Year of grant: 2015 Research Area: Náttúra og náttúrutilfeingi Project type: Verkætlan Project title: Virulentur ILA virus aftur ávístur í Føroyum. Grant number: 0444 Project manager: Debes Hammershaimb Christiansen Institution/company: Heilsufrøðiliga Starvsstovan Other participants: Dr. Knut Falk, Dr. Maria Aamelfot, Dr. Iveta Matejusova og Dr. Alistair J. A. McBeath Project period: Planned: 1.7.2015-30.12.2017 Actual: 1.7.2015-31.12.2021 Total budget: 1.855.000 Grant from the FRC in DKK: 425.000 Project description: Original Atlantic salmon aquaculture is one of the most important industries in the Faroe Islands and has increased from the beginning in the 1970s to a production of approximately 86.000 tons of a value of more than DKK 2500 million in 2014. Infectious salmon anaemia (ISA) disease epidemics have raged most major Atlantic salmon producing countries and resulted in an almost total collapse of the Faroese farming industry ten years ago. Lack of national management strategies on the prevention and control of ISA played a major role in the devastating consequences of the ISA epidemics. Based on current knowledge on ISA most countries have implemented national management strategies. However knowledge on ISA virus (ISAV) functional characteristics, aetiology, epidemiology and pathogenesis is incomplete including the association between non virulent and virulent ISAV variants (Håstein et al. 1999). We and others have shown that a non-virulent ISAV, designated ISAV-HPR0 (Mjaaland et al., 2002), is highly prevalent and only causes a transient and non-clinical infection localized to the gills in farmed Atlantic salmon (Christiansen et al. 2011, Lyngstad et al. 2012). It has been hypothesized that new virulent variants of ISAV have arisen in aquaculture through known and unknown genetic changes in ISAV-HPR0. Although central in the development of ISA disease, the factors driving the ISAV-HPR0 evolutionary dynamics as well as the putative functional differences between ISAV-HPR0 and virulent ISAV, is virtually non-existent. Last year a new virulent ISAV variant re-emerged at a Faroese marine farming site. Preliminary studies have demonstrated that the isolate is of relatively low-virulence. Furthermore the Atlantic salmon originated from a freshwater pre-smolt farm which was tested ISAV-HPR0 positive. Thus the epidemiologically linked material provides us with a unique opportunity to clarify if the ISAV-HPRO at the pre-smolt farm is the ancestor to the re-emerged virulent ISAV strain at the marine site and which mutations are required for the transition of non-virulent to virulent ISAV variants. The present project suggests to (I) perform whole genome sequencing of the re-emerged virulent ISAV and associated fresh-water ISAV-HPR0 variants to elucidate the link between non-virulent and virulent ISAV, (II) perform a longitudinal infection challenge of pre-smolt to study the evolutionary dynamics of this low-virulent ISAV variant. In the very fast growing farming industry on the Faroe Islands updated risk assessment of new ISA outbreaks including knowledge on ISAV biology is highly demanded by the authorities and the farming industry. Final The virulent Infectious salmon anaemia virus, designated ISAV-HPR is the causative agent of a systemic and lethal disease, infectious salmon anaemia (ISA), of farmed Atlantic salmon (Salmo salar L). In the present study we demonstrate that the non-virulent subtype of ISAV (ISAV-HPR0) is endemic in all three production stages of Atlantic salmon including brood fish, freshwater hatcheries/smolt farms and at marine grow-out sites where it causes a seasonal and non-clinical transient infection (Christiansen et al., 2011 and 2021). Previous work has shown that the virulent ISAV-HPR variant causes a systemic infection targeting endothelial cells. Here we show that ISAV-HPR0 has a different tissue tropism and causes an active infection and replication in the