cause overall darkening in color in fry, erosion of the fins, obvious secretions of mucus sometimes described as blue/grey slime, and emaciation (especially in young fish). Detachment of catfish barbels due to necrosis may occur. There are no reported cases of people becoming infected with these parasites by eating fish. These parasites are difficult to control in the wild. Formalin can be used in raceways at 250 ppm, up to 1 hour. In Earthen ponds: 15-25 ppm indefinitely.

Neascus spp. (Black spot)-Digenetic trematode:

These parasites encysts within the skin of many species of freshwater fish. The fish in turn, surround the cyst with a black-pigmented tissue giving rise to black spots. These parasites do not usually cause mortality, but can be very unsightly. There are no reported cases of people becoming infected with these parasites by eating fish. There is no known direct control for these parasites. Indirect control has been attempted by controlling the snails that serve as intermediate hosts to the parasites.

Clinostomum marginatum (Yellow grub)-digenetic trematode:

This parasite is found in many species of freshwater fish in small numbers. Infestations are seldom extremely heavy but individual fish may be highly parasitized. The grubs are often discovered in the musculature of infected fish by fishermen and considered very unsightly. Clinostomum spp. has been reported to develop in the oral cavity of humans who eat infected fish that are under cooked.

There is no known control for this parasite. Indirect control has been attempted by controlling the snails that serve as intermediate hosts to the parasite.

Proteocephalus ambloplitis (Bass tapeworm)-Cestode:

Although bass tapeworm is capable of infecting many species of fish, only heavy infections with adult tapeworms are believed to be detrimental. However, the plerocercoids can be much more damaging. The plerocercoids can migrate through the body cavity and internal organs causing adhesions to develop. Severe adhesions in the reproductive organs caused by plerocercoids can render female fish incapable of normal egg production. The migrating plerocercoids may actually kill small fish by penetrating a vital organ. There are no reported cases of people becoming infected with this parasite by eating fish. There is no known control for this parasite.

Posthodiplostomum spp. (White grub)-Digenetic trematode:

These parasites exist in small numbers in many freshwater fish species. The larval forms appear as white cysts in many internal organs of fish. When the cercariae penetrate the fish , they can cause hemorrhaging at the point of entry. However, for the most part these parasites are not considered problematic, just unsightly. There are no reported cases of people becoming infected with these parasites by eating fish. There is no known control for these parasites. Controlling the snail population has been attempted.

Ceratomyxa shasta (CS)-Protozoan myxosporidian:

CS is common in anadromous fish in the Western U.S., particularly the Columbia River basin. It is capable of infecting wild and captively reared salmonid populations with juvenile rainbow and cutthroat trout being especially susceptible. The parasite manifests itself in the gut, liver, spleen, and muscle causing abdominal distension due to production of spore-containing fluid. Under no circumstances should eggs or live fish be transferred from contaminated areas. Treatment of the disease is not yet possible. Contaminated water supplies need to be filtered by ultra-violet treatment or be used to culture species other than salmonids.

Myxobolus cerebralis - "whirling disease" -Myxosporidian:

This parasite infects the head cartilage of salmonids and is responsible for major economic impacts to trout culture in Europe and America. The parasite damages the cartilage and impairs balance, often causing the fish to swim in circles. The parasite is also capable, in the advanced stages of the disease, of infecting the spine, causing deformities and a change in color. Severely infected fish appear black and twisted posterior to the dorsal fin. The older the trout prior to exposure to the disease, the less the potential for mortality and severe infection. Fish at the alevins stage often suffer total mortality. Older fish will still become potential vectors for the disease by being relatively asymptomatic carriers. The myxosporidian spores are highly resistant to treatment and are long lived, making eradication, especially in earthen ponds, extremely difficult.

Viruses

Viruses are extremely small infectious agents that multiply only within the living cells of a host by using components of the host cells. They are made up of simple genetic material and are incapable of growing in size or undergoing typical cell processes like cell division. Some of the more problematic viruses are discussed.

Infectious Pancreatic Necrosis (IPN)-RNA Virus:

IPN is capable of infecting all ages of many salmonid species resulting in a chronic infection affecting many organs, particularly the kidney. This pathogen often causes severe mortalities in salmonid fry. IPN can be transmitted to the eggs from the adults (vertically) or through the water by fish excretions (urine or gametes) or by fish consuming dead fish or fish offal (horizontally). This virus has been isolated from North America, Europe, and Japan. There are no means of controlling infections in fish other than by avoiding the contraction of the virus. Eradication of the virus from a facility requires the sacrifice of all infected and presumed infected fish and proper sterilization.

Viral Hemorrhagic Septicaemia (VHS)-RNA Virus:

VHS is capable of infecting all ages of salmonids. The disease usually infects rainbow trout but can also infect Atlantic salmon, brown trout, and brook trout. It may result in acute disease in under two weeks or take up to a year to manifest itself in a population. The time period depends mostly on the temperature of the water with outbreaks usually occurring below 15 degrees Celsius. The disease, which manifests itself in most highly vascularized tissues, causes the most severe mortality in fingerling trout in their first winter but will also cause mortality at other ages.

