Biology of Fish II

Biology of Fish II


Written by: Robert B. Moeller Jr., DVM
March 22, 2000 part II

Bacterial Disease

1) Aeromonas hydrophila (Bacterial Hemorrhagic Septicemia)
  • A) Gram negative motile rods
  • B) Effects many freshwater species and usually is associated with stress and overcrowding.
  • C) The clinical signs and lesions are variable.
  • The most common finding is hemorrhage in skin, fins, oral cavity and muscles with superficial ulceration of the epidermis. Occasionally cavitary ulcers (similar to A. salmonicida) are observed. Exophthalmus and ascites are commonly observed. Splenomegaly and swollen kidneys are common. Histologically, multifocal areas of necrosis in the spleen, liver, kidney and heart with numerous rod shaped bacteria are observed.
  • D) Diagnosis is rendered by culturing the organism from affected animals: Remember this is a common water saprophyte with a great variation in virulence in serotypes.
  • E) Disease is transmitted via contaminated water or diseased fish.
2) Pseudomonas fluorescens
  • A) Short motile Gram negative rods with polar flagella.
  • B) Lesions similar to Aeromonas hydrophila with a hemorrhagic septicemia resulting in hemorrhage of the fins and tail and ulceration of the skin.
  • C) Pseudomonas anquilliseptica causes a serious problem in Japanese eels with a septicemia resulting in petechial hemorrhage on fins and tail and ulceration of the skin.
3) Vibrio
  • A) Gram negative rod, lives primarily in a marine
  • B) Vibrio septicemia: V. alginolyticus / V. anquillarum / V. salmonicida
    1. Septicemia has similar lesions to Aeromonas hydrophila.
    2. See hemorrhage in the skin of the tail and fins, ulceration of the skin, hemorrhage in the muscle and serosal surfaces. The spleen may be enlarged and bright red. Histologically may see necrosis of the liver kidney and spleen and occasionally the gut mucosa.
  • C) Ulcer Disease of Damselfish: V. damsela
    a. Deep skin ulcers and necrotizing myositis.
    b. Lesions similar to Aeromonas salmonicida.
  • D) Vibrio salmonicida: Hitra disease or Cold water vibriosis
4) Edwardsiella tarda (Edwardsiella septicemia)
  • A) Gram negative motile pleomorphic curved rod
  • B) The disease affects primarily channel catfish but also observed in goldfish, golden shiners, largemouth bass, and the brown bullhead. This organism is the most serious disease involving the eel culture of Asia.
  • C) The lesions are similar to A. hydrophila with small cutaneous ulcers and hemorrhage observed both in the skin and muscle. Muscle lesions often develop into large gas filled (malodorous) cavities. Diseased fish lose control over the posterior half of their body yet continue to feed.
5) Edwardsiella ictaluri (Enteric septicemia of catfish)
  • A. Gram negative motile pleomorphic curved rod
  • B. Disease affects primarily fingerlings and yearling catfish
  • C. Clinical signs of enteric septicemia of catfish closely resembles those of other systemic bacterial infections. The most characteristic external lesion is the presence of a raised or open ulcer on the frontal bone of the skull between the eyes (Hole in the head disease).
6) Aeromonas salmonicida (Furunculosis, Ulcerative disease of goldfish)
  • A. Gram negative non-motile short rod
  • B. This bacteria affects primarily salmonids but other freshwater fish can be affected.
  • C. Clinically the disease may present as a septicemia with hemorrhage in the muscles and other sites. The major lesion is a subcutaneous swelling that often causes an ulcerative dermatitis. In chronic disease these lesions may cavitate into the adjacent musculature. In the septicemic disease, there is splenomegaly, ascites, and swelling of the kidneys. Histologically, there is necrosis of the affected tissue with abundant colonies of bacteria and few inflammatory cells due to the bacteria's leukocytolytic exotoxin.
  • D. The disease is transmitted by contact with diseased fish, contaminated water, fomites, and infected eggs.
7) Yersinia ruckeri (Enteric red-mouth)
  • A. Gram-negative motile rod
  • B. The bacteria affects salmonids; rainbow trout are the most susceptible.
  • C. Clinically this disease manifest itself as a septicemia with exophthalmus, ascites and hemorrhage and ulceration of the jaw, palate, gills and operculum. Hemorrhage of the musculature and serosal surfaces of the intestines, splenomegaly, and kidney swelling are common. Histologically numerous bacterial colonies admixed with inflammatory cells are observed in many areas of necrosis involving the liver, spleen and kidney.
  • D. The disease is transmitted by contact with diseased or carrier fish, and contaminated water. Bacteria persist in asymptomatic non-salmonid fish and in some birds.
8) Streptococcus iniae
  • A. Beta-hemolytic Streptococcus (Note: Beta hemolysin may not be present in culture media in all cases leading to the possible belief that this bacteria is a non-pathogen.)
  • B. Disease of tilapia, hybrid striped bass and rainbow trout.
  • C. Major problem in the tilapia industry. Streptococcus iniae presents either as an acute fulminating septicemia or in a chronic form limited primarily to the central nervous system. The septicemic form may present with hemorrhage of the fins, skin, and serosal surfaces. Ulcers may appear. Microscopically, one observes a meningoencephalitis, polyserositis, epicarditis, myocarditis and/or cellulitis. Cocci are present in the inflammation. In the chronic form, granulomas are evident in the liver, kidney, and brain (meningoencephalitis). In the chronic disease, the brain is the best organ to culture.
  • D. Streptococcus iniae is a problem primarily of closed recirculating culture system. Probably associated with overcrowding and poor water quality - high nitrates. Depopulation, disinfection and restocking with disease free fish are the best means of elimination of the organism.
  • E. The bacteria is known to be a zoonotic agent. Individuals who have handled infected fish have develop cellulitis of the hands and endocarditis.

