Koi Herpes Virus - Spring Viremia of Carp -
What You Should Know*
Koi are generally hardy fish. They're descended
from the common carp and are tough, essentially
omnivorous fish with the ability to withstand a
range of living conditions.
As an ornamental specimen, the Koi is beautiful,
and sought after for it's highly strained color
Koi health and disease is essentially a balancing
act or "equilibrium" created between stocking
density, water-and-environmental conditions,
parasites, and the fish itself.
It was once said that "if you take care of the
environment, the fish will take care of
themselves". This was true until some of these
viruses started showing up with increasing
Introduction to the Viruses:
There are two known viruses of importance to Koi.
There are other viruses but these are important
from the perspective that they can quickly kill
the fish and are both highly contagious.
/ Spring Viremia of Carp (Rhabdovirus carpio)
/ Koi Herpes Virus
These viruses are similar and dissimilar. Some of
their differences and similarities are important.
Spring Viremia of Carp
Koi Herpes Virus
No. This virus was described in the literature
more than forty years ago.
No. KHV was reported in Japan fully ten
years before it's first outbreak or discovery
in Israel. The earliest documentation I can
find is from the 1980's
SVC has recently been shown to kill groups of
fish when experimentally injected with the
virus, earlier researchers maintained
that the SVC only allows opportunistic
bacterial infections which then can kill the
fish. Mortalities may be 20-30% if supportive
care is given and the environment is
KHV kills upwards of 70%-90% of exposed fish
which have not been previously exposed to KHV.
Yes - Spring
Yes - Spring and Fall
Endemic (native) to the USA?
Yes and No: The 'party line' is that the virus
had not formerly been found in North America
but there is emerging evidence that the virus
was indeed being encountered in fish kills in
Wisconsin almost a decade ago.
Reported "absence" of SVC from American waters
may have been due to a lack of testing. I
personally (ELJ) think that SVC is an endemic,
and highly morbid contributor to many of the
Springtime die-offs and illnesses we've seen
every year for the past two decades.
The problem is that testing for SVC can result
in quarantine or worse. Retailers are unlikely
to "step up to the plate" and endeavor to
discover this virus and limit its
Doubtful. This KHV virus seems to be infecting
"groups" of exposed fish which go on to infect
others, or simply die off en masse. It's own
virulence (aggressiveness) is probably
limiting it's morbidity.
Pale white lesions may result due to the
co-infection by bacteria. Fish may develop a
pink or red color in the skin as infection
Yes. Pale white lesions may appear in the
gills of affected fish. Excess slime,
especially on the head and nape of the fish
seems common. Body-color of the fish may
become blotchy and the internal organs may be
damaged or even liquefied.
People don't want to submit for, nor do some
labs want to test for; SVC because of the
maelstrom it causes. SVC is an RNA virus and
requires an extra step when using PCR
technology to diagnose it. When the virus is
not in a vulnerable host or is not in its
ideal temperature range for replication, it's
diagnosis is essentially impossible.
The PCR test and the other culture and
swabbing techniques available are quite
accurate for infected fish but false negatives
can occur. When the virus is not in a
vulnerable host or is not in its ideal
temperature range for replication, it's
diagnosis is essentially impossible.
Diagnosing "occult" (hidden) carrier-states of
KHV may be impossible with current technology.
Immune Carrier States?
Fish often survive SVC; but their carrier
state is unconfirmed.
Survivors of KHV are said to be clear of the
virus and cannot be re infected with KHV.
The lack of virus in post-infection specimens
is probably due to the difficulty in
detecting virus in asymptomatic fish or fish
outside the viruses' ideal range.
Kind of Virus
RNA virus, rhabdo (bullet) shaped.
DNA virus. (Herpes virus)
Can be cultured, there is a reverse PCR test
for this virus.
Can be cultured, can be detected via novel
nucleic acid tests (swabs), can be detected by
This morbid virus is reportable by law.
This highly virulent virus is not legally
reportable and is as yet unregulated.
Prevent exposure to the virus.
Prevent exposure to the virus.
Control - Treatment
SVC: If fish are supported in ideal
environments and secondary infections are
controlled through aggressive antimicrobial
therapy, including antimicrobial food and
injections, 70+ percent survival is possible.
