Gill diseases in marine salmon aquaculture with an emphasis on amoebic gill disease…

ROZAS-SERRI MA. 2019. Gill diseases in marine salmon aquaculture with an emphasis on amoebic gill disease. CAB Reviews 14(32):1-15.


Gill diseases are a growing health challenge in salmon farming worldwide, but many gaps remain in our knowledge. Gill diseases are generally complex and multifactorial disorders, often with presumable spatial and temporal distribution patterns, but are highly difficult to effectively prevent and control. The term complex gill disease (CGD) includes a wide range of clinical disease presentations on the gills of farmed salmon; usually, CGD presents from the end of summer to early winter. The pathogens involved include Neoparamoeba perurans, Tenacibaculum maritimum, Candidatus Piscichlamydia salmonis, Candidatus Branchiomonas cysticola, Desmozoon lepeophtherii (syn. Paranucleospora theridion) and viruses, such as the Atlantic salmon paramyxovirus (ASPV) and salmon gill poxvirus (SGPV). Amoebic gill disease (AGD) is perhaps the most significant disease in terms of gill health and economic impact. AGD results in high mortality, reduced production performance and impaired fish welfare. This review summarizes and analyses CGD research, outbreaks and treatment, with a focus on AGD, as well as on knowledge gaps and avenues for future research.


Transcriptomic profiles of post-smolt Atlantic salmon challenged with Piscirickettsia salmonis…

ROZAS-SERRI MA, PEÑA A, MALDONADO L. 2018. Transcriptomic profiles of post-smolt Atlantic salmon challenged with Piscirickettsia salmonis reveal a strategy to evade the adaptive immune response and modify cell-autonomous immunity. Develepmental and Comparative Immunology 81, 348-362.


Piscirickettsiosis is the main bacterial disease affecting the Chilean salmon farming industry and is responsible for high economic losses. The development of effective strategies to control piscirickettsiosis has been limited in part by insufficient knowledge of the host response. The aim of this study was to use RNA sequencing to describe the transcriptional profiles of the responses of post-smolt Atlantic salmon infected with LF-89-like or EM-90-like Piscirickettsia salmonis. Enrichment and pathway analyses of the differentially expressed genes revealed several central signatures following infection, including positive regulation of DC-SIGN and TLR5 signalling, which converged at the NF-kB level to modulate the pro- inflammatory cytokine response, particularly in the PS-EM-90-infected fish. P. salmonis induced an IFN-inducible response (e.g., IRF-1 and GBP-1) but inhibited the humoral and cell-mediated immune responses. P. salmonis induced significant cytoskeletal reorganization but decreased lysosomal protease activity and caused the degradation of proteins associated with cellular stress. Infection with these isolates also delayed protein transport, antigen processing, vesicle trafficking and autophagy. Both P. salmonis isolates promoted cell survival and proliferation and inhibited apoptosis. Both groups of Trojan fish used similar pathways to modulate the immune response at 5 dpi, but the transcriptomic profiles in the head kidneys of the cohabitant fish infected with PS-LF-89 and PS-MS-90 were relatively different at day 35 post-infection of the Trojan fish, probably due to the different degree of pathogenicity of each isolate. Our study showed the most important biological mechanisms used by P. salmonis, regardless of the isolate, to evade the immune response, maintain the viability of host cells and increase intracellular replication and persistence at the infection site. These results improve the understanding of the mechanisms by which P. salmonis interacts with its host and may serve as a basis for the develop- ment of effective strategies for the control of piscirickettsiosis.


Comparison of gene expression in post-smolt Atlantic salmon challenged by LF-89-like…

ROZAS-SERRI M, PEÑA A, ARRIAGADA G, ENRÍQUEZ R, MALDONADO L. 2018. Comparison of gene expression in post-smolt Atlantic salmon (Salmo salar L.) challenged by LF-89-like and EM-90-like Piscirickettsia salmonis isolates reveals differences in the immune response associated with pathogenicity. Journal of Fish Diseases 41(3), 539-552.


