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Viral hemorrhagic septicemia
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VHS disease in a gizzard shad
Electron Micrograph of VHSV
Viral hemorrhagic septicemia (VHS) is a deadly infectious fish disease caused by the Viral
hemorrhagic septicemia virus (VHSV, or VHSv). It afflicts over 50 species of freshwater and marine
fish in several parts of the northern hemisphere.[1] VHS is caused by the viral hemorrhagic
septicemia virus (VHSV), different strains of which occur in different regions, and affect different
species. There are no signs that the disease affects human health. VHS is also known as "Egtved
disease," and VHSV as "Egtved virus."[2]
Historically, VHS was associated mostly with freshwater salmonids in western Europe, documented
as a pathogenic disease among cultured salmonids since the 1950s.[3] Today it is still a major
concern for many fish farms in Europe and is therefore being watched closely by the European
Community Reference Laboratory for Fish Diseases. It was first discovered in the US in 1988 among
salmon returning from the Pacific in Washington State.[4] This North American genotype was
identified as a distinct, more marine-stable strain than the European genotype. VHS has since been
found afflicting marine fish in the northeastern Pacific Ocean, the North Sea, and the Baltic
Sea.[3] Since 2005, massive die-offs have occurred among a wide variety of freshwater species in
the Great Lakes region of North America.
Contents
[hide]

1 Virus taxonomy

2 Molecular Virology

3 Virus subtypes

4 Evolution

5 Great Lakes Type IV-b
o
5.1 Great Lakes regional distribution
o
5.2 Government regulation

6 Transmission

7 Symptoms

8 Diagnosis
o
8.1 Field diagnosis
o
8.2 Gross pathology (non-laboratory)
o
8.3 Histopathology (microscopic tissue analysis)
o
8.4 Virology (definitive testing)
o
8.5 Prevention

