A brief introduction:

Carp aquaculturists in the United States and around the globe are worried about the emergence of carp edema virus disease (CEVD). Koi, as well as wild and cultured varieties of common carp (Cyprinus carpio), are susceptible to the carp edema virus. A sick fish may have erosive or hemorrhagic skin lesions with swelling (edema) of the underlying tissues, which is why the disease is known as "viral edema of carp" (Oyamatsu et al. 1997a). The disease has also been referred to as “koi sleepy disease” (KSD) because infected fish become lethargic and unresponsive (Miyazaki et al. 2005). The severity of disease is greatest in juveniles, which may hang just under the surface of the water before succumbing, while adult fish may lie motionless on the bottom of the pond/tank. 


First characterized from Japanese koi in 1974, carp edema virus disease/koi sleepy disease has been shown to be widespread throughout Japan where koi are cultured (Murakami et al. 1976; Ono et al. 1986; Amita et al. 2002). The international trade in koi has likely led to the global spread of CEVD/KSD with outbreaks documented in imported koi in North America and Europe (Hedrick et al. 1997; Way and Stone 2013; Haenen et al. 2013). In the United States, CEVD/KSD has been associated with outbreaks in imported and domestic koi in the following states: California (in the years 1996 and 2010), Washington (2005), North Carolina (2005), Georgia (2005 and 2010), and Florida (2014 and 2015) (Hedrick et al.1997; Waltzek et al. 2014). Carp edema virus has been detected widely across Europe in koi from at least Austria, Czech Republic, France, Germany, the Netherlands, and the United Kingdom (Haenen et al. 2013; Way and Stone 2013; Jung-Shoroers et al. 2015; Lewisch et al. 2015; Vesely et al. 2015). The virus has recently been detected for the first time in the southern hemisphere on koi farms in Brazil (Viadanna et al. 2015). Carp edema virus has been detected in common carp within Austria, the Czech Republic, Italy, Poland, the Netherlands, and the United Kingdom (Way and Stone 2013; Lewisch et al. 2015; Pretto et al. 2015; Vesely et al. 2015; M. Reichert personal communication; O. Haenen personal observation).


What is CEV?


Carp edema virus is a large, double-stranded DNA virus that belongs to the poxvirus family (family Poxviridae). Koi sleepy disease/carp edema virus disease should not be confused with carp pox, an infectious disease that causes wart-like growths on the skin in common carp varieties, which is caused by a herpesvirus (Cyprinid herpesvirus 1). Although CEV has not been grown in the laboratory by cell culture, poxvirus-like particles have been visualized in infected gill epithelial cells by transmission electron microscopy (Ono et al.1986; Hedrick et al. 1997; Oyamatsu et al. 1997a; Miyazaki et al. 2005).


What are the symptoms of CEVD/KSD?


The common name "koi sleepy disease" indicates the presence of behavioral abnormalities in CEVD/KSD-infected larger carp and koi, including lethargy and unresponsiveness, with the fish often lying motionless on their sides or bellies for long periods of time unless disturbed (Figure 1). If the “sleepy” carp or koi are disturbed, they may swim for a short period of time, but soon settle back into an inactive state on the bottom of the tank (Miyazaki et al. 2005). In CEVD/KSD, weakened juveniles may swim slowly and aimlessly along the margins of the pond/tank or hang motionless just under the surface of the water (Oyamatsu et al. 1997a). However, it should be noted that one of the authors (A. Goodwin personal observation) has observed a similar “sleepy” presentation in fish due to sudden (acute) swim bladder under-inflation after stressful harvest or handling. Infected fish will often refuse to eat, which further weakens the fish. Diseased juvenile koi may have extensive erosions or hemorrhages of the skin with edema (swelling) of the underlying tissues (Miyazaki et al. 2005).



The other external signs of infection include sunken eyes (enophthalmos) and pale, swollen gills (Figure 2, Haenen et al. 2013). These similar external signs (i.e., sunken eyes and gill lesions) may also be observed with another significant viral disease of common carp known as koi herpesvirus disease (KHVD) caused by Cyprinid herpesvirus 3 (Hartman et al. 2013). No grossly visible internal disease signs have been reported with CEVD/KSD. The disease is typically observed at water temperatures between 15–25°C (59–77°F) in koi, and at 6–10°C (43–50°F) in common carp and can kill up to 75–100% of juvenile koi during an outbreak (Hedrick et al. 1997; Miyazaki et al. 2005; Way and Stone 2013).

How is a diagnosis of CEVD/KSD reached?

In addition to an appropriate case history and clinical signs, wet mounts of the gills may provide further supporting evidence of CEVD/KSD. In the early stages of the disease, the gill epithelial cells at the tips of the gill filament proliferate, resulting in a thickening or “clubbing” appearance (Ono et al. 1986; Miyazaki et al. 2005). In CEV-infected fish, the proliferation may extend to the base of the gill filament and impair gill function. Thickened gills are not a problem exclusive to CEVD/KSD, and may also be caused by other factors such as poor water quality. Also, as with many fish diseases, there may be secondary or opportunistic infections that occur in CEV-infected carp and koi (Haenen et al. 2013; Lewisch et al. 2014).


Observation of microscopic abnormalities from stained slides prepared from formalin-fixed gill and skin tissues (histopathology) can further support a CEVD/KSD diagnosis. Typical lesions observed by light microscopy include: 1) hypertrophy (increased size) and hyperplasia (increased numbers of cells) of the gill epithelium with associated inflammation and 2) extensive destruction of the skin and edema (swelling) of the underlying tissues (Oyamatsu et al. 1997a; Miyazaki et al. 2005). A definitive diagnosis requires specialized laboratory testing such as Polymerase Chain Reaction (PCR) that detects the DNA of CEV (Oyamatsu et al. 1997b). Recently validated PCR tests have been developed and are available at the University of Florida’s Wildlife and Aquatic Animal Veterinary Disease Laboratory in Gainesville (Waltzek and Viadanna unpublished). Transmission electron microscopy of gill tissues may also be used to demonstrate hypertrophied (enlarged) gill epithelial cells containing large spheroid poxvirus-like particles (Figure 3). To date, the virus has not been propagated in cell culture (Oyamatsu et al. 1997a).


