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Carp edema virus (koi sleepy disease)

By the koi.wiki editorial team · Updated

Key facts: Carp edema virus (CEV) is an unclassified poxvirus that causes koi sleepy disease (koi sleepy disease syndrome, KSD), a condition marked by lethargy and gill damage in koi and common carp (Cyprinus carpio)[1].
Infection with carp edema virus (CEV) is recognized by the World Organisation for Animal Health (WOAH) as an emerging disease of koi and common carp, first detected in Japan in the 1970s[1]. The virus is classified as an unclassified poxvirus within the family Poxviridae, distinct from the herpesviruses that cause koi herpesvirus (KHV) and carp pox, and from the rhabdovirus that causes spring viremia of carp (SVC)[1][2][3].The disease is commonly called koi sleepy disease because affected fish become extremely lethargic, often lying on the pond bottom or drifting near the surface[1]. According to the US koi dealer Kodama Koi Farm, the name reflects this resting behavior rather than any visible wounds[4].

Causative agent and classification

CEV is described by WOAH as an unclassified poxvirus of the family Poxviridae[1]. It was first detected in Japan in the 1970s, and its genome was later published in GenBank[1]. This places CEV in a different viral family from koi herpesvirus (an Alloherpesviridae member) and spring viremia of carp virus (a Rhabdoviridae member), so the three diseases are caused by unrelated pathogens despite some overlapping clinical signs such as lethargy[2][5][1].

Host range and transmission

Common carp and koi are susceptible to CEV, and several other species, including bleak, crucian carp, European perch, Prussian carp, roach and tench, are potential vectors[1]. Koi carp are reported to be among the most susceptible, though some genetic strains of carp show greater resistance[1]. Outbreaks have been observed in both juvenile and adult koi and common carp[1].Co-habitation studies show that direct horizontal transmission is an important route of spread, and because the gills are the main site of viral replication, vertical transmission is considered unlikely[1]. Infected fish populations, farmed and wild, are the only established reservoirs of infection, and the biophysical characteristics of the virus are not well understood, making the role of indirect transmission via equipment difficult to assess[1].According to Kodama Koi Farm, a US koi dealer, young tosai (one-year-old koi) are the age group most often affected, and some Japanese breeders allow controlled exposure of young koi so that natural immunity can develop before the fish are sold, a practice the dealer compares to childhood immunization, though it is not itself a vaccine[4]. The same source states that immunity is not always complete, and the virus can become active later when a koi experiences significant stress such as a new pond, shipping, temperature change, poor water quality, parasites or other illness[4].

Geographical distribution

Infection with CEV has been reported in Europe, North America, South America and Asia[1]. The common carp is one of the most widely cultured freshwater fish, with global production of 4.5 million tons in 2019, 97% of it from aquaculture, and introduction of the virus has been shown to cause significant mortality and to affect wild populations as well as farmed stock[1].

Clinical signs

The main organ affected by CEV is the gills[1]. The primary behavioral sign is lethargy and unresponsiveness[1]. Gross lesions include swollen gills or gill necrosis, enophthalmos (sunken eyes), skin lesions at the base of the fins or around the mouth, and inflammation of the anus[1]. Microscopically, lesions are found primarily in the gills, with hyperplasia and clubbing of secondary gill lamellae, partial or total occlusion of the interlamellar space, edema in epithelial cells of secondary filaments, and mild infiltration with eosinophilic granular cells[1].Kodama Koi Farm describes affected koi as resting motionless on the pond bottom or floating quietly near the surface, showing little interest in food, reacting slowly to movement, and sometimes isolating from other koi[4]. The dealer notes these symptoms are most commonly observed in younger koi, although fish of other ages can occasionally be affected[4].

Risk factors and affected life stages

Disease has been associated with international trade and may be linked to the stress of shipping[1]. CEV was originally considered to occur at temperatures between 15°C and 25°C (59°F–77°F), though it has also been reported at lower temperatures[1]. Kodama Koi Farm similarly notes that koi sleepy disease is most often observed during cooler water temperatures, particularly in spring when pond conditions fluctuate, and that stress triggers such as sudden temperature changes, poor water quality, overcrowding, transportation and introducing new fish without quarantine can increase risk[4].