9) Flexibacter columnaris (Columnaris disease or Saddleback disease)

  • A. Gram-negative slender rods (3-8 microns)
  • B. The disease is a serious disease of young salmonids, catfish and many other fish.
  • C. This is a highly communicable disease. Lesions usually first appear as small white spots on the caudal fin and progresses towards the head. The caudal fin and anal fins may become severely eroded. As the disease progresses, the skin is often involved with numerous gray-white ulcers. Gills are a common site of damage and may be the only affected area. The gill lesions are characterized by necrosis of the distal end of the gill filament which progresses basally to involve the entire filament.
  • D. Flexibacter columnaris infections are frequently associated with stress conditions. Predisposing factors for Columnaris disease are high water temperature (25oC-32oC.), crowding, injury, and poor water quality (low oxygen and increased concentrations of free ammonia).
  • E. Flexibacter maritimus: cause similar problems in salt water environment.
  • F. Flexibacter psychrophilus causes Cold Water Disease or Peduncle disease. Fish develop dark skin, hemorrhage at the base of fins, and anemia with pale gills with increase mucus. Hemorrhage into the muscles is common. Periostitis of cranial and vertebral bones is common in chronic cases. A chronic meningoencephalitis occasionally is observed with abnormal and erratic swimming.
10) Bacterial Gill Disease
  • A. Bacterial gill disease is caused by a variety of bacteria. Flexibacter columnaris, Cytophagia psychrophilia and various species of Flavobacterium (all are gram negative rods) are the primary bacteria involved in this disease.
  • B. Fry are the most susceptible to the disease, however, all ages may be affected. Clinically the fish become anorectic, and face the water current. Prominent hyperplasia (mucus and epithelial) of the gills is evident on gross and microscopic examination. Microscopically one observes proliferation of the epithelium which result in clubbing and fusion of the lamella. Necrosis of the gill lamella occurs in serious cases.
  • C. Overcrowding, accumulation of metabolite waste products (particularly ammonia), organic matter in the water, and an increase in water temperature may all be predisposing factors.
11) Cytophaga psychrophila (Rainbow Trout Fry Anemia)
  • A. Gram negative filamentous bacteria
  • B. Occurs primarily in rainbow trout fry. Fish develop abdominal distention, exophthalmus, increased pigmentation, lethargy, loss of balance, pale gills, and occasional cutaneous ulcers and necrosis of tail fins. Epidermal hyperemia and increase mucus secretions are common. Splenomegaly and hepatomegaly are common with multifocal necrosis of the liver spleen and kidney.
  • C. Transmission is believed to be by direct contact with contaminated water and is an indication of poor water quality and overcrowding.
12) Renibacterium salmoninarum (Bacterial Kidney Disease)
  • A. Gram positive nonmotile diplobacillus.
  • B. This is a serious disease of salmonids. Brook trout are the most severely affected species.
  • C. The disease follows a slow course with clinical signs not present until the fish is well grown. The fish may exhibit exophthalmus, skin darkening, and hemorrhage at the base of the fins. Cutaneous vesicles and ulcers may develop in mature trout "spawning rash". Abscesses, cavitation and contraction of muscles is occasionally observed. Splenomegaly and swelling of the kidney and liver with abundant ascites fluid is commonly observed. The large swollen kidney and spleen have numerous white nodules visible in the parenchyma. Numerous granulomas (containing gram positive bacteria) are observed in the kidney and may be also present in the spleen, heart and liver.
