What is it?
Gyrodactylus are small, leech-like parasites. Over 400 species have been described, from fish and frogs, in fresh and salt water. In Norway, catastrophic losses of Atlantic salmon were seen following the introduction of G. salaris to the country in the 1970s. As of 2001, 41 Norwegian rivers have been infected and their salmon populations effectively exterminated. These parasites are remarkable in that they give birth to live young. The daughter parasite is the same size as the mother, and inside this daughter there is already a developing grand-daughter, in a 'Russian doll' arrangement.
Where and When Might it Occur?
Only Atlantic salmon are severely affected by this parasite, although G. salaris has been reported from Atlantic salmon (Salmo salar), rainbow trout (Oncorhynchus mykiss), Arctic char (Salvelinus alpinus), North American brook trout (S. fontinalis), grayling (Thymallus thymallus), North American lake trout (Salvelinus namaycush) and brown trout (Salmo trutta). Salmon from Scottish rivers have also been shown to be susceptible to this parasite.
The parasite can be seen under low power magnification, for example, with a good hand lens. Without magnification, heavily infected parr appear greyish, with excess mucus, and possibly concurrent fungal infections.
They attach to the host by the attachment organ, or opisthaptor at one end of the body, and feed using glands at the other end. Attachment can cause large wounds and feeding can damage the epidermis, allowing secondary infection. G. salaris can build up to very high infection intensity of several thousand parasites on a single salmon parr.
Rotenone treatment has been used to eradicate G. salaris from some rivers in Norway. This kills all fish in the river, and re-stocking is carried out from eggs and juveniles collected prior to treatment. Not all rotenone treatments have been successful, and such drastic treatment is only possible in short rivers with favourable biological and geographical conditions. The parasite cannot survive full strength sea water, so natural migration of fish is unlikely to spread infection.
An intensive sampling programme has proved the absence of G. salaris within Great Britain. G. salaris is notifiable within Great Britain and is a listed disease under Schedule 1 of the Aquatic Animal Health (Scotland) Regulations 2009.