The disease becomes apparent in three stages. The first stage is characterized by high mortality in fish that are dark, lethargic and have hemorrhages at the base of fins and gills. The second stage is recognized by very dark colored, anaemic fish. Pale gills and exophthalmos is common. The third stage is characterized by atypical swimming behavior (looping) and a swollen and discolored kidney. This disease is widespread in many European countries. In the United States, thus far, the disease has only been found in the state of Washington. There are no methods to control or treat this disease once introduced.

Infectious Hematopoietic Necrosis (IHN)-RNA Virus:

This virus is found in the wild in salmonid populations on the Pacific coast of North America. It can infect chinook and sockeye salmon and rainbow trout. The virus causes the most severe mortality in yolksac fry to two-year old fish and may inflict up to 80% mortality in infected populations when the water temperature is at 10 degrees Celsius. Infected fish exhibit lethargy and sporadic hyperactivity, dark coloration, anemia, exophthalmia, abdominal distentions, long off-white casts trailing from the rectum, and hemorrhages at the base of fins. Disease in hatcheries is common and capable of transmitting epizootic conditions to wild populations. Epizootics have occurred throughout the United States due to transport of infected fish from the west coast. The disease has also been reported in Japan. Fish can contract the virus through direct contact with or ingestion of infected fish. Ovarian and Seminal fluids have the highest concentration of viral agents.

The virus can not be eliminated from infected or diseased fish. A rise in water temperature to 15 C will diminish acute levels of disease but only as long as the elevated temperature persists. The only way to avoid disease to prevent contamination by introduced fish, eggs and water supply.

Channel Catfish Virus Disease (CCV)-DNA Virus:

This disease occurs in fry and fingerling channel catfish when the water temperatures rise to 25-30 degrees Celsius. Other Ictalurids may also serve as carriers for the virus. Fish surviving the disease may serve as viral carriers. Effected fish will display a loss of equilibrium, spiral swimming movement, and assume a vertical position in the water column. Gill, skin, and internal organ hemorrhages and abdominal distension occur. The disease effects the liver, spleen, kidney and digestive tract. The virus is shed by diseased fish. Control of the disease is only possible by preventing infected fish or water supplies from contacting uninfected populations.

Bacteria

Bacteria are all single prokaryotic cells that break down organic material for their energy needs. All bacteria present on fish will be killed if the fish are properly cooked. Some of the more common bacterial pathogens found in freshwater fish are described here.

Motile Aeromonas spp.:

The bacteria belonging to the genus Aeromonas, may under certain adverse environmental conditions, cause serious conditions in most species of fish. The bacteria may also infect frogs, turtles and snakes. These bacteria are found in soil and most natural waters. Aeromonas spp. has been credited with causing several diseases in fish, including motile aeromonas septicemia, bacterial hemorrhagic septicemia, tail/fin rot, redsore disease and others. Clinical symptoms include, the presence of small surface lesions, local hemorrhages particularly in the gills and vent, ulcers, abscesses, exophthalmia and abdominal distension. Internally, there may be accumulation of ascitic fluid, anemia, and damage to the organs, especially the kidney and liver. Terramycin can be used to control diseases in salmonids and catfish in aquacultural settings. The dosage regimen is 2.5 to 3.75 g/100 lb/day for 10 days.

Pseudomonas spp.:

The bacteria belonging to the genus Pseudomonas, are present in most natural waters and infect most species of fish. These parasites are considered opportunistic pathogens, causing disease when the host is subjected to some type of stress. Pseudomonas spp. have been credited with causing pseudomonad septicemia, red spot disease, fin/tail rot, and others. Clinical symptoms include, hemorrhages in the mouth region, opercula, and ventral side of the body. Small petechial hemorrhages can occur through out body cavity. Organs such as the liver and kidney may also be affected. Terramycin can be used to control diseases in salmonids and catfish in aquacultural settings. The dosage regimen is 2.5 to 3.75 g/100 lb/day for ten days.

Flexibacter psycrophilus:

This bacteria infects salmonid fishes and causes "Coldwater Disease", also referred to as bacterial coldwater disease, peduncle disease, or low temperature disease. The precise reservoirs of the bacteria are unclear, but it may occur naturally in the aquatic environment. This disease occurs when water temperatures are 12 C or below, however some outbreaks have occurred at temperatures of up to 16 C. This disease primarily affects juvenile fish, but infections can occur in yearlings. Skin and muscle lesions are the most common signs of this disease. Lesions are frequently seen in the peduncle area, but they may also occur on other areas of the body surface. If the fish survive long enough, they may suffer the loss of their caudal fins. Following a severe epizootic, at least two disease conditions may be observed. In the first, fish appear lethargic and later on spinal deformities and chronic mortality occur. In the second case, fish display a spiral swimming behavior, dorsal swelling, and dark pigmentation on one side of the body. There is no approved treatment for this disease.

