Viral diseases

These microorganisms cannot be seen with the naked eye. They cause many diseases in fish. Symptoms include swollen, fluid-filled body cavity, bulging eyes, bloody fins, bloody spots on the skin or base of scales, sores, and lesions, etc.

Lymphocystis Disease:

1. Iridovirus
2. Observed in most freshwater and saltwater species
3. Clinically fish are presented with variably sized white to yellow cauliflower 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 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 terroir gut )
     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 spiralling 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 broodstock and lowering water temperature to less than 19°C during an outbreak to lessen the mortality.

Epithelioma papillosum (Fish Pox)

1. Herpesvirus cyprini
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 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 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.

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 10°C.

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 low hematocrits 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.