Written by: Robert B. Moeller Jr., DVM
March 22, 2000 part I
The author of this lecture wishes
to thank Drs. Floyd, Bowser, Plumb, Herman, Wolke, Migaki, Harshbarger,
Schmale, Smith, Colorni Leard, Chen, Muench, Hedrick and Wedemyer for
supplying photographs of the various fish diseases to the Registry of
BIOLOGY OF FISH
Fish have some unique anatomical and physical characteristics that are
different from mammals, however, they still possess the same organ systems
that are present in other animals. All fish are poikilothermic and must be
able to adapt to changes in water temperature. Fish live in a variety of
temperatures ranging from less than 0°C to hot geothermal springs. Yet,
each species of fish must live in its particular specific temperature
range. Abrupt temperature changes in the water can be lethal to fish.
Organ systems of fish vary to some extent from that of mammals due to
the aquatic environment they live in. The following are some of the
Fish do not have a keratin layer over the epidermis. These animals are
covered by a cuticle composed of mucus, mucopoly-
saccharides, immunoglobulins and free fatty acids. The epidermis is
composed of a stratified squamous epithelium of variable thickness (4-20
cells thick). The outermost epidermal cells (Malpighian cell layer) retain
the capacity to divide. Other cells present in the epidermis are goblet
cells (responsible for secreting the cuticle), large eosinophilic club
cells or alarm cells (present in most species of fish), eosinophilic
granular cells (unknown function), leukocytes and macrophages.
The dermis is composed of an upper stratum spongiosum and a deeper
stratum compactum. Numerous melanophores, xanthophores, and iridophores
(give fish their silvery color) are observed scattered throughout the
dermis. Scales are calcified plates originating in the dermis and covered
by the epidermis. There are two types of scales: ctenoid scales and
cycloid scales. Ctenoid scales of elasmobranchs have spicules extending
from the external surface giving these fish a rough sandpaper-like
texture. Cycloid scales of teleost fish have a smooth outer surface and
are laid down in concentric rings which makes them useful in determining
the age of some fish. Scales also represent a source of calcium for fish;
some fish will utilize the calcium in the scales in preference to the
calcium in their skeleton during times of starvation or prespawning
The gills consist of four holobranchs which form the sides of the
pharynx. Each holobranch has two hemibranchs projecting from the gill
arch. The hemibranch are composed of rows of long thin filament called
primary lamella. The primary lamella have their surface area increased
further by the secondary lamella that are semilunar folds over the dorsal
and ventral surface. Gas exchange takes place at the level of the
secondary lamella. These are lined by epithelial cells bounded by pillar
cells. A thin endothelial lined vascular channel lies between the pillar
cells and is the site of gas exchange, removal of nitrogenous waste and
some electrolyte exchange.
The pseudobranch lies under the dorsal operculum. This
organ is a gill arch with a single row of filaments. The function of the
pseudobranch is unknown, however it is believed that this structure
supplies highly oxygenated blood to the optic choroid and retina and may
have thermoregulation and baroreceptor functions.
There is no true adrenal gland present in most fish (exception is
sculpins). The adrenal cortical tissue is represented by the interrenal
cells. These cells are pale eosinophilic cuboidal cells associated with
major blood vessels in the anterior kidney. Both glucocorticoid and
mineralocorticoid are secreted.
The adrenal medullary cells (chromaffin cells) may vary in location.
These cells are usually found with the sympathetic ganglia in clumps
between the anterior kidney and spine or in the interrenal tissue.
The thyroid follicles are very similar to mammalian thyroid tissue.
Thyroid follicles are distributed throughout the connective tissue of the
pharyngeal area and may be observed around the eye, ventral aorta, hepatic
veins and anterior kidney. It is important to realize that thyroid tissue
can be widely distributed. Many times pathologist have erroneously
considered this distribution of normal thyroid tissue to represent
metastasis from a thyroid follicular cell tumor.
The endocrine pancreas is present in most fish as islet of Langerhans
and is associated with the exocrine pancreas. In some species the islets
are very large and may be grossly visible (Brockman bodies). During the
spawning season the size and number of islet will increase in some fish.
These should not be confused with an adenoma.
The parathyroid glands are absent in fish, their function is taken over
by other endocrine organs. (Corpuscles of Stannius)
This gland lies ventral to the esophagus in the transverse septum
separating the heart from the abdominal cavity. This organ secretes
calcitonin (lowers serum calcium levels) which acts with hypocalcin
(secreted by the corpuscles of Stannius) to regulate calcium metabolism.
Corpuscles of Stannius
These are islands of eosinophilic granular cells located in paired
organs on the ventral surface of the kidney. This organ secretes a protein
called hypocalcin (teleocalcin) which acts with calcitonin to regulate
This is a neurosecretory organ found on the ventral aspect of the
distal end of the spinal cord. These bodies are composed of unmyelinated
axons terminating on a capillary wall. The function of the urophysis is
The pineal gland is a light sensitive neuroendocrine structure which
lies in the anterior brain and is a well vascularized organ. This gland
secretes melatonin which may play a role in controlling reproduction,
growth, and migration.
