Animal Diseases –
Aquaculture / Fish

Shrimp diseases

Infectious hypodermal and hematopoietic necrosis virus (IHHNV)Top

Epidemiology
The IHHNV is a linear single-stranded DNA with an estimated size of 4.1 kb. IHHNV is considered to be a member of the family Parvoviridae, genus Brevidensovirus. Most penaeid species can be infected with IHHNV, including the principal cultured species, P. monodon (black tiger shrimp/prawn), P. vannamei (Pacific white shrimp), and P. stylirostris (Pacific blue shrimp).

A large portion of the IHHNV genome has been found to be inserted in the genome of some genetic lines of P. monodon. There is no evidence that this variant of IHHNV is infectious.

Signs
IHHNV causes the chronic disease runt-deformity syndrome (RDS) in P. vannamei in which reduced, irregular growth, and cuticular deformities—rather than mortalities—are the principal effects. IHHNV infection in P. monodon is usually subclinical, but RDS, reduced growth rates, and reduced culture performance have been reported in IHHNV-infected stocks.

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Infectious myonecrosis virus (IMNV)Top

Epidemiology
IMNV is a totivirus, and closely related to giardia lamblia virus, a member of the family Totiviridae. The genome of IMNV consists of a single, double-stranded RNA molecule of 7560 bp. The principal host species in which IMNV is known to cause significant disease outbreaks and mortalities in farmed populations is P. vannamei.

Signs
Shrimp in the acute phase of IMNV will present focal to extensive white necrotic areas in striated (skeletal) muscles, especially in the distal abdominal segments and tail fan, which can become necrotic and reddened in some shrimp. Severely affected shrimp become moribund, and mortalities can be high immediately following a “stress” event and continue for several days.

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Taura syndrome virus (TSV)Top

Epidemiology
The genome of TSV consists of a linear, positive-sense single-stranded RNA of 10,205 nucleotides. TSV was listed as an unassigned species in the family Dicistroviridae in the most recent report of the International Committee on Taxonomy of Viruses.

The principal host species in which TSV can cause significant disease outbreaks and mortalities are Penaeus vannamei and P. stylirostris.

Signs
Infected shrimp display anorexia, lethargy, and erratic swimming behavior. They also present opacification of the tail musculature, soft cuticle, and, in naturally occurring infection, a red tail due to the expansion of the red chromatophores. Mortality during this phase can be as high as 95%.

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Yellow head virus (YHV)Top

Epidemiology
Yellow head virus (YHV) and other genotypes in the yellow head complex are classified by the International Committee on Taxonomy of Viruses as a single species (Gill-associated virus) in the genus Okavirus, family Roniviridae, order Nidovirales. YHV (genotype 1) is one of six known genotypes in the yellow head complex of viruses and is the only known agent of yellow head disease.

YHV is highly infectious for most known species of cultivated penaeid shrimp. Natural infections have been recorded from black tiger shrimp (P. monodon) and P. setiferus.

Signs
The gross signs of shrimps with YHV are:

  • An initial period or exceptionally high feed consumption followed by abrupt cessation of feeding and increased mortality in the population (not always seen).
  • Lethargic or moribund shrimp accumulate at the pond surface and edges with slow to erratic swimming behavior.
  • The region of the cephalothorax may be yellowish due to the light yellow color of the underlying and abnormally soft hepatopancreas (not always seen).
  • Overall body color always of abnormally light or bleached appearance.

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White spot syndrome virus (WSSV)

Epidemiology
White spot syndrome virus (WSSV) is a double-stranded rod-shaped DNA virus that belongs to the family Nimaviridae, genus Whispovirus. WSSV has an extremely wide host range. The virus can infect a wide range of aquatic crustaceans, especially decapod, including marine, brackish and freshwater prawns, crabs, crayfish, and lobsters.

Signs
The clinical signs of infected penaeid shrimps are:

  • Sudden reduction in feeding, exhibiting lethargic behavior
  • Red discoloration in P. monodon, L. vannamei, and L. stylirostris.
  • Soft, loose shells
  • White spots 0.5 to 2 mm under cuticle (less common in Western Hemisphere penaeids)
  • Up to 100% mortality within 3 days of onset of disease signs.

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Fish diseases

Infectious salmon anemia virus (ISAV)Top

Epidemiology
ISAV is an enveloped virus, with a genome consisting of eight single-stranded RNA segments with negative polarity. The morphological, physiochemical, and genetic properties of ISAV are consistent with those of the Orthomyxoviridae, and ISAV has been classified as the type species of the genus Isavirus within this virus family.

Natural outbreaks of ISA have only been recorded in farmed Atlantic salmon, and in Coho salmon (Oncorhynchus kisutch) in Chile. In Atlantic salmon, disease outbreaks are mainly reported in seawater cages, and only a few cases have been reported in the freshwater stage, including one case in yolk sac fry.

Signs
The clinical signs of ISAV are:

  • Increased mortality
  • Lethargic fish
  • Gill pallor due to anemia
  • Hemorrhage in the anterior eye chamber
  • Exophthalmia

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Koi herpes virus (KHV)Top

Epidemiology
Koi herpes virus disease (KHVD) is a herpes virus infection capable of inducing a contagious and acute viremia in common carp (Cyprinus carpio) and varieties such as koi carp and ghost carp. The virus is highly contagious and may cause up to 100% mortality.

KHV is a double-stranded DNA virus of size 295kb in the family Herpesviridae with unclassified genus.