  • D. Transmission of the disease is believed to be via direct contact with contaminated fish. It is believed that the organism enters through the epidermis and then becomes a systemic disease.
13) Mycobacterium species (Tuberculosis)
  • A. Gram positive, acid fast rods (M. marinum, M. chelonei and M. fortuitum are the most common Mycobacterium species involved.)
  • B. All species of fish are affected. This disease affects both saltwater and freshwater aquariums.
  • C. Clinical signs of tuberculosis are quite variable. The most common signs are anorexia, emaciation, vertebral deformities, exophthalmus, and loss of normal coloration. Numerous variably sized granulomas are often observed in various organs throughout the body. Often numerous acid-fast bacteria are observed in the granulomas.
  • D. The aquatic environment is believed to be the source of initial infection with fish becoming infected by ingestion of bacterial contaminated feed or debris. Once an aquarium is infected with this disease, it is difficult to remove except by depopulation of the aquarium and disinfecting the tank. Remember this is a zoonotic disease (atypical mycobacteriosis).
  • E. Atypical mycobacteriosis may manifest itself as a single cutaneous nodule on the hand or finger or may produce a regional granulomatous lymphadenitis of the lymphatics near the original nodule. Occasional local osteomyelitis and arthritis may also occur.
14) Nocardia sp.
  • A. Gram-positive filamentous rod (weakly acid-fast positive)
  • B. The organism is a problem with mostly aquarium fish. However, it is occasionally observed in cultured salmonids.
  • C. Clinically this is a chronic disease characterized by raised granulomatous masses in the mouth, jaw, gills and skin (The mouth and jaw are the most common sites). Dermal masses eventually ulcerate. Numerous white raised nodules (granulomas) are often observed in the viscera.
  • D. The exact route of transmission is unknown. However, it is felt that entry through wounds and abrasions is the most common source of infection. (Ingestion of the bacteria has been known to cause the disease.)
15) Flavobacterium sp.
  • A. Gram-negative rods
  • B. Usually a problem for individual fish. This disease is a cause of concern to primarily hobbyist and producers of ornamental fish. (Mollie granuloma, Mollie madness, Mollie popeye)
  • C. Infected fish are usually emaciated and pale. Multifocal white nodules are observed in the visceral organs, the retina and choroid and the brain. These nodules may be cystic or mineralized. Histologically the nodules are granulomas with a caseous center, a thin peripheral rim of macrophages and lymphocytes and a fibrous capsule.(Must be differentiated from Mycobacterium)
  • D. The mode of transmission is unknown.
16) Epitheliocystis (Chlamydial infection)
  • A. Obligated intercellular parasite. Organism stain red with Macchiavello stain.
  • B. These organisms have been observed in many species of fresh water and marine fish. Mortality occurs most commonly in heavily infected juvenile fish.
  • C. Clinically infected fish may be asymptomatic or show respiratory distress or excessive mucus secretions. Multiple white cysts are observed on the gill lamella and skin. Histologically, the cyst consists of distended epithelial cells with numerous basophilic organism.
  • D. The means of transmission is unknown.

