Topic-ONCORHYNCHUS MASOU VIRUS DISEASE

BYAbishaFC&RI

Tuticorin

ONCORHYNCHUS MASOU VIRUS DISEASE

Oncorhynchus masou virus disease (OMVD) is an oncogenic and skin ulcerative condition coupled with hepatitis among salmonid fish in Japan, and probably in the coastal rivers of eastern Asia that harbour Pacific salmon.

Electron microscopy of negatively stained Salmonid herpesvirus 2.

ONCORHYNCHUS MASOU VIRUS DISEASE

Aetiological agent, agent strains caused by the Salmonid herpesvirus 2 (SalHV-2), first described as an oncogenic virus isolated from Oncorhynchus masou .

This virus is more commonly known as Oncorhynchus masou virus (OMV)

synonyms include the

nerka virus Yamame tumor virus (YTV) Oncorhynchus kisutch virus (OKV ) coho salmon tumor virus (CSTV or COTV) coho salmon herpesvirus (CHV) rainbow trout kidney virus (RKV) rainbow trout herpesvirus (RHV )

SalHV-2 is a recognized species in the family Alloherpesviridae and the genus Salmonivirus.

is believed to belong to the Family Herpesviridae, based on an icosahedral diameter of 120-200 nm, and enveloped, dsDNA properties.

Importance

Oncorhynchus masou virus disease (OMVD) is an economically significant disease of farmed salmonid fish (salmon and rainbow trout) in Japan.

This infection also occurs in wild fish.

In young fish, OMVD is a systemic disease with a high mortality rate.

Surviving salmon often develop cutaneous tumors, particularly around the mouth.

In chinook salmon

Host and geographic distribution 3 herpes virus – from salmonid fishes in Japan

nerka virus in Towada lake, Akita and Aomori Prefecture (NeVTA )Oncorhynchus masou virus (OMV) From Masou salmon or YamameYamame tumor virus (YTV) from Yamame

Host Species Epizootics of OMVD have occurred in pond culture of masu salmon, coho salmon (O. kisutch) kokanee salmon (O. nerka).

Juvenile rainbow trout (O. mykiss), yearling rainbow trout and adult rainbow trout have also been shown to be susceptible.

Chum salmon (O. keta) were susceptible to SalHV-2 in laboratory challenges

Amago salmon (O.rhodurus) and iwana salmon (Salvelinus pluvius) were not susceptible

Epizootiology By the beginning of the 1980s, OMV was widely distributed in wild masou salmon in the northern part of Japan.

In 1988, OMVD was diagnosed in pond cultured coho salmon and marine net pen reared coho salmon in Tohoku district, Japan

OMVD of coho salmon was successfully controlled

In 1991, OMVD was first reported in rainbow trout and

is now a major problem in rainbow trout pond culture in central Japan.

Observations from naturally occurring disease and experimental infections indicate that fish from one to five months old are most susceptible

3month old fish more susecptible than 5 month old fish7 month old fish- infected but not die

5month old chum salmon & Yamame-expt infected by intraperitonaeal injection of YTV

Mortality and morbidity

Epithelial perioral Tumors develop 4to 13 months after exposure to OMV or YTV

In recent years, these epizootics have primarily impacted rainbow trout of 100-500g

mortality has reached more than 80% in some cases.

Environmental factors Epizootics have been limited to rainbow trout, and reported in juvenile, yearling to large rainbow trout.

OMVD occurs in winter or at temperatures below 15°C where well water is used for culture in fresh water

Electron microscopy of cultured Salmonid herpesvirus 2.

Transmission SalHV-2 is transmitted by1. diseased fish 2. asymptomatic carriers.

This virus is shed in the feces, urine, sexual products at spawning, probably in skin mucus.

. Transmission is by direct contact or through the water. “Egg-surface associated” transmission probably occurs.

SalHV-2 can also be spread by living vectors and fomites.

Vectors No vectors or virus reservoirs have been identified. Known or suspected wild aquatic animal carriers No carriers of this virus have been identified

Disease pattern Transmission mechanisms The transmission of OMV is horizontal and possibly ‘egg-surface associated’.

Horizontal transmission may be direct or vectorial, with water being the major abiotic factor.

