Piscirickettsiosis (SRS) in the North-East Atlantic area
SRS is a serious problem for the Chilean salmon farming industry. In recent years, the disease has become increasingly significant in Ireland and Scotland and has also appeared at several Norwegian farms. Researchers do not yet know why SRS is on the rise in the Northeast Atlantic. To increase knowledge of the disease, the two-year, FHF-funded PisciNOR research project was established. This article presents some of the preliminary results.
By: Hamish Rodger1, Anne Berit Olsen2, Meritxell Díez-Padrisa1, Ana Herrero1, Samantha White3, Silvia Soares3, Patricio Bustos4, Marcos Mancilla4 and Duncan J. Colquhoun2
1 PatoGen Fish Health Services Ltd., Ireland
2 Norwegian Veterinary Institute, Norway
3 Marine Institute, Ireland
4 ADL Diagnostic Chile, Chile
Primarily infecting salmonid species, piscirickettsiosis or SRS (salmon rickettsial syndrome/septicaemia) is caused by the bacterium Piscirickettsia salmonis, although as we are now becoming increasingly aware, other closely related bacterial species may also be involved. Piscirickettsiosis has also occasionally been reported in other farmed marine fish species including lumpfish, sea bass, and turbot. While SRS has been identified in most salmon-producing countries, the severity of infection experienced varies significantly across different geographical regions. It is considered the most important infectious disease in salmon farming in Chile, but its clinical significance has traditionally been lower in the Northern Hemisphere. It is, however, now considered a major threat to Irish salmon farming and, to a lesser but increasing extent, to Scottish salmon farming.
Comparison of SRS in Chile and the North Atlantic
Similarities between the Chilean and North-East Atlantic situations include long-standing recognition of the disease. In Norway, a condition then termed necrotic hepatitis in salmon (NHL), subsequently confirmed as SRS, was observed in the autumn of 1988 and initial SRS cases were reported from Ireland in 1991 and Scotland in 1995. While disease consistent with SRS had been observed as early as 1981 in Chile, it was first confirmed in 1989. Major differences between the European and Chilean situations included mortality levels, which in Norway, Ireland and Scotland were, with few exceptions until recently, usually insignificant or low level compared to the clinical impacts experienced in Chile.
While SRS has remained the most important bacterial disease in salmon farming in Chile, the situation in the North-East Atlantic appears to have changed in recent years. Following initial diagnoses in all three North Atlantic countries, only sporadic cases were observed with very low-level clinical impacts for many years. While this primarily remains the case in Norway, there has, since 2019, been a significant increase in the clinical impact, need for antibiotic treatments, and number of sites affected in Ireland and Scotland. In these countries, since 2023, Piscirickettsia spp. have been detected annually in more than 25 marine sites, with fish in the majority of these sites exhibiting clinical signs of SRS.
Although generally only sporadically diagnosed in Norway, there have been occasional years with a substantial number of sites affected e.g. 17 sites in 2002 and at least seven salmon farms affected in the north of the country in 2024. The 2024 situation, combined with an increased awareness of the worsening situation in neighbouring countries and the recent (2025) identification of piscirickettsiosis in farmed turbot in the south of Norway, gives cause for concern. As a precaution, piscirickettsiosis was re-listed as a notifiable disease in Norway in June 2025 (Category G).
SRS outbreaks in Chile, and to some degree in the Atlantic countries, are often reported after significant variation in sea temperature, following algal blooms or another environmental stressor such as a delousing bath. SRS mortality has a strong seasonal component, with mortalities significantly higher in warmer months and in the presence of lice infestations. In Chile, piscirickettsiosis is controlled through improved biosecurity and fish husbandry, regulation, surveillance, mandatory vaccination, selective breeding, and antibiotic use. Despite this, the disease remains a significant challenge for salmon farming with estimated costs of over US $700 million per annum (associated with quality downgrades, loss of market, loss of growth, mortalities, costs of treatment, vaccines, mortality disposal, etc.) attributed to the disease.
The strongest determinants of piscirickettsiosis prevalence in Chile have been shown to be the proximity to infected sites, the number of infected farms in upstream waters, and the severity of disease (total mortality), followed by seawater salinity and temperature. In addition to vaccination, genetics, and legislative regulation in Chile, good farming practices and improved biosecurity have proven vital to reduce impacts of SRS. Maintenance of net-pen cleanliness, reduced growing time at sea, daily mortality/moribund fish removal, effective lice control, accurate mortality classification, effective predator (sea lion) control, and high standards of smolt quality are all priority areas. While piscirickettsiosis cases in Norwegian aquaculture have rarely, if at all, required antibiotic treatment, both metaphylaxis and functional feeds have been applied with some success in management of SRS in Chile. Antibiotic treatment is also now utilised in Scotland and Ireland as required. Antimicrobial therapy applied when SRS weekly mortality >0.03% is identified in Chilean literature as a risk factor for earlier onset of outbreaks compared to initiation at <0.01%. Early treatment has proven critical as appetite declines in infected fish. Further, it is reported that salmon stocked in the autumn in Chile are at a higher risk of earlier SRS. Several studies have identified an association between the presence of Piscirickettsiaceae and amoebae in coastal, riverine, and estuarine sediments. A recent publication has confirmed the ability of P. salmonis to survive and multiply within free living marine amoebae.