Mycotic Diseases

1) Saprolegniasis
  • A. Caused by various groups of aquatic fungi; primarily Saprolegnia, Achlya, and Aphanomyces.
  • B. Saprolegniasis affects all species and ages of freshwater and estuarine fish.
  • C. Clinically, affected fish develop white to brown cotton-like growths on skin, fins, gills and dead eggs. This organism is an opportunist which will usually grow over previous ulcers or lesions. Diagnosis is by finding broad nonseptate branching hyphae that produce motile flagellated zoospores in the terminal sporangia.
  • D. In the Atlantic menhaden, gizzard shad, and some other marine fishes, this fungus may present as an ulcerative mycosis which may progress to a deep necrotic lesion involving the muscle. Histologically there is an intense granulomatous inflammation with broad (7 to 14 micron), nonseptate hyphae.
  • E. Most fish die due to osmotic or respiratory problems if the affected area of skin or gills is large.
  • F. The fungi are normal water inhabitants that invade the traumatized epidermis. Improper handling, bacterial or viral skin diseases, and trauma are the major causes of the disease. It is interesting to note that temperature has a significant effect on the development of infections. Most epizootics occur when temperatures are below the optimal temperature range for that species of fish.
2) Branchiomycosis (Gill rot)
  • A. Caused by two species Branchiomyces sanguinis and B. demigrans.
  • B. Primarily a problem in carp, rainbow and brown trout, and eels.
  • C. Affected fish usually show respiratory distress. There is prominent gill necrosis caused by thrombosis of blood vessels in the gills. Histologically, the identification of nonseptate branching hyphae with an intrahyphal eosinophilic round body (apleospores) in and around blood vessels of the gill is diagnostic.
  • D. The disease occurs most commonly in overcrowded ponds with abundant organic matter and high ammonia levels. Usually warm water temperatures (20-25C) bring about the disease.
3) Ichthyosporidiosis
  • A. Ichthyophonus hoferi; large 10-250 micron spores which may germinate to form large hyphae (similar to the hyphae of Saprolegnia).
  • B. This fungus infects all species of fish.
  • C. Clinically the fish are emaciated with small round occasionally ulcerated black granulomas in the skin. Scoliosis is occasionally observed. Internally, numerous granulomas are observed in many visceral organs. Microscopically, the lesion consists of granulomas with encysted large PAS-positive spores. Occasionally large irregular shaped hyphae are observed.
  • D. Transmission is unknown, but believed to be due to ingestion of contaminated feed.
4) Exophiala sp.
  • A. Exophiala salmonis and E. psychrophilia; these fungal organisms have hyphae that are septate, irregular in width and branched.
  • B. This disease is observed in many species of fresh and saltwater fish. E. salmonis has become an organism of increased importance in caged cultured salmonids.
  • C. Clinically the fish become darker and lethargic, with erratic and whirling swimming behavior. Occasionally dermal nodules are present. Numerous round yellow to white granulomas are present in visceral organs (liver, kidney, spleen) with prominent enlargement of the posterior kidney common. Histologically, branched, irregular width, septate hyphae are present in the lesions.
  • D. Transmission is unknown.