How are fish infected with CEV?

In Japan, CEVD/KSD typically appears in juvenile koi (young-of-the-year) during the rainy season after they have been moved from earthen nursery ponds to clear water concrete-lined ponds for grading. Even though the source of the virus is unknown, it is believed that juvenile koi are exposed before or during the stress of capture and subsequently display symptoms of the disease. As mentioned above, in the absence of CEV, fish may also be found lying on the bottom of the tank. It occurs after harvest from ponds when the water temperature is over 27°C (80°F) and affects nearly 100% of the fish within one hour. The majority of CEVD/KSD outbreaks occur after infected fish are added without quarantine to established ponds of koi that are presumed to be na*ve, that is, they have never been exposed to CEV. The mortality rate of nave koi or common carp exposed to CEV by bath challenge begins as soon as day 6 and continues until day 16 (Oyamatsu et al., 1997a). According to the study mentioned above, the virus may be transmitted through contact with contaminated water. It is likely that diseased fish that shed virus into the water from gill and skin lesions constitute an important mode of transmission. Currently, it is unknown whether CEV is vertically transmissible (i.e., occurs in eggs or sperm of breeders) and whether CEV-exposed fish are able to clear the virus after infection or can harbor the virus as carriers. It is also unknown how long the virus remains infectious in water (Oyamatsu 1997a).


What fish are susceptible to CEV?

Carps and koi (Cyprinus carpio) are the only species known to be susceptible. Goldfish (Carassius auratus) have not yet been exposed to CEV in a controlled laboratory setting. There is evidence that goldfish are susceptible to viruses that affect carp/koi, such as the rhabdovirus spring viremia of carp (Petty et al. 2012). A similar poxvirus-like agent has been partially characterized in farmed ayu (Plecoglossus altivelis) (Wada et al. 2008), farmed Atlantic salmon (Salmo salar) (Nylund et al. 2008; Gjessing et al 2015), and a managed Cape seahorse farm (Hippocampus capensis) (Waltzek et al. 2011). Currently, little is known about the genetic relationships between CEV and other fish poxviruses. Genetic sequencing of CEV strains from around the world in common carp and koi has revealed that they most likely represent a single novel poxvirus species (Way and Stone 2013; Waltzek et al. 2014).


What is the effect of water temperature on CEV?

The temperature of the water plays a critical role in the development of CEVD/KSD. Japan experiences outbreaks of CEVD/KSD in young koi from late June to late July, when water temperatures are between 15°C (59°F) and 25°C (77°F) (Oyamatsu et al. 1997a), although many fish also develop the disease in the spring and fall (Miyazaki et al. 2005). In the United States and the United Kingdom, outbreaks typically occur at similar water temperatures (listed above) (Way and Stone 2013; Waltzek et al. 2014). In the UK, however, a novel strain of CEV has been associated with disease episodes in wild common carp during the winter and early spring at much lower temperatures, between 6 and 9°C (43–48°F) (Way and Stone, 2013), and similarly in the Netherlands (O. Haenen personal observation). CEVD/KSD in common carp and koi have also been reported at lower water temperatures between 7–15°C (44–59°F) in Austria (Lewisch et al. 2014).


What can be done to prevent CEVD/KSD?

Ask the supplier if there have been any major unexplained population losses before acquiring any CEVD/KSD-susceptible fish. Even though diagnostic tests for monitoring and verifying CEV have only recently been developed, ask suppliers if CEV testing has been done and request lab result documentation. Having a good working relationship with your fish suppliers is the best way to prevent CEVD/KSD. Koi producers in Japan believe 0.5% (5g/L) salt can help prevent disease after the young-of-the-year fish move from earthen nursery ponds into cement-lined ponds for grading (Seno et al. 2003; Miyazaki et al. 2005; Way and Stone 2013). A few producers also avoid harvesting when water temperatures are permissive for the disease (15–25°C or 59–77°F). Always isolate and test sick fish showing suspicious clinical signs. Remove dead fish from ponds immediately to prevent disease transmission. Depopulate ponds harboring CEV to prevent further mortalities and ongoing spread to naïve individuals. Preventative depopulation is especially highly recommended for production facilities. Koi owners should understand the potential for ongoing disease and spread and monitor for signs of trouble. Best health management practices should be followed to minimize the risk of CEVD/KSD outbreaks. To keep fish healthy and better able to resist disease, quarantine and test new arrivals; avoid crowding and stressful events; maintain good water quality; and provide proper nutrition.


Quarantine (separation from other uninfected koi) is the most dependable method to avoid the introduction of pathogens into a pond or facility (Yanong and Erlacher-Reid 2012). To implement an effective quarantine procedure, all new fish must be kept in a separate system, ideally in a different building or area from the resident fish. Feed, handle, and maintain resident fish first, then treat the new fish to avoid introducing pathogens to resident fish. The quarantined fish require dedicated equipment such as nets, buckets, and siphon hoses that are used only for them. In addition, everyone must use foot baths and hand washes when entering and when leaving the quarantine area. Fish should be quarantined for a minimum of 30 days. For CEV, new koi should be quarantined in water that is a constant temperature (between 15–25°C) for at least 30 days.

Is CEV a risk to humans?

Fish viruses do not infect humans and thus CEV is not harmful to humans.