Red vent syndrome in wild Atlantic salmon Salmo salar in Scotland is associated with Anisakis simplex sensu stricto (Nematoda: Anisakidae).

Simultaneous reports were received between June and July 2007 of wild Atlantic salmon Salmo salar with red, swollen, bloody vents returning to geographically diverse rivers in Scotland. By the end of September the condition, colloquially known as 'red vent syndrome' (RVS), was reported from >50 rivers across Scotland. Fish were generally in good overall condition but the vent area showed mild to severe lesions. External characteristics of the syndrome included a swollen, raised, haemorrhagic vent and surrounding tissues, with erosion of the skin, scale loss and moderate to severe bleeding in more advanced cases. Predominantly, the fish affected were 1-sea-winter grilse; however, RVS was also recorded in 2-sea-winter salmon and sea trout S. trutta. High numbers of the nematode Anisakis Type I larvae were found infesting the discrete region of the vent, a localisation that is reported as novel for the parasite. The hypothesis that this is a different species than that commonly found in the body cavity and viscera was investigated through molecular studies. These studies failed to show evidence that the parasites infesting the vent were different from those in the body cavity, i.e. all were identified as A. simplex sensu stricto. No other disease agent was found associated with the lesions or was isolated systemically, and no mortality or prevention of spawning was recorded during the 2007 season. Possible causes, including warming environments in the North Atlantic, are hypothesised as playing a role in the development of RVS in Atlantic salmon.