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

https://doi.org/10.1016/j.jcpa.2018.07.004Get rights and content

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 naïve fish, and epidemiological investigations are required urgently to elucidate the roles of these pathogens and other factors in CGD.

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 performance, direct mortalities and, where medication is required, the cost of treatment. However, gill diseases 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, particulate 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 parasites, viruses and bacteria, and the presence of non-infectious organisms, including harmful phytoplankton 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 propagating the pathogens for use in in-vivo challenge studies.

Section snippets

Complex Gill Disease

The difficulty in establishing the cause of multipathogen and multifactorial gill disease has resulted in inconsistent classification. As such, no specific case definition exists currently. The term ‘proliferative gill inflammation’ (PGI) was used to describe recurrent gill disease outbreaks occurring in autumn in salmon farms on the southwest coast of Norway and mostly affecting smolts that had been transferred to the sea the previous spring (S1). Histological changes included epithelial cell

Pathogens Involved in Complex Gill Disease

Several agents have been associated with CGD, but the exact role each contributes to the pathogenesis remains unknown. For example, Atlantic salmon paramyxovirus was detected in fish with PGI, but more recent studies have shown it is not consistently associated with the disease. Salmon gill poxvirus (SGPV) was described in sea-reared salmon in Norway in 2008, although poxvirus has been considered to contribute to gill disease cases in fresh water since the 1990s. Currently, SGPV is considered a

Other Factors Involved in Complex Gill Disease

Although all risk factors associated with CGD have yet to be determined, some environmental conditions and handling procedures have been proposed to play a key role in the initiation of the disease. Harmful phytoplankton blooms (HABs) can precede CGD outbreaks, presumably by causing physical damage or irritation of the gill epithelium and facilitating the ingress of infectious agents. HABs can induce excess mucus production, small focal thickenings along the gill filaments and gill haemorrhage.

Conclusion and Future Studies

Definitive diagnosis of individual outbreaks of gill disease in Atlantic farmed salmon is often difficult as it is a multifactorial disease. None of the three principal pathogens most frequently associated with CGD (i.e. SGPV, D. lepeophtherii and Ca. Branchiomonas cysticola) have been cultured successfully in-vitro, so confirmation of their individual roles in the pathogenesis of CGD remains challenging and is the focus of present research. The occurrence of clinical CGD is mainly during

Acknowledgments

AH was funded by Moredun Scientific and Fish Vet Group. MPD was funded by the Rural and Environment Science and Analytical Services division of the Scottish Government.

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