What is CEV?
Carp Edema Virus, or CEV, is a virus that affects carp. It causes a condition often called Koi Sleepy Disease, because infected fish become extremely slow, weak, and often gather near the edges of the lake.
CEV was first found in UK carp in 2012 after several sudden fish deaths. Since then, more cases have appeared, and work is being done to understand how common the virus is and how it spreads.
What does CEV do to carp?
Carp with CEV often show:
- Lethargy (very slow, sleepy movement)
- Hanging around the margins
- Not responding to anglers or disturbance
- Severe gill damage, including swelling, clubbing and patches of dead tissue
The gill damage makes it hard for carp to breathe, especially when water quality is poor. Some infected fish also show internal organ stress or inflammation.
Because these signs can look similar to other illnesses or poor water quality, proper investigation is important.
When does the virus appear?
In Japan, CEV outbreaks mostly happen in spring and summer (15–23°C).
But in the UK, CEV has appeared in very cold water too — even at 3°C.
This means the virus might cause problems at any time of year, making it harder to predict.
How do we manage it?
As soon as CEV is suspected:
- Fish movements are restricted
- A full investigation (lab testing + water quality checks) is carried out
- The aim is to prevent spreading the virus to other fisheries
There is no treatment, so protecting other waters and improving conditions for the fish is vital.