External Protozoal Diseases

1) Ichthyophthirius multifiliis ("Ich" or White Spot Disease)
  • A. The largest protozoan parasite of fish. The trophozoite are up to 100 microns diameter, ciliated and contain an oval horseshoe shaped nucleus.
  • B. This is a disease of aquarium and hatchery reared fish.
  • C. Clinically fish become hyperactive with fish flashing and cutting against rocks or sides of aquariums. As the trophozoites enlarge they cause hyperplasia of the epidermis with white spots forming on the skin and gills. Severely infected fish may have respiratory problems and die. Histologically there is epidermal hyperplasia with the encysted trophozoite present in the epidermis.
  • D. The life cycle is direct. Encysted trophozoites (trophont) leave the fish and settle to the bottom of the tank. The trophozoites (tomonts)divide into numerous tomites (theronts) which are released to infect the skin of the fish. The life cycle takes approximately 4 days to complete. However, it can be sped up by increasing the water temperature.
  • E. The only way to treat the disease is by interrupting the life cycle of the parasite. Removal of fish from the infected water for 3 days (25C) will usually interrupt the life cycle (Tomites live only 48 hours at 26C). One must treat the water to kill the tomites to prevent spread of the disease (Malachite green, formalin, methylene blue, or KMnO4). Remember, these treatments only kill the tomites and not the trophozoites that are encysted in the fish.
  • F. Cryptocaryon irritans is the salt water equivalent to Ichthyophthirius.
2) Ichthyobodo necator (Costiasis)
  • A. Piriform shaped protozoa 6-12 microns long with two short and two long flagella. These are stalked protozoa which attach to the skin or gills.
  • B. This disease is observed in most aquariums and hatchery raised fish. This disease occurs primarily in cold waters (10C) and affects very young fish when they are just beginning to eat food.
  • C. Clinically the fish may flash, produce abundant mucus over the skin (blue slime disease) and/or show respiratory distress (flaring of gills). Histologically the parasites are attached to the epithelial surface of the skin or gills.
  • D. Transmission of the parasite is by direct contact with the protozoa. This protozoa is a free swimmer so it can swim and then attach to the host where it undergoes binary fusion for reproduction.
3) Trichodina sp. (Trichodiniasis)
  • A. This disease is caused by a group of peritrichal ciliated protozoans. The organisms are saucer-shaped, 50 microns diameter, with rows of cilia at either end and a macro and micro nucleus. When viewed dorsoventrally, the parasite appears as an ornate disk with a characteristic ring of interlocking denticles forming a circle in the middle of the organism. (Trichodina truttae is considered to be a specific pathogen for salmonids).
  • B. These are observed on most fresh and saltwater fish. This protozoa is relatively common on many fish and is not always associated with disease.
  • C. Clinically fish usually exhibit flashing and become lethargic. There is an increase in mucus production causing a white to bluish haze on the skin. The skin may develop ulcers and the fins may fray. If the gills are involved, the fish may have severe respiratory distress. Histologically, masses of organisms are attached by adhesive discs and denticles of exoskeleton to the epidermis. The underlying epithelial cells undergo necrosis. There is secondary hyperplasia and hypertrophy of the gill epithelium.
  • D. Transmission is by direct contact with infected fish and or contaminated water.
4) Tetrahymena corlissi
  • A. Normally a free-living oval ciliated 50-70 micron long protozoa.
  • B. The organism has been known to affect the fry of various cultured fish (Guppy "Guppie killer" and Northern pike).
  • C. Clinically, one may observe necrosis and hemorrhage of the skin. In severe cases the fish have rupture of the body walls and the fish eviscerate. Histologically one observes massive invasion of the musculature by this organism. (The ventral abdominal wall is severely affected.)
  • D. This is a free-living protozoan that only becomes a problem at times of overcrowding and poor water quality. (water having a high organic matter content)
5) Dinoflagellates (Velvet disease, Coral fish disease)
  • A. Dinoflagellate 100 microns diameter containing chromatophores and a single eccentric nucleus. When free swimming they are 20 microns diameter contain a transverse flagellum in the transverse furrow and a longitudinal flagellum in the longitudinal sulcus. Several species of dinoflagellate are involved:
    1) Oodinium - Velvet disease
    2) Amyloodinium - Coral fish disease
  • B. Problem in aquarium and cultured fish.
  • C. Clinically, fish flash in the water and become depressed with lateral opercular movement. A shimmering heavy yellow colored mucus secretion over the skin and gills is observed. Histologically, large oval organism (80 microns diameter) with multiple chromatophores and a single eccentric nucleus are attached to epithelial cells by a pseudopodia.
  • D. Transmission is by direct contact with infected fish, and contaminated water.
6) Epistylis (Red sore disease)
  • A. Branched stalked ciliated protozoan (Heteropolaria sp.).
  • B. Found primarily in wild populations of scaled fish.
  • C. Clinically, one observes ulcers or cotton-like growth on the skin, scales and spine resulting in a red-colored lesion. In catfish the lesion involves the spines and bones which underlie the skin of the head and pectoral girdle. This protozoan parasite has also been observed on eggs.
  • D. This ciliated protozoan is primarily a free living protozoan that lives on aquatic plants and is believed to be an opportunist. Outbreaks have occurred in catfish and salmon that have been maintained in water high in organic content.
7) Glossatella
  • A. This disease is caused by the ciliated protozoan Apiosoma which has a barrel-shaped body with cilia at the distal end and a large rounded macronucleus.
  • B. This organism usually is not a problem but can affect many species of fish.
  • C. The organism can appear on the gills or skin causing increased mucus production and hyperplasia. Severe infections of the gills will cause respiratory problems.
  • D. This disease is a problem when fish are exposed to poor water quality.

Part III

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