Complex Gill Disease: an Emerging Syndrome in Farmed Atlantic Salmon (Salmo salar L.)

Summary Gill disorders have become a significant problem during the marine phase of farming Atlantic salmon ( Salmo salar L.). The term complex gill disease (CGD) includes a wide range of clinical gill disease presentations generally occurring from the end of summer to early winter on marine Atlantic salmon farms. The gross and histological lesions observed are the resultant culmination of exposure to a mixture of environmental insults, pathogenic organisms and farm management practices. None of the three principal agents purportedly associated with CGD ( Desmozoon lepeophtherii , salmon gill poxvirus or Candidatus Branchiomonas cysticola) have been cultured successfully in-vitro, so individual in-vivo challenge studies to identify their pathogenesis have not been possible. Studies of cohabitation of single pathogen-infected fish with naive fish, and epidemiological investigations are required urgently to elucidate the roles of these pathogens and other factors in CGD.