Sub-clinical infection of farmed Atlantic salmon Salmo salar with salmonid alphavirus— a prospective longitudinal study

Abstract

A prospective longitudinal study of salmonid alphavirus infection in farmed Atlantic salmon Salmo salar L. was initiated in post-transfer smolts on a UK farm in July 2004 and continued for 320 d. Sampling was concentrated on a single caged population (C4) with serum and tissue samples collected and tested for viraemia, virus neutralising (VN) antibodies and viral nucleic acid by real time RT-PCR and by histopathology; 380 sera collected between Days 0 (D0) and 139 (D139) were consistently negative for both viraemia and VN antibodies. The first evidence of infection was detected on D146, when 4 out of 20 fish were found to be viraemic and 1 of 20 to be antibodypositive. On D153 only 2 of 20 fish was viraemic and 1 antibody positive. At the next sampling (D158) no viraemic or antibody positive fish were detected. Thereafter, one or two viraemic fish were detected on 6 occasions, including on D320. The prevalence of antibody-positive fish remained low (0 to 5%) until D192 after which time it rose irregularly to a peak of 57.9% on D320. Real time RT-PCR testing of sera was more sensitive than screening for viraemia, detecting a peak of 35% positive on D153 before declining. Histological lesions diagnostic for pancreas disease (PD) were observed at D146 and D153 only. In addition, mild cardiac and to a lesser extent brain lesions were frequently found after virus was detected, but not in earlier samples. No clinical signs or mortalities attributable to PD occurred throughout the study. This is the first detailed report of sub-clinical infection and highlights the usefulness of longitudinal surveys and the detection of virus and antibodies as diagnostic and epidemiological tools. KEY WORDS: Salmonid alphavirus · Pancreas disease · Longitudinal survey · Sub-clinical infection · Viraemia · Serology · RT-PCR Resale or republication not permitted without written consent of the publisherDis Aquat Org 72: 193-199, 2006 ological studies in Ireland between 1989 and 1994 found that mortality levels on sites varied widely from a low of 0.1% to as high as 63% (Crockford et al. 1999). However, overall there was a significant downward trend in PD-related mortalities during this period that continued in the second half of the 1990s. A similar observation was reported from Scotland (Bruno 2004). However, in recent years PD has re-emerged as a significant cause of mortality and economic loss in the Irish industry and there is evidence of a similar trend in both Scotland and Norway (G Ritchie pers. comm.).Both field and experimental infections of salmon with SAV isolates from outbreaks of PD are characterised clinically by sequential histopathological changes that include pancreatic and cardiomyocytic necrosis and skeletal myopathy (McLoughlin et al. 1996(McLoughlin et al. , 2002. Because of the difficulties inherent in isolating the virus from tissues of affected fish (Nelson et al. 1995), traditionally, these histopathological changes have been the basis on which PD is diagnosed. More recently, tests capable of scree...

Identification of a wild reservoir of salmonid alphavirus in common dab Limanda limanda, with emphasis on virus culture and sequencing

“…It is also plausible that amplification and spread of SAV from infected fish farms could lead to the establishment of potential reservoirs of re-infection through the complexities of trade (McLoughlin et al 2003). The saithe Pollachius virens occurs in the vicinity of sea cages (Bruno & Stone 1990), and later studies have shown virus-neutralising antibodies, but have failed to record disease, suggesting that inter-species transmission from wild to farmed fish, or vice versa, can take place (Graham et al 2006). Finally, in terrestrial alphaviruses, transmission of infection is linked to an arthropod vector; however, in the fish to fish transmission of SAV, as demonstrated by Boucher et al (1995), no vector is needed as a routine route of infection, and none has been identified to date.…”

“…Several studies have shown heart tissue to be one of the most appropriate tissues for detecting SAV during all stages of infection (also persistent infections) (Graham et al 2006, Christie et al 2007, Andersen et al 2007; for this reason, only heart tissue was processed for molecular testing. Approximately 5 mg of homogenised tissue was used for ex traction of total RNA (MagAttract M48 RNA Tissue Kit, Qiagen) using the M48 BioRobot (Qiagen).…”

Coastal salmon farms attract large and persistent aggregations of wild fish: an ecosystem effect

“…P. virens, G. morhua and S. scombrus in particular were observed next to the cage walls and were therefore often within 1 to 2 m of the caged salmon. The most abundant farm-associated species, P. virens, may act as a natural reservoir of the salmonid alphavirus (SAV; Graham et al 2006) and carries infectious pancreatic necrosis virus (IPNV), though at relatively low prevalence (1.1%; Wallace et al 2008). If pathogens or parasites are shared and transmitted among salmon and these 4 species, residence at specific farms followed by movements to adjacent farms may contribute to the propagation of outbreaks, in an analogous way to the dispersal of bird flu to domestic birds by migrating wild birds (Chen et al 2005).…”

Seasonality and heterogeneity of live fish movements in Scottish fish farms

“…However, this study did not include seasonality of movements. Subclinical infections can go unnoticed (Bruno 2004, Graham et al 2006, Lyngstad et al 2008, Murray et al 2010. Performing clinical tests increases the change of detecting subclinical infections and movements can be stopped when a farm tests positive.…”

“…Therefore, movements from source farms known to be infected with a notifiable disease are prohibited (Joint Government/Industry Working Group 2000). However, notifiable and other infections can go undetected (Murray & Peeler 2005, Graham et al 2006, Lyngstad et al 2008. Therefore, pathogens may spread through live fish movements before pathogens are detected (Jonkers et al 2010).…”