Assessment of infectious salmon anaemia virus prevalence for different groups of farmed Atlantic salmon, Salmo salar L., in New Brunswick

Abstract

Infectious salmon anaemia (ISA) virus (ISAV) has been causing disease in New Brunswick since 1996. As a control measure, all fish in an outbreak cage are killed. The objective of this study was to compare ISAV prevalence in cages experiencing an outbreak with healthy cages from the same farm, neighbouring farms and distant farms. Atlantic salmon from five different groups were tested using an RT-PCR test. Groups included moribund fish from a cage experiencing an outbreak (A), healthy fish from an outbreak cage (B), healthy fish from a negative cage from a farm experiencing an outbreak in a different cage (C), healthy fish from a negative farm near an outbreak farm (D), and healthy fish sampled at a negative farm located in an area with only negative farms (E). Apparent prevalences (standard error) for the different groups (A-E) were 0.94 (+/-0.026), 0.41 (+/-0.062), 0.29 (+/-0.040), 0.08 (+/-0.037) and 0.08 (+/-0.037), respectively. All groups were significantly different (P < 0.002) from each other except for groups B and C and groups D and E. Because the prevalence of the virus was significantly higher in the outbreak cage (B) compared with other sites, early harvest of outbreak cages will remove one source of virus. However, ISA negative cages (C) that remain on the positive farm may potentially act as a viral reservoir.

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

“…In Scotland, confirmed sites are depopulated, but in other countries depopulation may be restricted to cages with significant mortality. McClure et al (2004) showed that the prevalence of ISAV in non-moribund fish was the same whether or not mortality occurred in a sampled cage, providing ISA was present on the farm. Depopulation was undertaken on the basis of epidemiological evidence to provide a policy of eradication for which requirements to achieve are described in EU Council Directive 2006/88/EC.…”

Within-farm spread of infectious salmon anemia virus (ISAV) in Atlantic salmon Salmo salar farms in Chile

“…Introduction of ISAV into a salmon holding may have different sources and may affect a variable proportion of fish in different cages (McClure et al 2004). In general, when fish in one or more cages are initially infected, ISAV spreads among susceptible cages directly through seawater (Løvdal & Enger 2002) via the excretion or secretion of infected fish mucus, blood, tissues, or feces (Thorud & Djupvik 1988, Nylund et al 1994, Totland et al 1996, Rimstad et al 1999.…”

A systematic surveillance programme for infectious salmon anaemia virus supports its absence in the Pacific Northwest of the United States

“…We used an EpiTools epidemiological calculator (http://epitools.ausvet.com.au) to estimate a sample size of 332 fish necessary to achieve 0.95 probability of detecting ISAV, presuming 90% sensitivity and perfect specificity, present at a threshold prevalence of 1% or greater in any of the aggregate populations. This 1% detection threshold exceeds (is lower than) levels expected for HPR‐deleted prevalence in Atlantic salmon in outbreak settings (Gustafson, Ellis, Merrill, Robinson, & MacPhee, ; McClure, Hammell, Dohoo, Nerette, & Hawkins, ; Plarre et al., ) and, to address less susceptible species, also exceeds (is lower than) the 2% default described in the OIE Aquatic Code (OIE ; Plarre et al., ). We also repeated analyses for a range of test sensitivities to explore the impact to conclusions should actual sensitivities vary across HPR‐deleted genotypes or due to method modifications to the Snow assay.…”