Host and geographic distribution Diagnosis and contol

2. ONCORHYNCHUS MASOU VIRUS DISEASE

3. 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.

4. 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 )

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

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

7. 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

8. 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 Yamame
Yamame tumor virus (YTV) from Yamame

9. 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

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

11. 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

12. In 1991, OMVD was first reported in
rainbow trout and
is now a major problem in rainbow trout
pond culture in central Japan.

13. 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 fish
7 month old fish- infected but not die
5month old chum salmon & Yamame-
expt infected by intraperitonaeal
injection of YTV
Mortality and morbidity

14. 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.

15. 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

16. Electron microscopy of cultured Salmonid herpesvirus 2.

17. Transmission
SalHV-2 is transmitted by
1. 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.

18. 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 possib
‘egg-surface associated’.
 Horizontal transmission may be direct or vectoria
with water being the major abiotic factor.

19. 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

20. 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

21. Infectivity of OMV reduces
rapidly over the period of 1
week in rearing water, river
water or pond water,
compared with filter
sterilised rearing water,
bacteria living in
environmental water play an
important role in virus
inactivation.
Survival outside the host (i.e. in the natural
environment)

22. 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

23. 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 be
Susceptible

24. 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.

25. OMV-infected
hepatocytes with many
virions in a cytoplasm
showing degenerated
mitochondria and
fragmented

26. 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.

27. External signs
Diseased 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.

28. 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

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

30. Liver section of Salmonid herpesvirus 2 infected chum
salmon.

31. Electron micrographs of
OMV-infected cells.
(A) OMV-infected
hematopoietic cell showed
many nucleocapsids in a
nucleus
and enveloped virions in a
cytoplasm.
In this cell, an enveloped
virion was present in the
expanded space between two
nuclear membranes.
Electron dense chromatin
was increased and attached
on the inner nuclear
membrane. m: outer
nuclear membrane

32. 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.

33. 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.

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

35. 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.

36. 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

37. 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) Nodula
lesion consisted of cells with
basophilic nucleus in the
liver of survived fish in infectivit
experiment.

38. 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.

39. Virological Test
For 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

40. 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

41. 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

42. A polymerase chain reaction (PCR) method has been
accepted for confirmation of OMV
PCR 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.

43. 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

44. 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

45. 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.

46. 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.

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

48. 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.