Detection of infectious salmon anaemia virus (ISAV) by RT-PCR after cohabitant exposure in Atlantic salmon Salmo salar

Abstract

A reverse transcription polymerase chain reaction (RT-PCR) was used to study the early phase of infectious salmon anaemia virus (ISAV) infection in Atlantic salmon Salmo salar L. The detection threshold for the RT-PCR was estimated to be 0.01 to 0.1 TCID 50 . A protocol that closely mimics the conditions in populations of farmed salmon was used. The major port of ISAV entry was most likely the gills, but oral entry could not be excluded. The gills and heart were RT-PCR positive 5 d post infection and there was a rapid viraemic spread of the virus after entry. Ten or more days post infection, most organs yielded RT-PCR positive samples. The viral load of the fish followed a 2-phase curve with the first maximum at approximately 15 d and a minimum around 25 d. After 25 d, there was a steady increase in viral load until all sampled organs eventually became positive. In an experiment in which the transportation of material from field to diagnostic laboratory was simulated, the transportation of whole fish was found to be optimal for the performance of RT-PCR. KEY WORDS: Infectious salmon anaemia virus · ISAV · RT-PCR · Detection of ISAV Resale or republication not permitted without written consent of the publisherDis Aquat Org 47: [175][176][177][178][179][180][181] 2001 viruses in their hosts. However, there are indications that ISAV can cause long-term infection, based on observations in farms with infected fish, and also in experimental settings (Nylund & Jakobsen 1995). Screenings and surveys of ISAV infection in salmon with a clinically healthy appearance would require a highly sensitive detection method as it can be expected that the viral load in such fish would be low.There have been some studies of the early pathological changes in ISA (Falk et al. 1995, Speilberg et al. 1995. However these studies have been based on experiments in which fish have been injected intraperitoneally with ISAV. The initial phase of a primary ISAV infection in Atlantic salmon resulting from a natural route of infection is not well described. The primary target organs of ISAV and the rate with which the infection spreads in a host are not known. However, in the initial phase of a natural ISAV infection, i.e. soon after the introduction of the virus to a farm, the infectious dose that the individual fish would be exposed to and the viral load in the infected fish would presumably be low, and present diagnostic procedures may not be sufficiently sensitive to detect these low levels.To address the problem of a sensitive diagnostic procedure for ISAV, an RT-PCR procedure was optimised and used to monitor the early spread of ISAV within its host after cohabitant infection. MATERIALS AND METHODSExperimental infections. Atlantic salmon presmolts with an average weight of approximately 40 g were used in the experimental infections. The fish originated from a hatchery where ISA had never been diagnosed. During the experiments, the fish were kept in fresh water at 8 to 10°C in 150 l tanks with a water flow of 1 l min -1 . All s...

Correlation of Virus Replication in Tissues with Histologic Lesions in Atlantic Salmon Experimentally Infected with Infectious Salmon Anemia Virus

“…This indicates that the virus has only a single peak viremic period, and that fish that survive this period survive the infection. However, some authors 31 reported a second peak viremic period that resulted in 100% mortality within the same infected group. A bimodal peak of mortality is also believed by some authors 31 to be the result of a second round of infection from the high levels of virus shed by fish that were initially infected.…”

“…However, some authors 31 reported a second peak viremic period that resulted in 100% mortality within the same infected group. A bimodal peak of mortality is also believed by some authors 31 to be the result of a second round of infection from the high levels of virus shed by fish that were initially infected. This is more frequently seen with waterborne or cohabitation challenges.…”

“…13,40,44 Our observations support other findings that used RT-PCR to show that the virus might be present and replicate in the heart earlier than in other organs. 31 We previously reported that the heart shows the most ISH signals when compared with other tissues in fish mortalities due to experimental ISAV infection. 34 This study confirms those observations and further shows that the virus mRNA in the heart increases as the infection progresses, as evidenced by the intense ISH signals until the peak viremia period followed by a decline.…”

Experimental infection models and susceptibility of Atlantic salmon Salmo salar to a Scottish isolate of infectious salmon anaemia virus

“…The PCR method is a more sensitive method than tissue culture for diagnosing ISAV (Devold et al 2000, Mikalsen et al 2001, this issue) and, therefore, was chosen for confirmation of whether mortality could be attributed to ISA. The deaths of the 2 fish that tested negative by RT-PCR could not be attributed to ISAV and were not included in further analysis.…”

Comparative aspects of infectious salmon anemia virus, an orthomyxovirus of fish, to influenza viruses

“…The major target cells for virulent ISAV are endothelial cells lining blood vessels of all organs, including sinusoids, endocardium and endothelial macrophages [44][45][46]. The gills are assumed to be the major port of virus entry, but oral entry cannot be excluded [47,48]. Non-virulent ISAV with a HE of the HPR0 variant is often found to present in the gills and to a lower extent in other organs.…”