Diagnosis and WOAH status

High levels of lethargy and mortality in carp, associated with gill edema and necrosis, are considered suspicious for CEV infection[1]. Several PCR methods have been reported for detection, and qPCR methods have shown the best performance for confirmatory testing[1]. Infection with CEV is considered by WOAH to meet the definition of an "emerging disease" and should be reported accordingly[1]. A fish veterinarian or diagnostic laboratory is generally required to confirm the cause of lethargy and gill damage, as similar signs can occur with other conditions; see the general approach described for diagnosis of koi disease and signs of illness.

Treatment and control

WOAH notes that restricting movement of carp from farms or fisheries where the virus is known to occur limits spread, along with generic biosecurity measures such as cleaning and disinfection of equipment, vehicles and staff[1]. Mortality in infected carp can be significantly reduced when fish are immersed in a 0.5% saline bath[1].Kodama Koi Farm states there is no universally accepted treatment protocol for koi sleepy disease, and management approaches vary among breeders and dealers depending on outbreak severity, water temperature, water quality and the presence of secondary infections[4]. The dealer describes salt as a supportive treatment used to reduce stress and support osmoregulation, without eliminating the virus itself, and oxolinic acid as an antibacterial sometimes used for secondary bacterial infections rather than the virus[4]. General supportive measures described by the dealer include isolating severely affected fish, monitoring ammonia, nitrite, nitrate, pH and temperature, maintaining aeration, and allowing time for recovery, which can range from days to weeks[4]. Prevention measures cited include quarantining new koi before introduction to a pond, maintaining water quality through proper filtration, and minimizing handling stress[4].

Comparison with other koi viral diseases

CEV is one of several notifiable or emerging viral pathogens of koi and common carp. Koi herpesvirus (KHV) is a double-stranded DNA virus of the family Alloherpesviridae, closely related to the viruses causing carp pox and goldfish hematopoietic necrosis, and there is no effective treatment to rid fish of KHV once infected[2]. Spring viremia of carp (SVC) is caused by a Rhabdoviridae virus and is listed as a notifiable disease by WOAH, with signs including external hemorrhaging, pale gills and ascites[5][3]. In Canada, SVC is a federally reportable disease[3]. Carp pox, by contrast, is a skin disease that rarely kills fish and tends to resolve as water warms[6]. Readers should consult the notifiable koi diseases page for reporting requirements that may apply to specific viral infections.

Common questions

What is koi sleepy disease?
Koi sleepy disease (koi sleepy disease syndrome, KSD) is caused by carp edema virus (CEV), a poxvirus that damages the gills of koi and common carp and causes marked lethargy[1].
Is CEV the same as koi herpesvirus (KHV)?
No. CEV is an unclassified poxvirus, while KHV is a double-stranded DNA herpesvirus of the family Alloherpesviridae; they are unrelated pathogens that can both cause illness in koi[1][2].
At what water temperature does CEV occur?
CEV was originally associated with temperatures of 15–25°C (59–77°F), though it has also been reported at lower temperatures, and a US koi dealer notes outbreaks are often seen in spring as pond temperatures fluctuate[1][4].
Is there a cure for CEV?
There is no universally accepted treatment protocol; a 0.5% saline bath has been shown to significantly reduce mortality, and biosecurity measures help limit spread, according to WOAH[1]. A US koi dealer describes supportive measures such as salt therapy and management of secondary bacterial infections rather than a cure for the virus itself[4].
Can other fish species spread CEV?
Yes; species such as bleak, crucian carp, European perch, Prussian carp, roach and tench are considered potential vectors of CEV, even though koi and common carp are the most susceptible[1].

References

  1. ^ Carp edema virus disease card (WOAH, 2022). World Organisation for Animal Health. woah.org/app/uploads/2023/03/a-woah-cev-disease-card-nov-2022.pdf. Retrieved November 2026.
  2. ^ UF/IFAS EDIS VM113: Koi Herpesvirus Disease (KHVD). University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/VM113. Retrieved November 2026.
  3. ^ Spring Viraemia of Carp - inspection.canada.ca. Canadian Food Inspection Agency. inspection.canada.ca/en/animal-health/aquatic-animals/diseases/repo…. Retrieved November 2026.
  4. ^ What is Koi Sleeping Disease? KSS, KSD, Carp Edema Virus (CEV) Symptoms & Treatment. Kodama Koi Farm (koi dealer). kodamakoifarm.com/koi-sleeping-disease/. Retrieved November 2026.
  5. ^ Spring viraemia of carp (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Spring_viraemia_of_carp. Retrieved November 2026.
  6. ^ SRAC Publication No. 4710: Herpesviruses in Fish. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/4710%20Herpesviruses%20in%20Fis…. Retrieved November 2026.