Signs
The clinical signs of ISAV are:

  • Lethargic or erratic behavior
  • Loss of balance
  • Loss of mucus resulting in dry, rough patches
  • Sloughing of mucus and sunken eyes
  • Necrotic patches of dead tissue in gills
  • Often with secondary infection of bacteria and fungi

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Infectious hematopoietic necrosis (IHN)Top

Overview
The IHN disease belongs to the Rhabdoviridae family and is an OIE notifiable disease. It is prevalent within salmonid populations in the Pacific Northwest and Alaska and has been reported in Asia and Europe, with the majority of epizootics attributed to the importation of infected eggs or fry.

While Arctic char and lake trout appear to be resistant to IHN, the disease has been in found in various subspecies of trout (cutthroat, steelhead, and rainbow) and salmon (sockeye, chinook, kokanee, and Atlantic).

Outbreaks of IHN typically occur at water temperatures of 10–12°C and typically impact fry (fish <2 months old), which can experience a mortality rate of >90% if afflicted with an acute version of the disease. In addition to age, density, water temperatures, and overcrowded transportation conditions are additional risk factors.

Signs
Sick animals may display a variety of signs ranging from pale gills, darkened coloration, mucoid feces, and distended abdomens, as well as lethargy with sporadic hyperexcitability.

Human health risk
It does not appear that IHN presents a zoonotic risk.

Economic impact
Acute outbreaks can yield high mortality rates, which can pose a significant economic impact in farm environments where rainbow trout are being reared. Additionally, Atlantic and Pacific salmon can be adversely affected by acute IHN outbreaks.

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Infectious pancreatic necrosis (IPN)Top

Overview
IPN is caused by a birnavirus and is a systemic, contagious disease that affects salmonid fry and fingerlings. IPN has been reported worldwide with the exception of Iceland and Australia and is a disease that is transmitted both vertically and horizontally.

A variety of species are highly susceptible to IPN, including rainbow trout, brook trout, sea bass, yellowtail, turbot, freshwater eels, and aquatic invertebrates (mollusks and crustaceans). Morbidity and mortality typically occurs in young animals (<3 grams).

Signs
Sick animals may display several visible signs, including corkscrew swimming patterns, darkened coloration, exophthalmia, and external petechiation. Affected fish may also be ataxic and anorectic.

Human health risk
It does not appear that IPN presents a zoonotic risk.

Economic impact
IPN can have a tremendous economic impact on cultured fish, particularly salmonids. In these instances the costs can be attributed to the high mortality rates of young fish that have been infected by the disease, or the destruction of infected stocks even in the absence of disease. Additionally, aquaculturists may experience economic hardship as a result of regulatory considerations that restrict the transport of IPNV carrier fish between local, regional, and/or national boundaries. More often than not, transport restrictions and/or the destruction of infected fish can be more economically damaging than the losses associated with direct mortalities.

Spring viraemia of carp (SVC)Top

Overview
SVC is caused by Rhabdovirus carpio and is an acute, virulent, and OIE notifiable disease that affects cultured carp. The disease impacts various species of carp (common, koi, crucian, bighead, silver, and grass) and has been reported in the former USSR, Europe, and USA.

Infected fish shed the virus through feces, urine, and gill mucus, as well as the exudates of skin blisters. Transmission is through the water or direct contact.

Infectious SVC may thrive in water for >4 weeks at temperatures of 10°C, and persist in mud for >6 weeks at temperatures of 4°C. There are several known vectors, such as aquatic arthropods (carp louse and leech) and fish-eating birds.

Signs
Sick animals may display a variety of signs that include, but are not limited to, darkened coloration, pale gills, hemorrhage, enlarged eyes, and swollen abdomen. Infected fish may also develop a protruding vent along with thick mucoid fecal casts.

Human health risk
It does not appear that SVC presents a zoonotic risk.

Economic impact
Given the highly contagious nature of this viral disease among farmed carp and the 90% mortality rate associated with the disease in young fish, SVC outbreaks can cause significant economic losses.

Viral hemorrhagic septicaemia virus (VHSV)Top

Epidemiology
VHS is a disease of farmed rainbow trout, farmed turbot, farmed Japanese flounder as well as a broad range of wild freshwater and marine species (such as Atlantic salmon) caused by VHSV, a virus belonging to the genus Novirhabdovirus, within the family Rhabdoviridae.

There are different strains of VHSV that may affect species differently. The virions are bullet-shaped, and contain a negative-sense, single-stranded RNA genome of approximately 11,000 nucleotides.

Signs
Disease signs can include:

  • Inactive or overactive behavior
  • Pale gills and internal organs
  • Bloated abdomen
  • Bulging eyes
  • Dark body color
  • Fluid in body cavity

On the other hand, fish with VHSV may not exhibit any external signs of infection. Internally, the liver, spleen, and intestines may be congested with hemorrhages. This is generally true for fish in chronic state of infection.

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Viral nervous necrosis virus (VNNV)Top

Epidemiology
Viral encephalopathy and retinopathy (VER), or viral nervous necrosis (VNN), has been reported as a serious disease of larval, juvenile, and sometimes older marine fish. It occurs world-wide, except for Africa. To date, the disease has been reported in at least 30 fish species, with the greatest impact being in sea bass, groupers, jack, parrotfish, puffer, and flatfish.

VNNV belongs to the family Nodaviridae, genus Betanodavirus. The virion consists of an icosahedral, non-enveloped virus with a diameter ranging from 20 to 34nm. The genome is composed of a bisegmented, single-stranded, positive-sense RNA.

Signs
All diseases are characterized by a variety of neurological abnormalities, such as erratic swimming behaviour (spiral, whirling, or belly-up at rest) and vacuolation of the central nervous tissues. Usually, there is also vacuolation of the nuclear layers of the retina. In general, younger fish have more severe lesions and higher mortality.

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