KOI HERPES VIRUS – SHOULD WE BE WORRIED
The mere mention of KHV is normally enough to drive
fear and dread into the hearts and minds of most koikeepers particularly if
there is any indication that your own koi are affected by this deadly and
indiscriminate killer. It was only
a few short years back that KHV emerged within the hobby as a major potential
health threat.
One of the
first instances that came to my attention was of a koikeeper in England who
had purchased koi from a particular dealer and then had a wipe out of most
of his koi collection due to KHV (Koi Herpes Virus). This hobbyist was so incensed by the reluctance of the supplying
koi dealer to accept responsibility for supplying koi infected with KHV that
the matter went to Court and the hobbyist documented the whole account of
the court case on a web site. It made
for interesting reading with all the claims and counter claims and expert
witnesses being called to give evidence. Little
did I realise at the time that this new threat to the wellbeing of our koi
would gradually become as prominent a health issue as it is today.
Now in Autumn 2006 I read regular reports on internet
bulletin boards of hobbyists experiencing massive koi losses through KHV,
reports and rumours of dealers affected and only a couple of weeks ago indications
that KHV has now been reported among certain breeders in both the north and
south of Japan. It has been well reported that Momotaro Koi Farm experienced
a KHV outbreak in Autumn 2005 which was a major shock to the industry given
their reputation as one of the top breeders in Japan. Their outbreak was a major set back for me as
at the time, I had three koi at Momotaro that had been purchased (via Yume
Koi) and had been left to grow on. All
three had been purchased some time before as 9 to 12 month old tosai and at
the time of the outbreak two were three year old sansai (expected to be around
28 inches) and one was a two year old nissai (expected to be around 24 inches).
I was intending to have them harvested and brought
home during 2006 and was looking forward to checking out whether their growth
and development had turned out as well as I had initially hoped at the time
of purchase. Unfortunately I was not
destined to enjoy these koi as I was devastated to learn that all three were
lost to the Momotaro KHV problem.
Although not legally obliged to replace these koi
(as koi left to grow on at breeders are usually left at the owner’s risk)
I was pleased to learn that Momotaro would provide replacements. However it
took them all of last Autumn and Winter to test their koi stocks for KHV,
identify and cull affected koi and resume trading by which time all surviving
stocks were due to be placed back out in the mud ponds for the 2006 growing
season. I was therefore unlikely to be offered replacements until Autumn 2006
at the earliest.
An early resolution to the replacement issue was found
when Mike Snaden of Yume Koi (the dealer through whom I had purchased the
koi) kindly offered replacements from his own stocks. However just to prove that lightening can strike
twice in the same place, within days of selecting replacements with Mike my
new koi were caught up in the KHV outbreak that devastated Yume Koi following
their return from the BKKS National Show in June. Fortunately I had not yet
collected the koi otherwise KHV would surely have been introduced to my home
koi stocks – a lucky escape as it turned out although having this second group
of koi involved in a KHV outbreak did not make me feel very lucky at the time.
During my two run ins with KHV I found there was a
general lack of available literature about the virus and in some quarters
a tendency for misunderstanding and mis-information surrounding this nasty
virus. For information I list below some of the key bits of information I’ve
managed to gather over the last year:-
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KHV was first identified in Israel in 1998 and
has now spread world-wide.
-
It is a virus that affects koi and common carp
but goldfish and crucian carp can also be infected.
-
It is highly contagious and affected stocks that
experience an outbreak may typically see 80% to 100% losses.
-
Once triggered the virus can spread with remarkable
speed resulting in frequent mortalities which may occur within 1 to
2 days following the onset of clinical signs.
-
There is currently no cure although vaccines
are being investigated and are being trialled in Israel.
-
Infection occurs through the gills and/or the
gut and propagates in the intestine and kidney of infected fish.
-
KHV can be difficult for koikeepers to diagnose
as symptoms vary from koi to koi but typical signs may include lethargy,
gill problems, sunken eyes, erratic swimming and a notch/depression
in the nose. Some outbreaks
also report heavy mucus production at the start of an outbreak causing
the mucus to slough off the fish in layers followed by the complete
opposite with no mucus evident as the disease progresses.