Animate vectors and inanimate objects also act in OMV transmission.

Clinically, infected juvenile rainbow trout and carrier adults are the reservoir of virus for water-borne transmission via ovarian fluid and egg-surface contamination to alevins.

No other reservoirs of virus have been identified

Incubation Period moribund rainbow trout were first seen 13 days after experimental infection. The period from infection to neoplasia varies from four to 18 months

Infectivity of OMV reduces rapidly over the period of 1 week in rearing water, river water or pond water, compared with filtersterilised rearing water,

bacteria living in environmental water play an important role in virus inactivation.

Survival outside the host (i.e. in the natural environment)

Stability of the agent (effective inactivation methods) OMV is stable at 5°C for more than 7 days,

but unstable at temperatures above 20°C or at partial-freezing temperatures.

It is sensitive to ultraviolet irradiation (103 μW/second/cm2), ozone or iodophor treatment.

also inactivated by alcohol and various other disinfectants, as well as heating (50°C, 1 minute) and freezing

Host factors Susceptible host species Epizootics of OMVD have been commonly occurred in juvenile rainbow trout.

Yearling rainbow trout and large rainbow trout have also been shown to be susceptible.

Masou salmon, chum salmon (O. keta), coho salmon and kokanee salmon have been shown to beSusceptible

Target organs and infected tissue

Initial infection by OMV manifests as a systemic and frequently lethal infection associated with haemorrhages.

Virus multiplication in the endothelial cells of blood capillaries, haematopoietic tissue and hepatocytes is observed.

OMV-infectedhepatocytes with many virions in a cytoplasm showing degenerated mitochondria and fragmented

Clinical Signs During the initial systemic infection, the clinical signs may include lethargy, anorexia, darkening of the body, skin ulcers and petechiae.

Many fish die.

External signsDiseased rainbow trout exhibit almost no external signs; although some fish manifest ulcerative lesions on the skin

Rainbow trout infected with Salmonid herpesvirus 2 exhibiting ulcerative skin lesions.

Internal signs Internal signs can include intestinal hemorrhaging, whiteA spots on the liver, skin ulcers and neoplastic tissue around the mouth parts or body surface

Coho salmon infected with Salmonid herpesvirus 2 exhibiting skin ulcers and white spots on the liver

Chum salmon infected with Salmonid herpesvirus 2 exhibiting hemorrhaging and white spots on the liver.

Liver section of Salmonid herpesvirus 2 infected chum salmon.

Electron micrographs of OMV-infected cells.

(A) OMV-infected hematopoietic cell showed many nucleocapsids in a nucleusand enveloped virions in a cytoplasm.

In this cell, an enveloped virion was present in the expanded space between twonuclear membranes.

Electron dense chromatin was increased and attached on the inner nuclear membrane. m: outernuclear membrane

Four to eighteen months later, some surviving fish develop epitheliomas (cutaneous carcinomas).

These tumors occur mainly on the jaws but also on the fins, operculum, cornea and body surface.

Tumors can persist for up to a year.

Infected rainbow trout may have very few external signs of disease other than skin ulcers, a darkened body and pale gills.

Fish that recover from OMVD often become carriers.

Four months after the first clinical signs, a varying number of surviving fish exhibit epithelioma

virus may be isolated from the tumor.

A carrier state frequently occurs, which may lead to virus shedding via the sexual products at the time of spawning.

Tumor on the jaw of chum salmon induced by Salmonid herpesvirus 2.

Tumor on the upper jaw of coho salmon induced by Salmonid herpesvirus 2.

Post Mortem Lesions Acute infections are characterized by edema and hemorrhages.

In salmon, the lesions may include skin ulcers, white spots on the liver and neoplasia around the mouthparts or on the body surface.

Tumors may also be found in the kidney.

Skin ulcers, pale gills with hemorrhages in the gill filaments, intestinal hemorrhages and white spots on the liver can occur in rainbow trout.

Swelling and hemorrhages of the spleen and kidneys have also been reported in this species

Disease Diagnostic Procedures Presumptive Diagnosis Certain relative key features such as life stage and species of fish, water temperature, disease signs, and disease history of the farm and stock of fish aid the diagnosis of OMVD.

