The Aetiology, Histopathology, and Ultrastructural Features of Perianal Erythema (Red Anus Syndrome) in the European Eel (Anguilla anguilla)

The European eel (Anguilla anguilla) is a critically endangered species. Red anus syndrome (RAS) is known to be associated with parasitic infections of the eel, particularly with Anguillicola crassus, but the full range of causative pathogenic organisms has not been systematically investigated. Here we examined the infective organisms and histopathological and ultrastructural features of seventy eels with RAS. In total, nine different pathogens were detected in association with RAS: Pseudomonas aeruginosa were present in twelve specimens (17%), the metacercaria of Euclinostromum heterostomum in three cases (4%), Gastrostome (Bucephalidae family) in seven cases (10%), A. crassus in forty-five cases (64%), Bothriocephalus in seventeen cases (24%), and Proteocephalus in twenty-three cases (32%). Yeast, amoeba, and myxobolus-like pathogens were seen in the anal skin in all cases when examined in combination with electron microscopy. Histopathologically, the lesions appeared as anoproctitis of varying severity from mild anusitis to severe haemorrhagic anoproctitis, with severe perianal oedema, haemorrhage, and proctoptosis. Gut inflammation ranged from mild catarrhal enteritis to severe haemorrhagic enteritis with mucosal sloughing. RAS is associated with a range of parasitic infections, not only A. crassus, some of which we describe here for the first time. Since RAS is not associated with direct invasion by parasites, it is likely that RAS is a secondary phenomenon caused by superadded infection on a background of generalised immunosuppression, or indirect local toxic effects. RAS may be used as a non-invasive indicator of underlying parasitic infection, but further investigations are required to establish the causative organisms for effective fishery management.