INFECTIOUS DISEASE: MINI REVIEW Complex Gill Disease: an Emerging Syndrome in Farmed Atlantic Salmon (Salmo salar L.) A. Herrero * , K. D. Thompson * , A. Ashby † , H. D. Rodger ‡ and M. P. Dagleish * *Moredun Research Institute, Pentlands Science Park, Bush Loan, Penicuik, † Fish Vet Group Ltd., 22 Carsegate Road, Inverness, Scotland, UK and ‡ Fish Vet Group Ireland, Unit 7b Oranmore Business Park, Oranmore, Co. Galway, Ireland 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 gener- ally occurring from the end of summer to early winter on marine Atlantic salmon farms. The gross and histo- logical 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 asso- ciated 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 na € ıve fish, and epidemiological investigations are required urgently to elucidate the roles of these pathogens and other factors in CGD. Ó 2018 Elsevier Ltd. All rights reserved. Keywords: Atlantic salmon; complex gill disease; pathology; salmon farming Introduction Gill disease in farmed Atlantic salmon (Salmo salar L.) during the marine rearing stage has become highly significant in recent years. In addition to gaseous exchange, fish gills are responsible for several additional critical physiological functions including osmoregulation, excretion of nitrogenous waste, pH regulation and hormone production. Compromised gill function can lead to significant economic losses due to poor food conversion perfor- mance, direct mortalities and, where medication is required, the cost of treatment. However, gill dis- eases are not notifiable, so the true extent of the problem remains unknown. The intimate contact between gills, which are very delicate compared with the integument, and the marine environment, results in their exposure to pathogens, toxins, partic- ulate matter and organisms present within the farm net-pens, making them particularly susceptible to infection and physical damage. Several pathogens have been associated with gill disease, including par- asites, viruses and bacteria, and the presence of non- infectious organisms, including harmful phyto- plankton such as Chaetoceros spp. and certain zooplankton species, such as Pelagia noctiluca, are also important threats to salmon health as they can predispose fish to gill disease. Gill disease can be caused by a single pathogen, such as amoebic gill disease (AGD) caused by Neoparamoeba perurans. However, clinical disease often presents as a complex syndrome and the primary aetiology can be difficult to establish. The exact pathogeneses, respective roles and contributions of each of the known and putative pathogens present in cases of complex gill disease (CGD) has yet to be elucidated, primarily due to the lack of in-vitro models capable of isolating and J. Comp. Path. 2018, Vol. 163, 23e28 Available online at www.sciencedirect.com ScienceDirect www.elsevier.com/locate/jcpa Correspondence to: M.P. Dagleish (e-mail: mark.dagleish@moredun.ac. uk). 0021-9975/$ - see front matter Ó 2018 Elsevier Ltd. All rights reserved. https://doi.org/10.1016/j.jcpa.2018.07.004