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Viral Diseases of Fish
by Robert B. Moeller Jr., DVM

California Animal Health and Food Safety Laboratory System
University of California

Lymphocystis Disease

  1. Iridovirus
  2. Observed in most freshwater and saltwater species.
  3. 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.
  4. 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.
  5. The disease gains entry through epidermal abrasions. The virus infects dermal fibroblasts.
  6. 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 occur.

Herpesvirus salmonis (Herpesvirus disease of Salmonids)

  1. Herpesvirus
  2. Disease is observed primarily in the fry of rainbow trout.
  3. Clinically the fish are lethargic with prominent gill pallor. Mucoid fecal casts are commonly observed trailing from vent.
  4. Lesions:
    1. Exophthalmus and ascites
    2. Low hematocrit and numerous immature erythrocytes
    3. Hemorrhage in eyes and base of fins
  5. 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 pathognomonic sign.
  6. Transmission of the virus is believed to be direct.
  7. Control is by avoiding exposing susceptible trout to the virus. If the disease occurs, raising the water temperature to 15°C or more will minimize losses.

Channel Catfish Virus

  1. Herpesvirus
  2. Observed in fry or fingerling channel catfish (less than 10-gram weight) during the summer when water temperatures are above 22°C.
  3. Clinically these fish usually show erratic swimming or spiraling followed by terminal lethargy. Mortality is very high.
  4. 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;
  5. Histopathology: Multifocal areas of necrosis and hemorrhage are observed in the posterior kidney, liver, intestines, and spleen.
  6. 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.
  7. Control of the disease is by sanitation, purchasing of virus free brood stock and lowering water temperature to less than 19°C during an outbreak to lessen the mortality.

Epithelioma papillosum (Fish Pox)

  1. Herpesvirus cyprini (Cyprinid herpesvirus 1)
  2. Non-fatal disease is observed in carp and other cyprinids
  3. Lesions: Elevation of the epidermis with the formation of white to yellow plaques over the body of the fish. Healed lesions usually turn black.
  4. Histopathology: There is epidermal hyperplasia with the epithelial cells occasionally demonstrating intranuclear inclusion bodies.
  5. Transmission is unknown, however, it is probably direct.

Infectious Hematopoietic Necrosis (IHN)

  1. Rhabdovirus
  2. The disease is observed in the fry of trout (rainbow) and salmon (Chinook and sockeye) with mortality up to 100%.
  3. Clinical signs and lesions:
    1. Fish become lethargic or hyperactive.
    2. The fish become dark due to increase in pigmentation.
    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.
  4. 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.
  5. 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 10oC and rare at temperatures above 15°C.

Viral Hemorrhagic septicemia

  1. Rhabdovirus
  2. Widespread and very contagious viral disease of rainbow trout. This is a serious disease of trout in Europe. Affects both salmonids in fresh water and seawater. Disease occurs in temperatures below 14°C.
  3. 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.
    3. Turbot, sea bass, and Atlantic salmon are commonly affected by similar viruses.
  4. Transmission is believed to be direct with contact of carriers and contaminated water and feed. Vertical transmission via the egg is not reported.

Spring Viremia of Carp (SVC) and Swim Bladder Infection virus (SBI)

  1. Caused by several subtypes of Rhabdovirus carpio.
  2. Disease occurs in carp and other cyprinids.
  3. 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.
  4. Transmission: Virus shed in feces and found in contaminated eggs.

Infectious Pancreatic Necrosis (IPN)

  1. Birnavirus
  2. Affects most salmonids primarily rainbow trout and brook trout. IPN has also been implicated in disease among several nonsalmonid fish.
  3. Clinical signs and lesions:
    1. IPN is characterized by a sudden explosive outbreak with high mortality.
    2. Affected fish become dark and rotate their bodies while swimming.
    3. Diseased fish usually have distended abdomens and exophthalmus.
    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.
  4. 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 observed.
  5. Virus can be transmitted vertically in the eggs.
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