This study investigated the prevalence of pseudobranchial X-cell pseudotumors in 2 year classes of wild juvenile cod and to a lesser extent in older cod in NW Iceland. The disease was also monitored in farmed wild-caught juvenile cod. The youngest wild cod detected that had X-cell infections were 6.5 to 13.0 cm in length, with prevalence reaching a maximum level of 7%. The highest prevalence (23%) was recorded in 18.5 to 27.0 cm fish. The mean prevalence in older cod (25.0 to 76.0 cm in length)

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... was 7% but decreased with increasing age. The mean prevalence of X-cell pseudotumors in farmed juvenile cod was ≤1% during winter rearing in land-based tanks. Higher prevalence (2 to 15%) was noted in dead and moribund fish. Diseased farmed fish were usually emaciated, and mortality due to the disease was noticeable. KEY WORDS: Pseudotumor · X-cell · Wild cod · Farmed cod · Prevalence · Disease Resale or republication not permitted without written consent of the publisher Editorial responsibility: Thomas Lang,