Data from the North-East Atlantic
In this study, data and information were compiled from 14 farming companies, covering 223 marine sites of Atlantic salmon (or rainbow trout) during 2024 and 2025 in Ireland, Norway, and Scotland. In addition, mortality data for farm sites in Scotland was obtained through monthly figures in the publicly available Salmon Scotland website https://www.salmonscotland.co.uk/. Results of PCR analysis for Piscirickettsia spp. from sampled Scottish farms were obtained from the PatoGen AS database for 2024 and 2025. A relatively low percentage of salmon sites screened for Piscirickettsia spp. were identified as positive in 2024 and 2025 respectively. No detections were identified in rainbow trout sites during the study period. Mortalities were not significant in any site with P. salmonis detection in Norway during the study period and data on site level mortality was not available for Ireland. In clinical cases from all three countries, the mean size of salmon affected was 1.1 kg (range 0.2 to 2.2 kg), held at a mean stocking density of 5.1 kg/m3 (range 1.8 to 10 kg/m3), and mean water temperature of 13.9°C (range 9.5 to 17°C). The mean duration of clinical outbreaks was 13 weeks (range 4-26 weeks) including antibiotic treated and untreated fish (Ireland and Scotland). Norwegian cases in 2024 were not treated. SRS outbreaks (including some co-infections with other diseases) were commonly reported following delousing or amoebic gill disease bath treatments or after exposure to harmful cnidarians (hydrozoans).
The causal agent/s
While various genotypes of P. salmonis appear to be the sole bacterial species associated with piscirickettsiosis in Chile and Norway (and possibly Canada, New Zealand and Australia), preliminary studies suggest that Irish Piscirickettsia are at the very least, dominated by a genetically distinct species provisionally termed ‘Piscirickettsia nova’. In Scotland there is currently evidence of involvement of both P. salmonis and ‘P. nova’. Piscirickettsiosis in Chile is associated with two main genogroups of P. salmonis (LF-89 & EM-90), neither of which has been detected in Norway. The relatively small number of Norwegian isolates examined so far appear to be most closely related to P. salmonis from Western Canada.
Diagnostics
For details regarding clinical signs, surveillance, diagnosis and culture for SRS, see here:
https://www.vetinst.no/sykdom-og-agens/piscirickettsiose
Management
Fallowing of marine sites before restocking is required in most farming regions and will assist with benthic recovery. It has been shown that P. salmonis remains viable in seawater at both surface and 5-metre water depth for prolonged periods and only declines to non-detection levels 50 days after fish have been removed.
Why has this disease emerged to significance in regions which previously only had a low-level sporadic issue in the past?
Emergence and/or spread of more virulent bacterial strains cannot be discounted and is presently under study. Common factors between regions include increased use of wellboats for bath treatments, increased mechanical delousing, increased frequency of periods of elevated seawater temperature including the marine heatwave experienced in Northern Norway in 2024, and frequent harmful plankton challenges. Increases in complex gill disease and compromised gill function, combined with possible reduced levels of biosecurity may exacerbate the situation. Mitigation of infection will not be easy, given the infection is difficult to treat and the causative agent probably survives in the environment (and on fish) for many weeks. Piscirickettsia spp. can enter the fish easily through skin and gills, particularly when compromised, and existing vaccines are only partially effective. Marine reservoirs are known to exist although some remain to be identified.
Requirements for the future, besides the fundamental importance of farming a healthy, robust fish under optimal conditions, include genetic surveillance of Piscirickettsia spp., rapid diagnostics, alternative therapies, and effective vaccines. It is highly likely that fish farmed in an open pen environment cannot be protected from natural Piscirickettsia infection. Hence development of effective vaccines against this disease alongside genetic selection for tolerance in farmed salmon represent the most promising avenues of research towards successful control of piscirickettsiosis in open pen sea-farmed Atlantic salmon.
Acknowledgements
The FHF is gratefully acknowledged for funding this work (FHF project 910068). The fish farmers, veterinarians, and biologists in Ireland, Norway, and Scotland who readily assisted with experiences and surveys details are also thanked for their time and efforts.
References
Bustos, P. & Mancilla, M. (2025) Chile & SRS: what has worked and what has not? Presentation at SRS Initiative, April 25, 2025, Galway, Ireland
Gaete-Carrasco, Á., Rosenfeld, C., Bassini, L., Lara, M. & Mardones, F. (2026) Risk factors associated with piscirickettsiosis outbreaks in farmed Atlantic salmon (Salmo salar L.) in Chile. Aquaculture, 625, 744273
Gómez, F. A., Milesi, B. & Marshall, S. H. (2026) Free-living amoebae and their role in Piscirickettsia salmonis transmission in Chilean salmon aquaculture: insights from in vitro and in vivo studies. Frontiers in Microbiology, 16: 1711258
