Written by: Robert B. Moeller Jr., DVM
March 22, 2000 part II
|1) Aeromonas hydrophila (Bacterial Hemorrhagic
- 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
|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.
- A) Gram negative rod, lives primarily in a marine
- B) Vibrio septicemia: V. alginolyticus / V. anquillarum / V.
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
- 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
- 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
- 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
- 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
- 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
|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
- 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
|12) Renibacterium salmoninarum (Bacterial Kidney
- 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
|13) Mycobacterium species (Tuberculosis)
- A. Gram positive, acid fast rods (M. marinum, M. chelonei and
M. fortuitum are the most common Mycobacterium species
- 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
- 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
|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
- 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
- D. The mode of transmission is unknown.
|16) Epitheliocystis (Chlamydial infection)
- A. Obligated intercellular parasite. Organism stain red with
- 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.
- 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.
- B. Primarily a problem in carp, rainbow and brown trout, and
- 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 (20-25°C) bring about the disease.
- A. Ichthyophonus hoferi; large 10-250 micron spores which may
germinate to form large hyphae (similar to the hyphae of
- 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
- 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
- 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
(25°C) will usually interrupt the life cycle (Tomites live only 48 hours
at 26°C). 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
|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
- B. This disease is observed in most aquariums and hatchery raised fish.
This disease occurs primarily in cold waters (10°C) 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
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
- 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
- D. Transmission is by direct contact with infected fish and or
|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
- 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
|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
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
|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.
- 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
- B. This organism usually is not a problem but can affect many species
- C. The organism can appear on the gills or skin causing increased mucus
production and hyperplasia. Severe infections of the gills will cause
- D. This disease is a problem when fish are exposed to poor water