Microbial and pathological findings in farmed Atlantic salmon Salmo salar with proliferative gill inflammation

• Published in: Diseases of Aquatic Organisms
• Main Authors: MOHAMMAD, S., Steinum, Terje Marken, HEUM, M., GRONTVEDT, R. Nygaard, FALK, K., KVELLESTAD, A., COLGUHOUN, D. J.
• Other Authors: Norwegian Veterinary Institute ,, 100534
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2010
• Subjects: Tarımsal Bilimler; Su Ürünleri; Veteriner Bilimleri; Sağlık Bilimleri; VETERİNERLİK BİLİMLERİ; Tarım ve Çevre Bilimleri (AGE); Bitki ve Hayvan Bilimleri; BALIKÇILIK; Norway; Atlantic salmon; Salmo salar
• Online Access: https://hdl.handle.net/20.500.12627/116654; https://doi.org/10.3354/dao02266

Proliferative gill inflammation (PGI) is an important cause of loss in seawater-farmed Atlantic salmon in Norway. Several microbes have been associated with PGI, including the commonly but not exclusively observed inclusions (epitheliocysts) within the gill lamellae related to infection with 'Candidatus Piscichlamydia salmonis'. Atlantic salmon transferred in the spring of 2004 to 12 sea water farms situated in mid- and southwest Norway were sampled throughout that year. Outbreaks of PGI, as evaluated by clinical examination, histology, and mortality data, were diagnosed in 6 of 7 farms in southwest Norway but not in the 5 farms studied in mid-Norway. Generally, mortality started 3 to 5 mo after seawater transfer and outbreaks lasted at least 1 to 3 mo. 'Ca. P. salmonis' was detected by real-time PCR only in fish from PGI-affected farms and our results indicate an association between 'Ca. P. salmonis' load and PGI severity. Likewise, although widely distributed in all 12 farms studied, epitheliocyst prevalence and number per fish as observed by histology appears associated with PGI prevalence and severity. However, the occurrence of epitheliocysts showed no association with molecular detection of 'Ca. P. salmonis', suggesting that at least 1 other organism is responsible for many of the observed inclusions. A microsporidian, Desmozoon lepeophtherii, was identified at high prevalence regardless of fish and farm PGI status, but at higher loads in fish with PGI. Our results support a multifactorial etiology for PGI in which 'Ca. P. salmonis', an unidentified epitheliocyst agent, and the microsporidian are contributing causes. No evidence for the involvement of Atlantic salmon paramyxovirus in PGI development was identified in the present study. High water temperatures and ectoparasites probably exacerbated mortality.