Epidemiological investigation into the re-emergence and control of an outbreak of infectious salmon anaemia in the Shetland Islands, Scotland

Abstract

Infectious salmon anaemia (ISA) is an orthomyxoviral disease, primarily affecting marine-phase farmed Atlantic salmon, which can result in high levels of mortality. ISA first emerged in Norway in the 1980s and subsequently has occurred in Canada, the USA, the Faeroe Islands and Chile. An outbreak occurred in Scotland in 1998-1999, but was eradicated at a cost of over £20M. The epidemiology of a new outbreak of ISA in the Scottish Shetland Islands during 2008-2009 is described. Six sites have been confirmed ISA-positive. Spread of the virus via transport of fish between marine sites, harvest vessels, smolts and wild fish appears to have been of little or no importance, with spread primarily associated with marine water currents. The use of management areas by Marine Scotland to control the event appears to have been effective in restricting spread to a small area. This localised outbreak contrasts with the 1998-1999 outbreak that spread over a wide geographic area with transported fish and harvest vessels. The development and application of industry codes of good practice, good husbandry and biosecurity practices, limited marine site-to-site movement of live fish and improved disinfection of vessels and processing plant waste that occurred subsequent to the 1998-1999 outbreak may explain the localised spread of infection in [2008][2009]. Depopulation of confirmed sites has been achieved within 7 wk (mean = 3.7 wk); however, it is likely that subclinical infection persisted undetected for months on at least 1 site. The origin of the 2008-2009 outbreak remains unknown. Potential sources include evolution from a local reservoir of infection or importation. Synchronous fallowing of management areas, with good husbandry and biosecurity, reduces the risk of ISA recurring. Movement of fish between sites in different management areas represents the greatest risk of regional-scale spread, should this occur.KEY WORDS: Infectious salmon anaemia · ISA · Epidemiology · Control · Eradication · Hydrodynamics · Harvest · Scotland Resale or republication not permitted without written consent of the publisherDis Aquat Org 91: [189][190][191][192][193][194][195][196][197][198][199][200] 2010 behind ISA spread and how current Scottish fish farming structures and practices led to the spatial limitation of such spread, thus creating the conditions under which ISA could be controlled and eradicated.In 2008, Scotland produced 128 606 metric tonnes (t) of farmed Atlantic salmon from 257 marine sites and 2628 t of sea-farmed rainbow trout Oncorhynchus mykiss from 9 sites (Walker 2009). A further 311 t of brown and sea trout (both Salmo trutta) were also produced, but separate statistics are not published for marine and freshwater farms. (Both O. mykiss and S. trutta can carry the virus but do not develop clinical ISA [OIE 2009]). A third of Scottish salmon production (42 593 t) came from the Shetland Islands.Scottish salmon production is divided for the purposes of disease control into management areas (MAs) (Marine Scotland ...

First field evidence of the evolution from a non-virulent HPR0 to a virulent HPR-deleted infectious salmon anaemia virus

“…Similar stressful farming conditions, including bad weather, handling, infection with sea lice, treatments for sea lice and other environmental stressors have in several studies been associated with decreased resistance to viral pathogens and increased disease susceptibility, including ISA [54,[56][57][58][59][60].…”

Bona Fide Evidence for Natural Vertical Transmission of Infectious Salmon Anemia Virus in Freshwater Brood Stocks of Farmed Atlantic Salmon (Salmo salar) in Southern Chile

“…Although this disease has been observed in most salmon-producing countries that report to the International Veterinary Sanitary Authority (3,4), the aggressiveness displayed in the extremely cold seawater of southern Chile, with cumulative mortality rates sometimes exceeding 90%, is an Achilles' heel for a blooming and promising industry (5)(6)(7). The disease was first described in Norway in 1984 (8), and devastating outbreaks have also been reported in other locations since the late 1990s, such as the Faroe Islands (9, 10), Scotland (11), the United States (12), and several locations in Canada (13,14). The sole etiological agent of the disease has been identified as a novel RNA virus of the Orthomyxoviridae family (infectious salmon anemia virus [ISAV]) and classified as the only member of a new genus, Isavirus (2).…”

Disease management mitigates risk of pathogen transmission from maricultured salmonids

“…In addition, elevated biosecurity standards, including control over the movement of fish and the movement and disinfection of staff (including clothing and personal equip ment) and larger equipment such as transport vehicles, have become widely adopted by industry. As a result, outbreaks of clinical ISA are now rare or absent in Scotland and New Brunswick in eastern Canada and a recent outbreak of IHN in western Canada was of shorter duration with lower mortalities compared with earlier outbreaks (Murray et al 2010, ICES 2013. It is worth noting a possible difference between the epidemiology of ISAV and SAV.…”

“…The establishment of more stringent on-farm and among-farm biosecurity measures have reduced the risk of infection as indicated by the reduced longevity, reduced mortality and spatial limitations of more recent outbreaks (Murray et al 2010, Mardones et al 2014. Maintaining fish in good health may reduce their susceptibility to low virulent strains and thus lower the rate of new outbreaks (Lyngstad et al 2011).…”

“…Rapid removal of sick or dead stock is also effective in limiting virus transmission (Jarp & Karlsen 1997). Coordinated fallowing within management areas combined with risk-based surveillance lessens the likelihood that virulent ISAV will persist and re-establish (Murray et al 2010). In Chile, disease control measures introduced in 2009 (summarized in Table 1) contributed to the recovery of the industry after the occurrence of ISA (Mardones et al 2014).…”