Renibacterium salmoninarum:

This bacterial pathogen causes bacterial kidney disease (BKD). BKD is typically chronic, characterized internally by a large edematous kidney that can appear gray and corrugated. The kidney often exhibits off-white lesions that vary in size and number. The lesions sometimes occur in other organs such as the liver and spleen. Externally, fish can appear normal or they can exhibit one or more of the following signs: exophthalmos, abdominal distension, skin petechiation, and vesicles in the skin.

Renibacterium salmoninarum has been detected in both wild and hatchery fish and may be transmitted both vertically and horizontally. Infections can occur at any life stage, but clinical signs are uncommon in fish less than six months old. Mortality has been reported between a wide range of temperatures. There is no approved treatment for BKD.

Flexibacter columnaris:

This bacteria, capable of infecting most species of freshwater fish, causes "Columnaris Disease". The disease affects fish of all ages and occurs in warmwater conditions (14 C and up). The precise reservoirs of the bacteria are unclear, but it is thought to occur naturally in the aquatic environment. When highly virulent strains of the pathogen are involved, fish may die without any gross clinical symptoms. With less virulent strains, external lesions may occur on the body surface and gills. On scaled fish, grayish-white lesions occur on the fins, head, and trunk. If the pathogen is present in large numbers, the lesions may appear yellow or orange, reflecting the yellow-pigmented cells of the bacteria. On scaleless fish, lesions appear to be a dark blue area covered by a milky veil with a red tinge around the margin. There is no approved treatment for this disease.

Aeromonas salmonicida:

This bacteria causes a condition known as furunculosis in many species of freshwater and marine fish. The disease was named furunculosis because of the formation of furuncules, boil-like lesions that occur in various tissues of the body. This organism was discovered almost 100 years ago, but the exact route of transmission is still not completely understood. The occurrence of furunculosis is generally seasonal, acute cases occur when water temperatures are about 20 degrees Celsius and chronic infections occur when temperatures are 13 degrees Celsius or lower. In acute cases, fish may darken and go off feed. Internally, the viscera are hemorrhagic, kidney tissue is very soft, the spleen is enlarged, and the liver is pale or mottled with petechiae. In chronic cases there is a more gradual onset of mortality. Externally, fish have skin lesions or furuncules, along with internal lesions.

In addition to furunculosis, A. salmonicida has also been implicated in other conditions, including carp erythrodermatitis and goldfish ulcer disease.

Romet-30 and Terramycin can be used to control furunculosis in salmonids in aquacultural settings. Romet-30 @ 50 mg/kg/day for 5 days. Terramycin @ 2.5 to 3.75 g/100 lb/day for 10 days.

Proliferative Kidney Disease (PKD):

PKD is a pathogen that commonly effects young trout. Very little is known about PKD and is origin. Some evidence exists to link the infection of fish with the presence of an amoeba. Whole populations of young trout can be infected, especially right after introduction into a growing facility. Affected fish become dark in color, anaemic, and have some abdominal swelling. Despite impaired growth, losses are usually minimal until some other stress (grading, treatment for other diseases) is introduced. Catastrophic losses soon follow. Because of the associated losses, attempted treatment is counterproductive and not yet possible.

Yersinia ruckeri - Enteric Redmouth (ERM):

ERM has been isolated in the United States, Canada, and Norway. An outbreak is characterized by escalating losses in cultured fish due to severe congestion and hemorrhage in the head tissues with erosion of the lower jaw being common. The spleen, kidney, and intestine can also be infected. This disease is capable of infecting all salmonids, isolations have also been made in goldfish, cisco, largemouth bass, emerald shiners, sturgeon, fathead minnows, and walleye. Crayfish and muskrats have also been found to harbor large numbers of Yersinia ruckeri cells.

Fungi

Fungi are single celled, multinucleate organisms that secrete digestive enzymes and gain nutrition through absorption of dissolved inorganic and organic food materials. Fungi are generally considered secondary symptoms in fish as they usually only occur where other trauma, such as injury or disease, have created an opportunity for fungal infection.

Saprolegnia spp.:

Saprolegnia is a ubiquitous fungus and inhabits all freshwater. It invades most species of fish that have been subjected to some type of stress. It is also capable of infecting insects and amphibians. Handling injuries, malnutrition, temperature shock, external parasitism, and spawning increase the susceptibility of fish to infection by Saprolegnia. There is potential for infection whenever fungal zoospores are present in excess of 23,000 spores/liter. Infections left unattended can result in high mortality. Saprolegnia appears as white cottony tufts of non-septate filaments on the skin and other surfaces of infected fish. Fish eggs are highly susceptible to this pathogen.

Treatment consists of formalin at 1000 to 2000 ug/L for 15 minutes for salmonid and esocid eggs. There is no approved treatment to control Saprolegnia on fish.

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Published: September 1995