Piscirickettsiosis is the main bacterial disease affecting the Chilean salmon farming industry and is responsible for high economic losses. The aim of this study was to describe and comparatively quantify the immune response of post-smolt Atlantic salmon infected by cohabitation with fish bearing LF-89-like and EM-90-like Piscir- ickettsia salmonis. The expression of 17 genes related to the immune response was studied in head kidney from cohabitant fish by RT-qPCR. Our results at the transcriptomic level suggest that P. salmonis is able to manipulate the kinetics of cytokine production in a way that might constitute a virulence mechanism that pro- motes intracellular bacterial replication in cells of Atlantic salmon. This strategy involves the creation of an ideal environment for the microorganism based on induction of the inflammatory and IFN-mediated response, modulation of Th1 polar- ization, reduced antigen processing and presentation, modulation of the evasion of the immune response mediated by CD8+ T cells and promotion of the CD4+ T-cell response during the late stage of infection as a mechanism to escape host defences. This response was significantly exacerbated in fish infected by PS-EM-90 compared with fish infected by PS-LF-89, a finding that is probably associated with the higher pathogenicity of PS-EM-90.


Comparative pathogenesis of piscirickettsiosis in Atlantic salmon (Salmo salar L.)…

ROZAS-SERRI M, ILDEFONSO R, PEÑA A, ENRÍQUEZ R, BARRIENTOS S, MALDONADO L. 2017. Comparative pathogenesis of piscirickettsiosis in Atlantic salmon (Salmo salar L.) post-smolt experimentally challenged with LF-89-like and EM-90-like Piscirickettsia salmonis isolates. Journal of Fish Diseases 40(10), 1451-1472.


Piscirickettsiosis (SRS) is the most prevalent bacterial disease in Chilean salmon aquaculture and is responsible for high economic losses. The aim of this study was to comparatively characterize the pathogenesis of SRS in post-smolt Atlantic salmon during the early and late stages of infection with Piscirickettsia salmonis LF-89-like (PS-LF-89) and EM-90-like (PS-EM-90) using a cohabitation challenge. The patho- genesis of cohabitant fish infected with the two isolates was relatively different due to cohabitant fish infected with PS-EM-90 showing higher cumulative mortality and shorter time until death compared with PS-LF-89 fish. PS-LF-89 caused an SRS infection characterized by kidney and liver lesions, whereas PS-EM-90 caused sys- temic and haemorrhagic disease characterized by kidney, liver, heart, brain, skeletal muscle and intestine lesions. Decreased serum concentration of total proteins and albumin as well as increased serum ALT, AST and creatinine levels in fish infected with both isolates confirmed that changes in liver and kidney function occurred dur- ing infection. Tissue damage, expressed as an SRS histoscore, showed a strong posi- tive correlation with the bacterial load expressed as abundance of P. salmonis 16S rRNA transcripts in the livers and kidneys of fish affected with either isolate, but the correlation was significantly higher in fish infected with PS-EM-90. The results contribute to improving the understanding of the bacteria–host interaction.


Piscirickettsiosis and Piscirickettsia salmonis in fish: a review…

ROZAS MA, ENRÍQUEZ VR. 2014. Piscirickettsiosis and Piscirickettsia salmonis in fish: a review. Journal of Fish Diseases 37:163-188.


The bacterium Piscirickettsia salmonis is the aetio- logical agent of piscirickettsiosis a severe disease that has caused major economic losses in the aquaculture industry since its appearance in 1989. Recent reports of P. salmonis or P. salmonis-like organisms in new fish hosts and geographical regions have increased interest in the bacterium. Because this gram-negative bacterium is still poorly understood, many relevant aspects of its life cycle, virulence and pathogenesis must be investigated before prophylactic procedures can be properly designed. The development of effective control strategies for the disease has been limited due to a lack of knowledge about the biology, intracellular growth, transmission and virulence of the organism. Piscirickettsiosis has been difficult to control; the failure of antibiotic treatment is common, and currently used vaccines show vari- able long-term efficacy. This review summarizes the biology and characteristics of the bacterium, including its virulence; the infective strategy of P. salmonis for survival and evasion of the host immune response; the host immune response to invasion by this pathogen; and newly described features of the pathology, pathogenesis, epidemiol- ogy and transmission. Current approaches to the prevention of and treatment for piscirickettsiosis are discussed.


