Category: Aquaculture | Fish Disease and Health

Fish pox: a skin disease of fishes

Fish pox, or carp pox, is one of the oldest known diseases of fish, recorded as early as 1563. Fish pox is a chronic skin disease that occurs among several species of propagated cyprinids. It is also called carp pox, epithelioma papillosum, and papillosum cyprini.

Etiology:
The etiological agent, of this disease is Herpesvirus cyprini which was first reported by B. Hofer in 1904.

Character:

  • Icosahedral DNA virus with a 110-113 nm capsid diameter.
  • With it’s envelop, it measures 190 nm in diameter.
  • Enveloped.

Geographical range and species susceptibility:

  • Fish pox has been reported from most European countries, Japan, Russia, Israel, Korea, Malaysia and U.S.A. It is found mainly in freshwater. It can also reported from marine and brackish water.
  • The common carp and agasi carp (i.e., koi, fancy carp) are primary hosts. This disease also reported from bream and some aquarium fish.

Signs and symptoms:
External:

  • Superficial lesion on body surface, usually smooth and firms.
  • The tumors are milky white to gray and are raised about 1 to 3 mm above the skin on the head, fins, or anywhere on the body surface.
  • Dark pigmentation on the affected areas.
  • Growth retardation.
  • Distended abdomen.
  • Exophthalmia.
  • Hemorrhage on the operculum and abdomen.

Internal:

  • Hyperplasia of epithelium cells/ epithelial hyperplasia.
  • Neoplasms with spinal deformities.

Histopathology:

  • Scaled areas of skin show hyperblastic epidermal cells.
  • Similar hyperplastic epidermal lesions develop on the fins, but there are no mucus or alarm substance cells in the epidermis.
  • Epidermal cells are disrupted.
  • Infected cells are elongated and radiated.
  • Mitoses are common in the outer layers.
  • Mucous cells are in papillae.
  • Chromatin is marginated.
  • Endoplasmic reticulum is enlarged and coarse.
  • Mitochondria become swollen.
  • Ribosomes are abundant.

Epizootiology:

  • Transmision of the disease is horizontally and/or vertically.
  • Release of virus from ruptured cells that have sloughed from the skin of infected fish.
  • Disease can be transmitted by cohabitation.
  • Disease can also be transmitted by rubbing the healthy carp with the infected lesion area of carp.
  • Outbreak of fish pox occurred during low temperature in winter or early spring.

Diagnosis:

  • Diagnosis can be made on the basis of finding the distinctive clinical signs in carps and other cyprinids.
  • Histologic examination can be done on epithelial hyperplasia.
  • A definite diagnosis can be made by electron microscopy.

Pathogenesis:

  • Nutritional deficiency and chemical conditions of the water appears to have a predisposing role.
  • In nature, the incubation time is about 1 year to more but in tissue culture it is as early as 14-18 days after infection.
  • Optimum incubation temperature is 200C and replicate at 150C.

Significance:
In the tumor state, fish pox is regarded as benign and has not been associated with mortality. The lesions slough from the skin with little apparent harm to the host. However, from an aesthetic point of view, infected fish are unsightly and undesirable for food or display, particularly ‘fancy carp’ in Japan. When very young carp are infected with the virus, high mortalities may occur and survivors of such outbreaks may develop tumors within a year of infection, although the growths usually regress to an inapparent condition.

Prevention:

  • Avoidance of overcrowding.
  • Follow good management practice.
  • Remove infected fish.
  • Disinfected nets and equipment should be used.

Control:

  • Experimental vaccines provide the most effective means of control.
  • Ampicillin: orally 100 mg/kg body weight.
  • Oxytetracycline: 50-150 mg/kg body weight/day.

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Research Student, MS in Aquaculture, Bangladesh Agricultural University, Mymenshingh, Bangladesh. Email: sajjadhossain33@gmail.com. More...

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