Atlantic Salmon Papillomatosis (retrovirus)

Volume 2, Issue 11
October 2000
Updated November 2002

Atlantic salmon Salmo salar living in Maine, Great Britain, Norway, Sweden, and possibly other parts of the United States and Canada can occasionally be caught with a benign epidermal tumors growing on their skin and scales.

Skin lesion an Atlantic salmon

Figure 2. Gross appearance of Atlantic salmon skin lesions from Atlantic Salmon Papillomatosis virus infection. Lesions are commonly circular, but they may coalesce.

The tumors appear as single or multiple proliferations that have a smooth to nodular texture and are translucent to white. They may occur at any site on the body surface and are 2 to 5 mm thick and up to 4 cm in diameter. The tumors are relatively harmless to the fish and eventually fall off allowing the skin to heal. Sometimes fish that are heavily infested with the tumors can succumb to other secondary bacterial, viral, or fungal infections.

In nature, the prevalence of papillomatosis may approach 55% in 1 to 2-year old freshwater hatchery salmon, 10% in 2 to 3-year-old salmon captured in salt water, and 1.5% in spawning adults. The tumors have also been found in feral Atlantic salmon. In younger, freshwater fish, the epidermal papilloma appears in lake summer and regresses in autumn.


Figure 1. Histological appearance of Atlantic Salmon Papilloma
virus lesion.

In the older, saltwater fish, the tumor persists for up to a year. No methods of treatment or control are known. The tumors are caused by a retrovirus. This has made research on the condition very difficult and expensive. Consequently not much is known about the virus’s pathogenicity. The name, Atlantic salmon papilloma is a misnomer. “Papilloma” is the medical term for “warts;” and warts are really caused by papilloma viruses. Nevertheless, “papillomatosis” is certainly a fun word to say like— hipster dufus, high-falutin’, discombobulate, pecksniffian, Zimbabwe, huffnagle, and zog.

Special points of interest:

Fish with Atlantic Salmon Papillomatosis cannot transmit the disease to people.

A papilloma is like a wart.

Fish with Atlantic Salmon Papillomatosis are safe to eat.

Papillomas eventually fall off.

Images were made possible by a grant from the Maine Outdoor Heritage fund.