The digestive system of fish is similar to the digestive tract of other
animals. Carnivorous fish have short digestive tracts when compared to
herbivorous fish. The stomach and intestines contain submucosal
eosinophilic granular cells. The function of these cells is unknown. Some
species of fish (Salmonids) have pyloric ceca which are occasionally
confused with parasites. These ceca secrete the digestive enzymes required
to digest some food. Fish without the pyloric cecae have digestive enzyme
production in the liver and pancreas. It is not possible to divide the
intestine into large and small intestine.
The liver does not have the typical lobular architecture that is
present in mammals. In many species of fish there are areas of exocrine
pancreas (hepatopancreas) that are present near the small veins off the
hepatic portal vein.
The pancreas is scattered in the mesentery, primarily near the pylorus.
Fish do not have lymph nodes. Phagocytic cells are present in the
endothelial lining of the atrium of the heart and in the gill lamella.
There are no phagocytic cells (Kupffer cells) in the liver.
Melanomacrophage centers are present in the liver, kidney and spleen.
Melanomacrophage centers increase in number during disease or stress.
The fish thymus is the central lymphoid organ. This organ is located
subcutaneously in the dorsal commissure of the operculum.
Fish have the ability to produce specific immunoglobulins (IgM only)
and have both delayed and immediate hypersensitivity. Fish have the
ability to produce virus neutralizing, agglutinating, and precipitating
antibodies. Both B and T lymphocytes are present.
The heart is composed of two chambers; one ventricle and one atrium.
Some authors also describe the sinus venosus as the third chamber and
bulbus arteriosus as the fourth chamber. Blood flows from the heart
through the ventral aorta and the afferent branchial arteries, to the
gills for oxygenation. Oxygenated blood returns via the efferent arteries
to the dorsal aorta. The dorsal aorta then carries the oxygenated blood to
the body. Some oxygenated blood also leaves the dorsal aorta and goes to
the pseudobranch to be highly oxygenated and then is sent to the retina
which has a high oxygen demand.
The kidneys of fish develop from the pronephros and mesonephros. The
function of the kidney is osmoregulation. In freshwater fish, the kidney
saves ions and excretes water. In saltwater fish, the kidney excretes ions
and conserves water. The majority of nitrogenous waste is excreted through
the gills. The other function of the kidney is hematopoiesis with
hematopoietic tissue located in the interstitium of the kidney. This
function is primarily in the anterior kidney but can be found throughout
the entire kidney.
SPECIAL SENSE ORGANS
Lateral line system
There are two types of lateral line organs. These are the superficial
neuromast and the two lateral line canal organs. There are two types of
superficial neuromast, these are located in pits in the epidermis located
primarily on the head. Their function is not completely known but is
believed to aid in movement and orientation.
The second lateral line organ is the lateral line canal system which
runs the entire length of the fish with continuous extensions over the
head. This organ is sensitive to hydrostatic stimuli and sound.
|1. Lymphocystis Disease
- A) Iridovirus
- B) Observed in most freshwater and saltwater species.
- C) Clinically, fish are presented with variably sized white
to yellow cauliflower-like growths on the skin fins and
occasional gills. Occasionally, this virus may go systemic
with white nodules on the mesentery and peritoneum.
- D) Histopathology: Fibroblast undergoes cytomegaly with many
basophilic cytoplasmic inclusion bodies and a thick outer
hyalin capsule. The inflammatory response is variable but is
usually a chronic lymphocytic inflammatory infiltrate.
- E) The disease gains entry through epidermal abrasions. The
virus infects dermal fibroblasts.
- F) The disease is self-limiting and refractory to treatment.
Nodules may last several months and cause infected fish to be
susceptible to secondary bacterial infections. Reinfection can
|2. Herpesvirus salmonis (Herpesvirus disease of
- A) Herpesvirus
- B) Disease is observed primarily in the fry of Rainbow trout.
- C) Clinically the fish are lethargic with prominent gill pallor. Mucoid
fecal casts are commonly observed trailing from vent.
- D) Lesions:
1) Exophthalmus and ascites
2) Low hematocrit and numerous immature erythrocytes
3) Hemorrhage in eyes and base of fins
- E) Histopathology:
1) Multifocal areas of necrosis of the myocardium, liver, kidney, and posterior
gut (leading to cast formation)
2) Syncytial cells involving the acinar cells of the pancreas is
considered to be a pathognomoni sign.
- F) Transmission of the virus is believed to be direct.
- G) Control is by avoiding exposing susceptible trout to the virus. If
the disease occurs, raising the water temperature to 15°C or more will
|3) Channel Catfish Virus
- A) Herpesvirus
- B) Observed in fry or fingerling channel catfish (less than 10 gram
weight) during the summer when water temperatures are above 22oC.
- C) Clinically these fish usually show erratic swimming or spiraling
followed by terminal lethargy. Mortality is very high.
