Carp Edema Virus and Koi Sleepy Disease

Koi sleepy disease is a gill infection of koi and common carp caused by carp edema virus, a poxvirus. Fish lie listless on the bottom with swollen gills; it is easily confused with KHV.

Koi sleepy disease (KSD) is a gill disease of koi and common carp (Cyprinus carpio) caused by carp edema virus (CEV), a poxvirus. Affected fish become lethargic, lie on the bottom or gather at the edges, and develop swollen, damaged gills; losses range from low to very high, and because the signs overlap with koi herpesvirus (KHV), only laboratory PCR testing can tell them apart.

Cause

CEV is a large, enveloped DNA virus of the family Poxviridae. Carp edema, viral edema of carp and koi sleepy disease are now understood to be the same infection. The disease was first seen in juvenile koi in Japan in the 1970s (the MSD Veterinary Manual gives 1974) and for a long time appeared limited to Japan. It has since been detected in Europe, including Germany, France, the Netherlands, Austria, the Czech Republic and England, where it was first found in fisheries in 2012; the koi trade is presumed to have spread it. CEV has only been reported in common carp and its ornamental varieties such as koi and ghost carp. The virus has not been grown reliably in cell culture.

Signs: lethargy and gills

Sleepy behaviour: fish lie motionless on the bottom, or gather at the pond margins or water surface, and respond poorly.
Severe gill damage: swollen, oedematous or pale gills, clubbing and fusion of the gill filaments, inflammation and gill necrosis, leading to oxygen shortage.
Sunken eyes (enophthalmos), loss of appetite and a thin appearance.
Skin changes, often around the mouth and at the fin bases, sometimes erosive or haemorrhagic.

Research on infected carp links the gill damage to a fall in blood sodium (hyponatraemia) and a build-up of ammonia in the blood, which may explain both the lethargy and the deaths. Secondary bacteria such as Flavobacterium branchiophilum and gill parasites are often present, but CEV appears to be the primary cause of the gill injury.

Outbreak conditions

Outbreaks have been recorded over a wide temperature range. A UK study summarises cases from 6 to 24 °C, with most between 19 and 24 °C; a Czech report gives typical clinical ranges of 6–12 °C in common carp and 15–25 °C in koi, and describes an outbreak in overwintering carp that began under ice at 2 °C, with deaths as the water warmed to 8 °C. In UK fisheries, CEV has been detected almost all year round at temperatures as low as 3 °C, whereas Japanese outbreaks typically fall between 15 and 23 °C. Low dissolved oxygen can worsen the impact. Several outbreaks followed the introduction of newly imported or purchased fish.

Mortality varies widely. The MSD Veterinary Manual describes it as usually low but variable, while European reports describe losses of up to 80–100% in juvenile koi and up to 100% in some outbreaks. Susceptibility differs between carp strains: koi were highly susceptible in infection trials, and in the Czech overwintering outbreak two farmed carp strains lost 30% and 60% respectively.

How it differs from KHV

KHV (cyprinid herpesvirus 3) is covered in the Koi Herpesvirus (KHV) article. The two diseases are easily confused: both can cause lethargy, gill necrosis, sunken eyes and skin lesions in koi and carp, and in the UK spring temperatures often overlap the ranges at which CyHV-1, CyHV-2, KHV and CEV cause disease. Differences that help, but do not replace testing:

Cause: CEV is a poxvirus; KHV is a herpesvirus.
Temperature: KSD has been recorded in cold water down to a few degrees Celsius; gov.uk gives 16–29 °C for KHV disease.
Behaviour: the marked sleepy lying on the bottom gives KSD its name.
Legal status: KHV is a WOAH-listed and, in the UK, notifiable disease; CEV is not on the WOAH list.

Diagnosis

Diagnosis relies on PCR or quantitative PCR for the CEV p4a gene on gill tissue or gill swabs, followed by sequencing, carried out by a fish-health laboratory. Laboratories usually test for KHV, spring viraemia of carp virus and other carp viruses at the same time. CEV DNA is not always detectable in every clinically affected fish, including the first fish to show signs, so several fish and repeat samples may be needed. Field tests (LAMP) on mucus or gill swabs are under development.

Management described by fish-health sources

There is no antiviral treatment. Studies from veterinary research groups report that keeping affected koi in water with about 0.5% sodium chloride restores blood sodium and ammonia excretion and can prevent most KSD-related deaths; this approach has been used by koi breeders in Japan and in clinical koi cases in the US. The same studies warn that salt-treated fish can keep the virus and that repeat outbreaks occurred without new fish being added. Salt treatment of koi or carp should be planned with an aquatic veterinarian after CEV has been confirmed, because KHV and other diseases need a different response; the veterinarian sets concentration and duration.

Quarantine new koi and carp, and do not mix fish from different sources without veterinary advice.
In the UK, movement of fish from affected fisheries has been restricted, and anglers are asked to check, clean and dry equipment.
Maintain good oxygenation, since low dissolved oxygen worsens losses.
Recovered fish may still carry the virus; the MSD Veterinary Manual notes that carrier status is unknown.