-
When exposed to the virus some koi develop a
strong immune response that may fight off the virus in certain instances
and survivors then carry the virus in a dormant state waiting to be
triggered at a later date.
-
Triggers known to activate the virus in dormant
carriers include stress and exposure to specific temperatures.
-
Virus outbreaks normally arise within a temperature
range of 17 - 26°C and there is some indication that carriers need to
be exposed to both high and low temperatures within this range in order
to trigger an outbreak.
-
KHV outbreaks may be stopped at temperatures
above 30°C as the virus reverts to a dormant state although it is not
yet known whether such survivors are cured or merely remain as latent
carriers.
-
The virus dies fairly quickly in empty ponds
with no koi although it is advisable to disinfect affected systems and
quarantine new fish.
-
Virus outbreaks may not have been introduced
by the last purchase/addition to the pond as the virus may have been
dormant in carrier koi for some considerable time.
-
KHV is not yet a notifiable disease unlike the
less common Spring Viraemia of Carp disease (SVC) which must be reported
to CEFAS ( the Centre for Environment Fisheries & Aquaculture Science)
if an outbreak is suspected.
-
One of the difficulties with KHV is that initial signs
are often similar to a number of other more frequently experienced koi related
ailments. However rapid and frequent koi mortalities will inevitably follow
and it should become fairly obvious that something other than normal parasite
or bacterial issues are involved. Unfortunately
by the time KHV is suspected it will be too late in most instances to prevent
losses. Diagnosis cannot be confirmed at home or by professional vets.
Specialist laboratories or koi health consultants will need to be engaged.
There are two fundamentally different laboratory tests
currently available to test for KHV:-
PCR (Polymerase Chain Reaction) tests have been available
since 2002 and are carried out on tissue samples. The test will establish if suspect koi have
contracted KHV. Unfortunately any
suspect koi (if not already dead) submitted for PCR test must be killed in
order to obtain the samples so the PCR test may not be appropriate if it is
one of your favourite koi that you suspect may have the virus unless you are
prepared to sacrifice it to obtain confirmation one way or the other.
A more recently developed test is the ELISA (Enzyme
Linked Immunosorbent Assay) test which examines a blood sample taken from
suspect live koi and will establish if the koi has developed antibodies to
the virus. Any koi that has been exposed to the active virus will develop
antibodies so a negative ELISA test is confirmation that the koi has not been
exposed to KHV and is clear of the virus.
By contrast a positive ELISA test does not automatically mean the koi
is infected with KHV but it indicates that it has been exposed to the virus
at some point and should be considered as a possible carrier with potential
to transmit the virus at some later stage if triggered to become active.
Both the PCR and ELISA tests are available in the
UK from organisations such as CEFAS a specialist laboratory testing facility
based in Weymouth which is part of DEFRA.
In fact CEFAS undertook the hundreds of ELISA tests performed on all
of the Yume koi during their KHV Summer outbreak. This included tests carried
out on my three replacement koi referred to above, one of which tested negative
on two separate occasions providing the confidence for me to collect the koi
a couple of weeks ago. Unfortunately
my other two koi tested positive and were part of the seventy or so koi that
Mike Snaden was forced to euthanize.
Unfortunately the ELISA test is not currently available
in Japan so while the koi farms may be using PCR tests on koi suspected of
KHV there must remain some questions about whether dormant KHV carrier koi
are passing through the koi farms undetected for onward sale and transmission
to UK dealers and hobbyists. I’m not
trying to frighten everyone from buying new koi from dealers as I’m sure most
stocks will be free of KHV if purchased from reputable and identifiable sources.
However until effective KHV vaccines are available, I will certainly have
more confidence in buying koi that have been subjected to an ELISA blood test
and shown to be negative. In the meantime my latest acquisition from Yume
Koi should be clear of KHV if the available reported science is correct.
It’s now in my main pond, with the rest of my koi and so far appears
to be in disgustingly good health - so was it a sensible move to introduce
this koi – only time will tell.
Please note that since I’m not scientifically or medically
trained I do not claim to be an expert on KHV. The above information has been
gathered from various sources and represents the facts and current science
as currently available. Further information
for those who are interested may be obtained from:-
I’d
like thank Rob for this article –
there’s nothing better than one of our members being able to pass on first
hand experience.