9 References

10 External links
Virus taxonomy[edit]
Viral hemorrhagic septicemia
virus
Virus classification
Group:
Group V ((-)ssRNA)
Order:
Mononegavirales
Family: Rhabdoviridae
Genus:
Novirhabdovirus
Species: Viral hemorrhagic
septicemia virus
VHSV is a negative-sense single-stranded RNA virus of the order Mononegavirales,
family Rhabdoviridae, and genus Novirhabdovirus.[5] (ICTV virus classification, unlike biological
classification, begins with the order.)[6] Its NCBI-assigned Taxonomy ID is 11287. Another related fish
rhabdovirus in the Novirhabdovirus genus is the Infectious hematopoietic necrosis virus (IHNV),
which causes infectious hematopoietic necrosis (IHN) disease in solmonidae.
The viral cause of the disease was discovered in 1963 by M. H. Jenson.[7] The virus is an enveloped,
bullet-shaped particle, about 180 nm long by 60 nm in diameter, covered with 5 to 15 nm
long peplomers.[2]
Molecular Virology[edit]
The genome of VHSV is composed of approximately 11-kb of single stranded RNA, which contains
six genes that are located along the genome in the 3′-5′ order: 3′-N-P-M-G-NV-L-5′, nucleocapsid
protein (N), polymerase-associated phosphoprotein (P), matrix protein (M), surface glycoprotein (G),
a unique non-virion protein (NV), and virus polymerase (L).
Reverse genetics is a powerful tool to study and characterize the previously unknown viral genes.
Reverse genetics system is currently available for VHSV.[8][9] A vaccinia virus free reverse genetic
system for Great Lakes VHSV (Genotype IVb) was developed by a research group from the
USA.[9] This system allows the investigators to explore the functional properties of individual viral
genes of VHSV in detail. This system was immediately utilized to characterize the non-virion (NV)
gene of Novirhabdoviruses. Even though it has been demonstrated that the NV gene is not
necessary for viral replication, it is highly essential for viral pathogenesis. A new role of NV protein
has been discovered and demonstrated that it inhibits apoptosis at the early stage of viral
infection.[10] This discovery unlocked the mystery of presence of NV protein in Novirhabdoviruses.
Virus subtypes[edit]
Different isolates (unique strains) of VHSV are typically grouped by genotyping. It is found that
genotype groups are divided more geographically than by host species. Earlier studies used different
numbering systems,[11][12] but the following system has come into common usage based on genotype
similarity based on sequencing of the N- and G-genes. Types I-III are enzootic to Europe, and Type
IV to North America, and Type I and type IV isolates are further subdivided, as follows:
Type
Prevalent host type and location
I-a
Farmed rainbow trout and a few other freshwater fish in continental Europe[13]
I-b
Marine fish of the Baltic Sea, Skagerrak, Kattegat, North Sea, Japan[1]
I-c
Farmed rainbow trout Denmark
I-d
Farmed rainbow trout in Norway, Finland, Gulf of Bothnia
I-e
Rainbow trout in Georgia, farmed and wild turbot in the Black Sea[14]
II
Marine fish of the Baltic Sea
III
Marine fish of the British Isles and northern France, farmed turbot in the UK and Ireland, and
Greenland halibut (Reinhardtius hippoglossoides) in Greenland[15]
IV-a
Marine fish of the Northwest Pacific (North America), North American north Atlantic
coast,[16] Japan, and Korea[1][17]
IV-b
Freshwater fish in North American Great Lakes region[17]
Type I-a was the only strain known from VHSV's discovery in 1963 until the late 1988, isolated to fish
farms in continental Europe, affecting primarily rainbow trout and occasionally brown trout or pike.[18]
In 1988, the first marine strain of VHSV, now designated type IV, was found in normal-appearing
salmon returning from the Pacific to rivers of Washington State. This strain and other marine strains
was not lethal to rainbow trout. The discovery prompted further studies, and by the mid-1990s,
marine VHSV was found in eight species along the northern North America's Pacific coast, and 14
species in and around the Atlantic's North Sea.[19] 1996 saw the first VHSV in Japan, among
Japanese flounder farmed in the Seto Inland Sea,[20] and different genotypes have appeared in
different areas since then.[11] Type IV was later found off North America's northern Atlantic coast, in
Atlantic herring (Clupea harengus)[21] mummichog (Fundulus heteroclitus), stickleback (Gasterosteus
aculeatus aculeatus), brown trout (Salmo trutta), and striped bass (Morone saxatilis),[15] as well as
dozens of freshwater species in the Great Lakes.
VHSV continues to be found in new geographical areas, in new species of fish. This is thought to
represent both the spread of the virus into new areas, as with VHSV egg and live fish transfers from
North America to Asia, or feeding of raw marine fish to inland farmed trout in Finland,[13] as well as
discovery of existing populations, as with an apparently well established marine reservoir in the
Black Sea.[14]
To keep track of the distribution of different VHSV genotypes, a database
called Fishpathogens.eu[22] has been created to store data on different fish pathogens (including
VHSV) and their sequences.
Evolution[edit]
This virus appears to have evolved in the Pacific Northwest of North America.[23] It appears to have
diverged into several strains ~300 years ago.
Great Lakes Type IV-b[edit]
The Type IV-b strain of VHSV has been spreading among freshwater fish in the Great Lakes region
since at least 2003, resulting in some massive die-offs since 2005 of many species in the affected
lakes.[1] Originally found off the Atlantic coast of Canada, it was considered a low mortality marine
strain. Its first detection in freshwater was in Lake Ontario in 2005, and then in an archived 2003
sample from Lake St. Clair.[24] The isolate was named MI03GL and was sequenced for its entire
genome.[25]
The North American genotype of the virus, in addition to moderate mortality to salmonid species,
including various varieties of trout, is also proving virulent among a wide variety of warm-water
species previously considered resistant to VHS. The Great Lakes region variant has killed lake trout,
steelhead trout, chinook salmon, yellow perch, gobies, emerald shiners, muskies, whitefish, and
walleye. While the European strain of VHSV is particularly deadly to rainbow trout, the Great Lakes
region variant affects the species only mildly, as is typical with primarily marine genotypes.
Great Lakes regional distribution[edit]
An archived 2003 sample from Lake St. Clair of Great Lakes muskellunge is the earliest confirmed
case of VHSV within the Great Lakes region.[17] Lake St. Clair connects to Lake Erie through
the Detroit River to the south, and to Lake Huron through the St. Clair River to the north. The sample
was not tested for VHS until 2005, after the disease was detected in Lake Ontario.[24]
2005 samples of Lake Ontario freshwater drum and Lake Huron lake whitefish were infected with
VHS. Initially classified as an unknown rhabdovirus, the Lake Ontario sample was confirmed to be
