Carp Pox

A very common problem encountered by koi keepers is carp pox, a viral infection caused by Cyprinid herpesvirus-1  which results in white plaques on the skin surface of koi. 

Typical carp pox lesions are white to grey in colour and smooth in texture often resembling melted candle wax. Lesions are most commonly found on the dorsum and fin tips but can be found elsewhere on the body.

Carp pox lesions are abnormal epithelial growths in the skin surface. Thankfully they are benign in nature and pose little risk to the health of the fish carrying the disease. Carp pox is much more of an aesthetics issue than a threat to health. That being said, rarely, these growths can grow to cover larger areas of skin and may become inflamed or infected by opportunistic bacteria.

 As carp pox is a viral infection, the disease is contagious and can spread throughout a pond. It is therefore assumed that if carp pox is diagnosed on one koi, all koi in that pond will have been exposed to the disease and are therefore carriers of the disease. This does not mean all koi will develop lesions on the body all at once. It’s way more common to have a small group of fish with clinical disease (with skin lesions) and the rest asymptomatic (without skin lesions).

 I often describe carp pox as being like cold sores in people, people will have the virus for life but when they are stressed or immune-compromised the cold sore comes out, this is very similar for carp pox. Often carp pox lesions will appear or become more pronounced during the winter period when immune systems are very slow or when the fish are stressed or challenged for other reasons such as poor water quality or over-crowding.

In most cases, lesions will reduce and even sometimes fully clear in the summer and some may disappear as the fish ages. There is no medical or physical treatment for carp pox lesions and instead the emphasis should be placed on supporting the immune system in order to reduce the appearance of lesions. This can be achieved by heating the pond through winter, supplementing with vitamin C and E and ensuring optimal water quality year round.