epithelial cells of the gills and skin of Atlantic salmon (Aamelfot et al., 2015, Petersen et al., 2023). The non-virulent ISAV-HPR0 has been proposed to be the progenitor and reservoir for all virulent ISAVs and thus represent a potential risk factor for the emergence of ISA disease. Here, we provide the first field evidence of genetic and functional evolution of ISAV-HPR0 to a low-virulent ISAV- HPR (Christiansen et al., 2017). In the immersion challenge and statistical modelling of viral replication kinetics we demonstrated that fish infected with low-virulent variants showed lower replication efficiencies and different immune responses than fish infected with high-virulent ISAV variant (McBeath et al., 2014a, 2014b). Taken together, the present work shows that mutations of the F and HE genes are key first steps in the within-host infection dynamics i.e. the functional shift of cell and organ tropisms (Christiansen et al. 2017). In general, a virus may be transmitted by horizontal or vertical routes, or both. The question whether ISAV is vertically transmitted has been, and still is, controversial. Here we demonstrate that ISAV-HPR0 is not transmitted vertically from parents to offspring. On the contrary, the geographical clustering of various HPR0 variants emphasizes the importance of horizontal transmission as a driving force of spreading HPR0 within and between the three production stages of Atlantic salmon (Christiansen et al., 2021). Project status: Liðug Project output: Publications outside the scientific community, i.e. lectures, periodicals, articles in newspapers, television and radio 2012 Invited speaker on ISAV-HPR0. Annual EURL Meeting of National Reference laboratories. Aarhus 2013 Invited speaker on Faroese Fish Health Situation. Annual meeting of the Faroese aquaculture industry 2014 Invited speaker on ISA re-emergence (26.02.2014). Annual meeting of the Faroese aquaculture industry. 2014 Invited speaker on Tracking ISAV-HPR0 Transmission. 5th freshwater symposium on Infectious salmon anaemia (24.10.2014). Puerto Varas, Chile. 2015 Invited speaker on Tracking ISAV-HPR0 Transmission in the Faroe Islands. 19th Annual meeting of the National reference laboratories for Fish Diseases. Copenhagen, Denmark, May 27th to 28th 2015. 2017 Oral presentation entitled “Evolution of ISAV-HPR deleted from an ISAV-HPR0 progenitor” at Frisk Fisk, Jan. 31st to Feb. 1st 2017, Bergen, Norway. 2017 Oral presentations entitled “Erfaringer med overvågning af ILA på Færøerne” og ”Kan ILA virus spres med stamfisken” at a workshop on control with ISA. Apr. 3rd to 4th, Tróndheim, Norway. 2017 Oral presentations entitled “Evolution of ISAV-HPR deleted from an ISAV-HPR0 progenitor” and “A new case of ISA in the Faroe Islands” 21st Annual NRL Workshop. May 30th to 31st. 2017 Copenhagen, Denmark 2017 Oral presentation entitled “Evolution of ISAV-HPR deleted from an ISAV-HPR0 progenitor”. 18th International EAFP Conference on Diseases of Fish and Shelfish. Sep. 4th – 8th , Belfast, Ireland. 2017 Invited speaker on “Aquaculture in the Faroe Islands”. Workshop in planning the blue future. Dec 3rd – 8th Hobart, Tasmania. 2018 Invited speaker on “Infectious salmon anemia in the Faroe Islands”. VIII International Veterinary Congress: One world – one Health. April 23rd – 25th 2018, Moscow, Russia. 2019 Invited speaker on “Is RAS and large smolt a risk factor for the spread of ISA?”. Annual meeting for the Faroese aquaculture industry. Mars 1st 2019, Tórshavn. 2019 Invited speaker on “ISAV on the Faroe Islands” Opstartsmøde for ny bekæmpelsesplan for ILA. 20. March 2019, Oslo, Norge. 2019 Invited speaker on “Is production of large smolt in RAS a risk factor for ISA”. 23rd Annual NRL workshop, DTU-Aqua, May 30th – 31st 2019, DTU-Aqua, CPH. 2022 Invited speaker on “Hvordan smitter ILAV-HPR0 mellem produktionsstadier af laks”. Frisk Fisk, Bergen 30th – 31st May 2022. 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