Parvicapsula pseudobranchicola n.sp. (Myxozoa), a myxosporidian infecting the pseudobranch of cultured Atlantic salmon (Salmo salar) in Norway

In March-June 2002 increased mortality was observed in seawater reared Atlantic salmon in a farm in Lyngen, Northern Norway. Diseased fish surfaced, appeared lethargic and swam disorganised. Affected fish were dark in colour and most had eye bleedings; some also showed cataracts and exophthalmia. Diseased fish had changes in the pseudobranchs, which in advanced cases was seen as a whitish "cheesy" covering. Microscopy revealed masses of myxozoan trophozoites and typical Parvicapsula spores. Parvicapsula pseudobranchicola n.sp. is disporous, and have asymmetrical curved spores in sutural view that measures 11.1-13.8 mu m in length. The clinical signs are in accordance with vision impairment, and it is suggested that the pseudobranch lesions may obstruct the blood supply to the choroids bodies of the eyes. Possible natural hosts for the infections in cultured salmon are discussed.