Fish pox, also known as carp pox or warts, is a chronic skin disease of carp and several related species of cyprinid (family of soft-rayed fishes) fish, including ornamental koi. The disease is caused by a virus, but contrary to its name, the causative agent is a herpesvirus, not a poxvirus. This particular agent is a cousin to the viruses that cause chickenpox and fever blisters in humans. Similar conditions of unknown cause have been reported in other species of fish.
Fish pox is one of the oldest recorded diseases of fish, first recognized in the Middle Ages, although a viral cause was not proven until the 1960s. The long history of the disease probably reflects the long relationship of carp aquaculture and man in Europe and Asia. The disease now occurs essentially everywhere carp are raised.
The disease is primarily cosmetic and light infections have little effect on the host. In severe cases, however, fish grow poorly, and death can occur. The virus invades cells that make up the epidermis, or superficial component of the skin. These cells, called epithelial cells, are induced by the virus to enlarge and proliferate. As this occurs, the highly ordered structure of the skin is disrupted, resulting in the formation of a benign tumor called a papilloma, otherwise known as a wart.
Signs of fish pox include the presence of glistening, smooth, flat, milky to tan, slightly raised plaques on the skin surface. The plaques are firm and small, but may coalesce to form larger, irregularly shaped lesions over one inch in size. The plaques are not permanent, but may persist for months before regressing. Once the plaques have disappeared, the affected sites may be become darkly pigmented destroying the value of ornamental fish. Occasionally, lesions may ulcerate and become secondarily infected by
Water temperature is an important factor in the incubation period and persistence of the skin lesions. The plaques, or papillomas, take about two months to appear at 50 degrees F. Therefore, outbreaks tend to occur at lower water temperatures during winter and early spring, then subside as temperatures rise into summer. The disease usually occurs only in fish over a year of age. Recurrence may occur in the same pond in subsequent years whenever environmental conditions are right.
Your veterinarian can make a presumptive diagnosis of pox based on the presence of characteristic skin lesions in koi and related species. To confirm the diagnosis, a biopsy of the skin lesions must be surgically removed and submitted to a pathology laboratory where the tissue is examined for the presence of microscopic changes typical of the disease, called viral inclusion bodies. The pathologist may recommend that a piece of skin be examined under an electron microscope to visualize the actual virus particles. Additional diagnostic methods, including virus isolation in cell culture or fluorescent antibody testing, can be employed, but it may be available only in specialized laboratories familiar with diseases of fish.
It has been suggested that certain external parasites, such as skin flukes and leeches, may be capable of transmitting the virus from fish to fish. Fish at risk of acquiring the infection should be examined for external parasites and treated appropriately. Antibiotic therapy may be needed to treat secondary bacterial infections.
There are no specific treatments that will eliminate the virus or cause the skin lesions to regress more rapidly. Fish with skin lesions characteristic of pox should be isolated immediately to limit spread of the disease. Make sure that the fish is eating and provide an environment that is as low in stress as possible by maintaining good water quality.
The virus appears to have high host specificity for cyprinids and unrelated species do not appear at risk. The disease can be transmitted directly from fish to fish, especially under crowded conditions. Transmission is facilitated by trauma to the skin, so avoid unnecessary netting or materials in a pond that may be abrasive to the skin.
The keys to prevention are avoidance and quarantine. When purchasing new fish, avoid the disease by inspecting closely for the presence of typical skin changes, but remember that it may take several months for the papillomas to become visible. For this reason, always quarantine new aquarium or pond fish for at least one month. This period of time will usually allow any disease signs to manifest before they have a chance to spread to your established fish population.
There is some evidence that highly inbred fish may be more susceptible.
While inbred fish may possess more desirable physical features, they are generally weaker and should be avoided.