KHV: Mortalities may be kept below 70% if the
fish are rapidly warmed to above 80 Degrees
To put the brakes on a late-summer outbreak,
you can let the temperature sail down into the
forties instead of heating, and the losses
will slow down as the virus is deprived of
it's ideal temperature range. Fish may still
die from prior damage done by the
Sooner or later, the fish will have to be
During an outbreak; if possible you can move
the fish as quickly as possible to
temperatures higher than 80 oF, or
lower than the seventies (in Fo)
The real issues concerning SVC are it's status as
a reportable virus. It's very possible that
many breeder and wholesale facilities (as well
as many residential ponds) have fish which harbor
this virus. Testing is currently possible, but is
not being undertaken on a widespread basis,
because of the cost, the lack of centralized and
unified regulation, and a reluctance of civilians,
and researchers to open that "can of worms".
Retailers concerned that their stocks could harbor
this virus would put themselves out of
business by soliciting SVC testing by a laboratory
and receiving a positive result
Fortunately, SVC isn't a terribly efficient killer
of fish and could be considered 'mild' at least
compared to KHV. Well-cared-for fish can often
survive the virus not unlike the way healthy
people survive the Influenza virus, and optimally
housed fish may not even break out with signs of
I for one do not spend much time worrying about
the SVC condition because I would neither subject
my customers to diagnosis (and potential
persecution caused by an SVC diagnosis), nor would
it change my treatment, which is antimicrobial
support "past" the ravages of the virus . I am, as
a healthcare provider to fish, almost alone with
this opinion *.
The real issues concerning KHV is it's
predilection for a narrow temperature range for
infection, and it's ability to hide when it's
outside those temperatures in asymptomatic (not
sick) fish. If you grind up a healthy-looking fish
which you think might have or be carrying KHV
looking for virus, you can easily miss the
diagnosis unless the fish is actually
viremic. When a fish is symptomatic
and sick with a KHV infection, the virus can
usually be cultured into certain cell lines,
detected by enzyme linked PCR tests, or even
detected by unique nucleic acids in it's
What it boils down to is this:
If you're considering buying some nice new fish
this Spring, how do you know the fish isn't
just sitting there; ready to explode with KHV as
soon as it hits seventy degrees Fahrenheit?
You don't have any security unless the fish
has been through the following cycle of
cold-then-warm, which are believed to be important
triggering events for KHV infections:
Warming, to the viruses ideal range in the
seventies (oF) allowing virus to
replicate and damage the fish.
So, a fish which has endured, and survived, a
temperate (North American) climate change from
winter to summer could be regarded as the safest
fish to buy but does not rule out that the fish
could be carrying the virus. Some dealers are
artificially inducing these cold-then-warm cyclic
changes in their recent imports to try and bring
these cases out of the woodwork before sale by
chilling and then warming the fish after
importation, creating a "mini" cycle.
Testing for KHV can prove the fish to be without
the virus and "not currently infected" but since
the carrier state is a relative "unknown" at the
present time, there is little security in a
negative KHV test in a healthy fish. A negative
KHV test in sick fish could be considered
much more reliable as most fish with active
infections have virus which is capable of
detection by available means.
Quarantine will become a necessity, not an
ideal, in 2003. This quarantine could arguably be
8-12 months to allow a complete "cold-warm-cold"
cycle in order to reveal occult KHV or SVC
It bears mentioning that it is the professional
opinion of most researchers and ornamental fish
health specialists in this field, that in the
interest of the health of our nation's Koi and
carp livestock, all individuals and
retailers suspecting that their fish might be
infected with SVC or KHV should request testing
for these infectious agents.
My (ELJ) position has been to recommend that
retailers and wholesalers decline SVC testing and
to destroy fish which might be infected. This
represents irresponsible behavior on the part
of the dealer and puts the hobby at risk because
it will hamper attempts to detect and eliminate
the SVC virus. However, the position is a result
of the following:
Currently, if your fish are diagnosed with SVC you
will be summarily bankrupted by the following
processes currently in place:
There is no financial compensation for
lost livestock which may be tested and
slaughtered. You will not be compensated
for lost business-days while under
quarantine. Requests to operate under a
new business name with new broodstock and
new production ponds will probably be (and
have been) denied.
No official process exists to formally
determine the length of impound and
quarantine. You may be under quarantine
for an indefinite period of time.
There are no mechanisms to protect your
identity and you may be informed of
your SVC infection along with the rest of
the industry, simultaneously.
There is no standard format, nor
standard interval for testing of your
peers or competitors, so you may be the
only organization subjected to the
penalties associated with reportable SVC
*Errors or omissions in the above are possible
but are unintentional. Some of the above is
based on hearsay, opinion or verbal exchanges
with researchers in the field. Newer
information may be available. Errors in fact
can be corrected with a much-appreciated fax
which could be sent to: 928 244 2772 If any of
the above information is proprietary or was
not intended for the public domain please
alert us using the above fax number.