- D) Lesions:
1) Hemorrhage at the base of the fins and skins;
2) Ascites; exophthalmos; and pale gills;
3) Kidneys swollen and pale with hemorrhage;
4) Spleen is enlarged and dark red;
5) Gills usually pale;
- E) Histopathology: Multifocal areas of necrosis and hemorrhage
are observed in the posterior kidney, liver, intestines, and spleen.
F) Infection is direct with transmission of the virus in the water or
feed. Piscivorous birds, snakes, or turtles may mechanically carry the
virus from pond to pond. Transovarian transmission has not been
conclusively demonstrated but is suspected. Survivors are persistently
infected and become carriers for life.
- G) Control of the disease is by sanitation, purchasing of virus free
broodstock and lowering water temperature to less than 19°C during an
outbreak to lessen the mortality.
|4) Epithelioma papillosum (Fish Pox)
- A) Herpesvirus cyprini
- B) Non-fatal disease is observed in carp and other
- C) Lesions: Elevation of the epidermis with the formation of white to
yellow plaques over the body of the fish. Healed lesions usually turn
- D) Histopathology: There is epidermal hyperplasia with the epithelial
cells occasionally demonstrating intranuclear inclusion bodies.
- E) Transmission is unknown, however, it is probably direct.
|5) Infectious Hematopoietic Necrosis (IHN)
- A) Rhabdovirus
- B) The disease is observed in the fry of trout (rainbow) and salmon
and sockeye) with mortality up to 100%.
- C) Clinical signs and lesions:
1) Fish become lethargic or hyperactive.
2) The fish become dark in color.
3) Exophthalmus, abdominal distension, and fecal cast.
4) Hemorrhage on skin and viscera primarily at base of fins, behind the skull, and
above the lateral line.
5) Anemia with pale gills.
6) Surviving fish may develop scoliosis.
- D) Histopathology: There is prominent necrosis of hematopoietic tissue
including melanomacrophages of the kidney, red pulp of the spleen and
hepatic parenchyma. Necrosis of the submucosal eosinophilic granular cells
is considered pathognomonic for IHN. (This lesion is observed in other
systemic viral diseases.) Intranuclear and intracytoplasmic inclusions are
occasionally observed in acinar and islet cells of pancreas.
- E) The virus is transmitted by direct contact with infected survivors or by feeding
contaminated feed. The virus is probably shed in
contaminated semen and eggs. The disease is most severe at 10C and rare
at temperatures above 15C.
|6) Viral Hemorrhagic septicemia
- A) Rhabdovirus
- B) Widespread and very contagious viral disease of
rainbow trout. This is a serious disease of trout in
Europe. Affects both Salmonid in fresh water and sea
water. Disease occurs in temperatures below 14C.
- C) Two forms of the Disease --- Acute and Chronic
1) Acute disease: High mortality in affected fish.
Fish have pale gills, dark body coloration, ascites, exophthalmus and erratic swimming behavior
(spiraling). Hemorrhage is a common finding in the eyes, skin, serosal surfaces of the intestines and
muscles. Necrosis of the hematopoietic and lymphoid elements of the anterior kidney and congestion and
necrosis of the hepatic parenchyma are histopathologic findings.
2) Chronic disease: See a slower prolonged mortality.
Fish become lethargic, have pale anemic gills, darken skin coloration,
exophthalmus, and distention of the abdominal cavity. Internal organs are commonly involved with
splenomegaly, hepatomegaly, and swollen kidneys Turbot, sea bass,
and Atlantic salmon are commonly affected by similar viruses.
- E) Transmission is believed to be direct with contact of carriers and
contaminated water and feed. Vertical transmission via the egg is not
|7) Spring Viremia of Carp (SVC) and Swim Bladder Infection virus
- A) Caused by several subtypes of Rhabdovirus carpio.
- B) Disease occurs in carp and other cyprinids.
- C) Clinical Signs and Lesions:
1) Loss of coordination and equilibrium.
2) Exophthalmus and abdominal distension (ascites).
3) Inflamed and swollen vent.
4) Edema and hemorrhage in many organs.
5) In SBI see pronounced inflammation and hemorrhage of swim bladder.
- D. Transmission: Virus shed in feces and found in
|8) Infectious Pancreatic Necrosis (IPN)
- A) Birnavirus
- B) Affects most salmonids primarily rainbow trout and brook
trout. IPN has also been implicated in disease among several
- C) Clinical signs and lesions:
1) IPN is characterized by a sudden explosive outbreak with high
2) Affected fish become dark and rotate their bodies while
3) Diseased fish usually have distended abdomens and
4) The presence of a gelatinous material in the stomach and
anterior intestine is highly suggestive of IPN; mucoid fecal
casts are common.
5) Infected fish commonly have a low hematocrit and hemorrhage
in gut, primarily in the area of the pyloric ceca.
- D) Histopathology:
- Histologically, there is necrosis of the pancreatic acini, gut
mucosa, and renal hematopoietic elements. A moderate
inflammatory infiltrate is usually observed around the
pancreatic acini. Hyalin degeneration of skeletal muscle is also
- E) Virus can be transmitted vertically in the eggs.