To isolate SalHV-2, affected fish tissues such as the liver, kidney and spleen are examined by standard cell culture techniques .

Histopathological signs of OMV-infected rainbow trout. (A) Nodular lesion consisted of cells with a basophilic nucleus in theliver of survived fish in infectivity experiment.

In the case of the tumor tissue, I. tissue is cut and II. disinfected with iodophore (50 ppm, 15 min),III. washed with Hanks’s BSS.

Tumor tissue must be prepared for the primary culture or co-culture with RTG-2 cells.

Virological TestFor the purpose of virological survey of mature salmonid fish, ovarian fluid is collected with the addition of equivalent volume of antibiotic solution and overnight incubation at 5°C.

Processed specimens must be inoculated onto the rainbow trout gonad cells (RTG-2) or Chinook salmon embryo cells (CHSE-214).

Cytopathic effect includes cell rounding and giant syncytium formation

Cytopathic effect of Salmonid herpesvirus 2 infected CHSE-214 cells

Plaque assay Plaque assay procedures which use a methyl cellulose overlay are also used for isolation and enumeration of SalHV-2.

After one subculture of primary culture cells, the culture medium should be subjected to confirmatory tests

Confirmatory Diagnosis

Confirmation of SalHV-2 in cell culture is accomplished by serum neutralization tests with polyclonal rabbit antisera or monoclonal antibodies .

An ELISA and indirect fluorescent antibody test (I-FAT) for SalHV-2 have also been reported .

The FAT was specific and reacted with all isolates of SalHV-2 tested and required less time than the ELISA to obtain a definitive diagnosis

A polymerase chain reaction (PCR) method has been accepted for confirmation of OMVPCR using the F10 primer, (5’-GTA CCG AAA CTC CCG AGT C)R05 primer (5’-AAC TTG AAC TAC TCC GGG G) amplifies a 439 base-pair segment of DNA from OMV strains isolated from masu salmon, coho salmon and rainbow trout, liver, kidney, brain and nervous tissues.

Sensitivity of this in PCR was 100.8 TCID50/ml.

The PCR test can distinguish SalHV-2 from Salmonid herpesvirus 1 (SalHV-1), with the later species producing an 800 bp product.

Laboratory tests Oncorhynchus masou virus disease can be diagnosed by virus isolation in cell cultures;

appropriate cell lines include oRTG-2 (Rainbow trout gonad) and oCHSE-214 (Chinook salmon embryo) cells.

Infections can also be diagnosed by co-culturing neoplastic tissues with salmonid cell lines.

The identity of the virus is confirmed by virus neutralization, immunofluorescence, enzyme-linked immunosorbent assay (ELISA) polymerase chain reaction (PCR) tests

Viral antigens can be identified directly in tissues by immunofluorescence or ELISA techniques.

PCR can be used to detect nucleic acids in tissues.

Serologic tests including virus neutralization, indirect immunofluorescence, ELISA may be available, but these methods remain to be validated for routine diagnosis

Control In areas where OMDV is not endemic, outbreaks are controlled by culling,disinfection, quarantines and other measures.

Where this disease is endemic, good biosecurity and sanitation decrease the risk of introducing SalHV-2 to a farm.

Fertilized eggs should be disinfected. Fry and alevins should be raised on premises that are completely separate from areas where SalHV-2 carriers may be found.

Fish of different species should not be mixed.

Management should also address virus entry on fomites and in water.

SalHV-2 is readily inactivated by many common disinfectants including iodophors, sodium hypochlorite and potassium permanganate solution.

It is also susceptible to ozonization of seawater, UV irradiation or electrolyzation.

This virus is relatively labile, particularly at warmer temperatures, and does not survive for more than 5 to 7 days in fish rearing water.

Public Health There is no indication that OMDV is a threat to human health.

Vaccination A formalin-killed OMV vaccine produced by an OMV strain isolated from rainbow trout is effective, and has been used successfully to reduce the numbers of OMV replicating in ovarian fluid.

Chemotherapy Anti-herpesvirus agent, acyclovir: 9-(2-hydroxyethoxymethyl) guanine was effective in inhibiting replication of OMV in both in vitro and in vivo experimental infections of chum salmon fry with OMV.

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