Shipping and the Spread of Infectious Salmon Anemia in Scottish Aquaculture

Alexander G. Murray, Fisheries Research Services Marine Laboratory, Aberdeen, United Kingdom; and University of Aberdeen, Aberdeen, United Kingdom; Ronald J. Smith, Ronald M. Stagg, Fisheries Research Services Marine Laboratory, Aberdeen, United Kingdom

Disclosures

Emerging Infectious Diseases. 2002;8(1) 

In This Article

Abstract and Introduction

Long-distance transport of pathogens plays a critical role in the emergence of novel diseases. Shipping is a major contributor to such transport, and the role of ships in spreading disease has been recognized for centuries. However, statistical confirmation of pathogen spread by shipping is usually impractical. We present evidence of invasive spread of infectious salmon anemia in the salmon farms of Scotland and demonstrate a link between vessel visits and farm contamination. The link is associated with vessels moving fish between sites and transporting harvest, but not with vessels delivering food or involved in other work.

Anthropogenic activity increases the incidence of infectious diseases, which in turn influence the populations and production of marine organisms, from free-living bacteria[1] to mammals[2,3]. The aquaculture industry has been strongly affected by diseases emerging from anthropogenic activities, and itself has played a critical role in their spread[4].

Infectious salmon anemia is an emerging disease causing severe damage to the salmon-farming industry in an increasing number of countries. The disease, first reported in Norway in 1984[5], has since been reported in Atlantic Canada (1996)[6]; Scotland (1998)[7]; the Faroe Islands and possibly Chile (1999)[8]; and most recently Maine, USA (late 2000)[9]; over the last few months (2001), infectious salmon anemia has spread rapidly in Maine[10]. In 1999, the annual cost of infectious salmon anemia was reported to be US$11M in Norway and US$14M in Canada, while in Scotland, the total cost of the epidemic of 1998-99 was US$32M[11]. It is too early to say what the cost of the disease will be in Chile and the United States, but both countries have large salmon-farming industries. In almost all cases, infectious salmon anemia has mainly affected Atlantic salmon, Salmo salar, but in Chile, deaths have been reported among Coho salmon, Oncorhynchus kisutch[8]. All deaths have been among farmed salmon.

Infectious salmon anemia is an emerging disease[12] caused by novel virulent strains of a virus that has adapted to intensive aquacultural practices and has exploited the associated traffic to spread both locally and internationally. Genetic[13,14] and phenotypic[15] differences suggest that this adaptation occurred independently in Europe and the Americas. The virus strains then aggressively expanded their geographic ranges.

Invading new areas is critical for the survival of exotic species[16], including pathogens[12,17]. Shipping has been identified as a major factor in movement of exotic species to coastal regions[16,18]. The role played by ships in the introduction and spread of Black Death (a virulent form of plague) in 14th-century Europe has been extensively chronicled[19]. Recently, huge numbers of bacteria (8.3 x 108 l-1), including Vibrio cholerae, the agent of cholera, and viruses (7.4 x 109 l-1) have been detected in ballast water of ships entering U.S. waters[17]. Given the increasing volume of shipping[18], introduction of pathogens to coastal ecosystems is likely to increase. The rapidly growing aquaculture industry, with its high densities of potential host monocultures, is based in such coastal ecosystems[4].

Numerical analysis of the role of shipping in spreading pathogens is usually not possible because of heavy ship traffic and the multitude of pathogen sources. In this article, we examine the role of shipping in the invasive spread of infectious salmon anemia among Scottish salmon farms.

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