VHSV in 2006, while the Lake Huron sample was confirmed to be VHSV in 2007.[17]
2006 saw mass die-offs from VHS in Lake Erie, the St. Clair River, the Detroit River, and the St.
Lawrence River, which connects the Great Lakes to the Atlantic Ocean.[17] VHS was further detected
in the Niagara River, which connects Lake Erie to Lake Ontario.[26] It was also found in a walleye dieoff in the landlocked inland Conesus Lake, the westernmost of the Finger Lakes in western New
York state.[27] This was the first case in the region outside waters contiguously connected to the
Great Lakes.
On May 12, 2007, the Wisconsin DNR announced the likely presence of VHS in Wisconsin's
inland Little Lake Butte des Morts. Preliminary tests of samples of freshwater drum collected on
May 2 were positive, and the announcement came amidst a die-off of hundreds of freshwater drum
there and on neighboring Lake Winnebago.[28] Preliminary tests later indicated VHS in specimens
from Lake Winnebago.[29] The lakes drain through the Fox River to Lake Michigan's Green Bay.
On May 17, 2007, the Michigan DNR confirmed the presence of VHS in the Michigan's inland Budd
Lake, a popular fishing destination is in the central part of Michigan's lower peninsula.[30] A major dieoff of VHS-positive muskies, bluegills, and black crappie began on April 30, 2007.[31]
On May 24, 2007, preliminary tests indicated the presence of VHS in a brown trout from Lake
Michigan, the second largest freshwater lake in the United States. Contamination in the lake had
been expected for months by experts, since the presence of VHS was confirmed in the connected
Lake Huron.[32]
On July 14, 2007, federal labs confirmed the presence of VHS in Skaneateles Lake, the second of
New York's Finger Lakes to test positive for the disease.[33] The disease caused a large die-off of
bass in the spring of 2007.
Government regulation[edit]
Before the Type IVb die-offs in the Great Lakes, few states regulated transfer of live species other
than for salmonids. Since 2005, new policies have been adopted concerning fish and egg transfer,
use of live bait, and water transfer, aimed at curtailing the spread to new lakes and rivers in the
region. As of July 13, 2007, new rules have been enacted in the Canadian province of Ontario,[34] and
US states of Michigan, New York, Ohio, Pennsylvania, and Wisconsin, while they are currently being
drafted in Illinois, Indiana, and Minnesota.[35] Additionally, the USDA's Animal and Plant Health
Inspection Service issued a federal order in the fall of 2006 barring the transfer of all live susceptible
species from the eight states bordering the lower Great Lakes, as well as importing such species
from The Canadian provinces of Ontario and Quebec.[36] [37]
Transmission[edit]
VHSV can be spread from fish to fish through water transfer, as well as through contaminated
eggs,[3] and bait fish from infected waters.[38] The emerald shiner is a particularly popular bait fish in
the Great Lakes region, and is among the species afflicted.[39]
Survivors of the disease can become lifelong carriers of the virus, contaminating water with urine,
sperm, and ovarian fluids.[1] The virus has been shown to survive two freeze/thaw cycles in a
conventional freezer, suggesting both live and frozen bait could be a transmission vector. In Europe,
the gray heron has spread the virus, but it does so mechanically; the virus is apparently inactive in
the digestive tract of birds.[2]
Symptoms[edit]
Fish that become infected experience hemorrhaging of their internal organs, skin, and muscle. Some
fish show no external symptoms, but others show signs of infection that include bulging eyes,
bloated abdomens, bruised-looking reddish tints to the eyes, skin, gills and fins. Some infected fish
have open sores that may look like the lesions from other diseases or from lamprey attacks.[40]
There may also be a nervous form of the disease where fish are constantly flashing and showing
abnormal behaviour.[41]
Diagnosis[edit]
Field diagnosis[edit]
Living fish afflicted with VHS may appear listless or limp, hang just beneath the surface, or swim
very abnormally, such as constant flashing circling due to the tropism of the virus for the brain.
External signs may include darker coloration, exophthalmia ("pop eye"), pale or red-dotted gills,
sunken eyes, and bleeding around orbits (eye sockets) and at base of fins.[17][18][42]
Genetics researchers at the Lake Erie Research Center at the University of Toledo are developing a
test that will speed diagnosis from a month to a matter of hours.
Gross pathology (non-laboratory)[edit]
VHSV is a hemorrhagic disease, meaning it causes bleeding. Internally, the virus can cause
petechial hemorraging (tiny spots of blood) in internal muscle tissue, and petechial or severe
hemorrhaging in internal organs and other tissues. Internal hemorrhaging can be observed as red
spots inside a dead fish, particularly around the kidney, spleen, and intestines, as well as the swim
bladder, which would normally have a clear membrane. The liver may be pale, mottled with
red hyperemic areas, the kidney may be swollen and unusually red, the spleen may be swollen, and
the digestive tract may be empty.[18]
External signs are not always present, but if they are, hemorrhaging on the skin's surface can
appear as anywhere from tiny red dots (petechiae) to large red patches.
Histopathology (microscopic tissue analysis) [edit]
Preliminary diagnosis involves histopathological examination, observing tissues through a
microscope. Most tissue changes can be observed as minor to major necrosis (cell death) in the
liver, kidneys, spleen, and skeletal muscle. The hematopoietic (blood-forming) areas of the kidney
and spleen are the initial area of infection, and should show necrosis. The gill may have thickened
lamellae, and the liver may have pyknotic nuclei. Skeletal muscle accumulates blood but does not
suffer much damage.[18]
Virology (definitive testing)[edit]
Electron microscopy can reveal the bullet-shaped rhabdovirus, but is not adequate for definitive
diagnosis.
The Manual or Diagnostic for Aquatic Animals, 2006, is the standard reference for definitive tests. In
most cases, cell culturization is recommended for surveillance, with antibody tests and reverse
transcription polymerase chain reaction (RT-PCR) and genetic sequencing and comparison for
definitive confirmation and genotype classification.[18][42]
Virus neutralisation is another important method of diagnosis, especially for carrier fish.
Prevention[edit]
Thoroughly cleaning boats, trailers, nets and other equipment when traveling between different lakes
and streams also helps. The only EPA-approved disinfectant proven effective against VHS
is Virkon AQUATIC (made by Dupont).[43] Chlorine bleach kills the VHS virus, but in concentrations
that are much too caustic for ordinary use. Disinfecting stations can be found at various inland lake
boat launches in the Great Lakes region.
References[edit]
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