Epidemiology of Amoebic Gill Disease (AGD) in Chilean salmon industry between 2007…

ROZAS MA, BOHLE HM, GROTHUSEN H, BUSTOS PA. 2012. Epidemiology of Amoebic Gill Disease (AGD) in Chilean salmon industry between 2007 and 2010. Bulletin of European Association of Fish Pathologists 32(5), 181-188.


The recent report of amoebic gill disease (AGD) and of Neoparamoeba perurans in Chile has made it necessary to develop practical tools that will be useful for carrying out epidemiological studies. N. perurans. The prevalence of AGD in At- lantic salmon was 55.7% (29/52 farms) and the epidemic curve was observed between May 2007 300 g reared in Los Lagos Region during summer and autumn season showed 3.7 (p=0.0004), 4.2 (p=0.0178) and 6.2 (p=0.0031) times greater risk to be AGD positive, respectively. The reduction of Atlantic salmon biomass reared in Chile closely related with ISA crisis could considerably have increased the infection pressure of N. perurans to rainbow trout (63.2%, 12/19 farms) and coho salmon (90.9%, 10/11 farms).


Development of PCR assay for detection of Neoparamoeba perurans…

ROZAS MA, BOHLE HM, ILDEFONSO RS, BUSTOS PA. 2011. Development of PCR assay for detection of Neoparamoeba perurans and comparison of histological diagnosis. Bulletin of European Association of Fish Pathologists 31(6), 211-218.


The recent description of Amoebic Gill Disease (AGD) and Neoparamoeba perurans in Atlantic salmon (Salmo salar) farmed in Chile has necessitated the development of more reliable and sensitive diag- nostic tests. Final diagnosis of infection is normally confirmed by histology. However, the correla- tion between gross gill lesions and histological lesions is generally unclear. In the current study, moderate concordance level (k=0.5319) between gross pathology and histology was observed. The sensitivity and specificity of gross pathology was 77.91% and 71.05%, respectively. Neoparamoeba spp. are considered morphologically indistinguishable therefore by using histopathology limits the capacity to characterise the causative agent and it can be time consuming. We developed a PCR assay to amplify the N. perurans 18S rRNA gene from gill clinical samples of AGD-affected fish. High concordance level (k=0.95) between PCR and histological examination was observed. The sensitivity and specificity of PCR assay was 94.64% and 97.06%, respectively. The PCR-based assay provides a rapid tool that will be useful to the diagnostic routine for AGD in Chile.


Amoebic gill disease (AGD) in Atlantic salmon (Salmo salar) farmed in Chile…

BUSTOS PA, YOUNG ND, ROZAS MA, BOHLE HM, ILDEFONSO RS, MORRISON RN, NOWAK BF. 2011. Amoebic Gill Disease (AGD) in Atlantic salmon, Salmo salar, farmed in Chile. Aquaculture 310, 281-288.


Between May and November 2007, three marine Atlantic salmon farms around Chiloé Island, Chile, reported mortalities in which affected fish presented with Caligus rogercresseyi infections and gross gill lesions characteristic of amoebic gill disease (AGD). Histological examination of the gills from affected fish confirmed the presence of AGD lesions. Trophozoites possessing one or more endosymbiotic Perkinsela amoeba-like organisms (PLOs) were observed in association with hyperplastic tissue. Further analyses were undertaken using a combination of PCR and in situ hybridization and the trophozoites were identified as Neoparamoeba perurans. Thus, our data indicate that N. perurans is a causal agent of AGD in Chile. However, it is possible that AGD was not the single cause of mortalities in the epizootics investigated here. The exceptionally high level of co-infection with Caligus rogercresseyi (maximum mean intensity 34, prevalence 100%), could have contributed to the production losses.