The Faroese researchers behind the study (see separate fact box) write in a Nature publication (Scientific Reports) that to their knowledge this is the first study that shows probable aerosol-transmitted infection of the fish viruses SGPV, ILAV-HPR0 and IPNV in recirculating aquaculture systems (RAS).
“We present the first field-based evidence of viable IPNV isolated from aerosols in a commercial RAS facility for Atlantic salmon, along with insights into infection dynamics, predictive value of anaesthetic water as a sample, and occurrence of multiple pathogens in aerosol samples. Furthermore, we show that pathogen abundance in aerosols is directly affected by levels in the water in the system,” they write in the article.
Unclear routes of infection
Although RAS facilities are closed systems that offer biosecure and water-saving operating conditions, the spread of infection remains a key challenge.
While horizontal transmission via water is well documented in RAS, the potential for aerosol-borne transmission has been little explored. A study conducted at two commercial Atlantic salmon hatcheries based on RAS has therefore looked into this issue.
“Due to high water retention in the RAS, potential pathogens can accumulate over time if water quality is not maintained optimally. This allows them to establish themselves in various RAS components – especially in biofilms, which are known to harbour pathogens,” the researchers write.
Vitenskapelig artikkel
Title: First field evidence of
aerosolised SGPV, ISAV-HPR0, and IPNV in Atlantic salmon RAS highlights
transmission and biosecurity risks
Published in: Scientific Reports
The researchers:
Dhiraj Krishna
- National Reference Laboratory for Fish and Animal Diseases, Faroese Food and Veterinary Authority, Torshavn, Faroe Islands
- Department of Veterinary and Animal Sciences, University of Copenhagen, Copenhagen, Denmark
Petra Elisabeth Petersen
- National Reference Laboratory for Fish and Animal Diseases, Faroese Food and Veterinary Authority, Torshavn, Faroe Islands
Maria Marjunardóttir Dahl
- National Reference Laboratory for Fish and Animal Diseases, Faroese Food and Veterinary Authority, Torshavn, Faroe Islands
Ingibjørg Egholm
- Hiddenfjord, Sandavágur, Faroe Islands
Debes Hammershaimb Christiansen
- National Reference Laboratory for Fish and Animal Diseases, Faroese Food and Veterinary Authority, Torshavn, Faroe Islands
Click here to read the scientific publication
Outbreaks in RAS are associated with several types of pathogens and are often triggered by environmental fluctuations, depending on the species produced.
The most important pathogens for freshwater smolts in the Faroese RAS include:
- non-virulent infectious salmon anaemia virus (ISAV-HPR0),
- infectious pancreatic necrosis virus (IPNV),
- piscine orthoreovirus (PRV-1),
- salmon gill pox virus (SGPV),
- Flavobacterium psychrophilum, as well as opportunistic fungi such as Saprolegnia parasitica
In aquaculture systems, pathogens can be transmitted horizontally (from fish to fish, or via water, feed and equipment) and vertically (from broodstock to offspring). The routes of transmission are not fully understood for all pathogens in aquaculture.
The researchers write that most cases of vertical transmission are due to contamination of egg surfaces rather than true transovarial transmission, and effective disinfection protocols are therefore essential.
"True vertical transmission has been documented for IPNV and Flavobacterium psychrophilum. Aerosol transmission of respiratory pathogens is well known in humans and terrestrial animals, but for fish pathogens – especially viruses – this has been little studied. There is some experimental documentation for aerosol transmission of non-viral pathogens such as Ichthyophthirius multifiliis, Amyloodinium ocellatum, Aeromonas hydrophila, A. salmonicida and Vibrio parahaemolyticus.
The researchers also point out that research has shown that marine aerosols, produced by bubbling in the ocean microlayer (SML), are rich in viruses and bacteria, and can spread pathogens over long distances – a phenomenon also known from sea spray, which can carry microorganisms and human pathogens.
“RAS facilities have components that can produce aerosols, such as deaerators (for CO2 removal) and drum filters (which form aerosols by water spray). These are similar to processes that form sea spray in the marine environment. In the event of an outbreak, pathogens can thus be continuously aerosolised and spread throughout the facility,” they write.
The trials
At hatchery 1, aerosolised pathogens were monitored with two aerosol collectors (Coriolis+ and Coriolis Compact, Bertin Technologies SAS, France), together with water and fish samples, to investigate the dynamics of infection. A sequential infection pattern was observed, starting with salmon gill pox virus (SGPV), followed by non-virulent infectious salmon anaemia virus (ILAV-HPR0), piscine orthoreovirus-1 (PRV-1), and sporadic findings of infectious pancreatic necrosis virus (IPNV) and Flavobacterium psychrophilum.
All pathogens were detected in aerosol samples, with the highest occurrence and concentration in the biofilter room compared to local deaerators.
The detection trends for SGPV and ILAV-HPR0 in the aerosols reflected those observed in fish and water.
Coriolis+ performed marginally better than Coriolis Compact in reflecting infection dynamics.
Viable IPNV was not isolated from the initial aerosol samples at hatchery 1, but bacterial cultures identified relevant colonies for RAS environments.
Targeted aerosol sampling for IPNV at hatchery 2, following an IPNV outbreak, produced IPNV-specific cytopathic effects in cell cultures from Coriolis Compact samples, marking the first field-based evidence of viable aerosolised IPNV from a RAS facility.
The study builds on previous work by introducing the use of anaesthetic water as a refined, non-invasive method for monitoring, while also providing the first field-based evidence of Atlantic salmon viruses in aerosols from RAS, pointing to the potential for airborne transmission and underscoring the need for stringent biosecurity measures.
Conclusion
The researchers write in their conclusion that previous studies have shown that the ocean's microlayer plays an important role in the spread of viruses and bacteria over long distances.
"Our findings build on the hypothesis of sea spray-mediated transmission, and suggest that RAS deaerators may contribute to aerosolisation and spread of fish pathogens in a similar way."
Given the RAS plants' large air throughput, number of systems, and continuous operation, the researchers' results indicate that pathogen-containing aerosols are continuously produced during outbreaks.
"The distance between RAS systems, as well as between hatcheries and sea-based farming locations, can thus pose a high risk of aerosol-transmitted infection, as air intakes and emissions are not disinfected.
They believe the isolation of viable IPNV – used as a “proxy” for non-vertically transmitted ILAV-HPR0 – highlights the biosecurity risk, since ILAV-HPR0 is often detected in hatchery and marine facilities without a known route of infection.
"With caution, based on the detection of viral RNA and successful isolation of IPNV from aerosols, it can therefore be suggested that aerosols have the potential to spread fish pathogens," they write.
They conclude that, in line with previous findings, the study shows that sampling anaesthetic water is more effective than regular water sampling for detecting pathogens, but they emphasise that the method is limited to mapping occurrence at the system level – not for diagnosing disease.
