Carp pox is one of the oldest recognized fish viral disease, caused by cyprinid herpesvirus-1.
The benign epidermal hyperplasia commonly known as carp pox or epithelioma papillosum affects primarily the common carp but also occurs in common carp and hybrids of common carp and goldfish (Carassius auratus). Intensive trafficking of fish has no doubt contributed to the spread of the disease, which is now enzootic in eastern and western Europe, Asia, and the Middle East. Eurasian cyprinids were introduced into North America in the 1870s, but the appearance of carp pox has been rare.
Pox lesions may occur on other species of fish, and are sometimes referred to as fish pox. Lesions typically are smooth and raised and may have a milky appearance. They are benign, non-necrotizing areas of epidermal hyperplasia on the fins, skin and lips. They are quite firm to the touch and the lumps can appear in grouped bunches or they can be singular and usually occur when the carp is in the winter months and the immune system is impaired due to low temperatures. The fact is that any event that causes the immune system to be impaired may trigger latent pox or make naïve fish susceptible to it Carp pox can be highly infectious in over crowded condition but, as mentioned, is not generally known as a killer of adult koi/carp. However in fry it can be quite devastating as young carp possess an immature and incomplete immune systems
Severe cases may result in development of papillomatous growths, and these may be a site of complicating bacterial infection. Generally, lesions are self-limiting and of minimal clinical significance.
Carp pox is not life threatening and usually goes away of its own accord when temperatures rise, heat can be helpful. The practise of cutting pox lesions off should be discouraged as this could aid further spread of the disease by releasing viral particles.
Carp pox can be a significant problem with koi because the aesthetic quality, and hence the market value, of the animal is severely compromised. For the serious koi enthusiast, carp pox-affected fish should be culled, preferably during quarantine.
Surgical removal of pox lesions has not been rewarding.