Abstract
Salmonid alphavirus (SAV) is the OIE-listed, viral cause of pancreas disease (PD) in farmed Atlantic salmon. SAV is routinely detected by PCR–methods while typical histopathological lesions are additionally used to confirm the diagnosis. Field evaluation of diagnostic test performance is essential to ensure confidence in a test's ability to predict the infection or disease status of a target animal. For most tests used in aquaculture, characteristics like sensitivity (Se) and specificity (Sp) at the analytical level may be known. Few tests are, however, evaluated at the diagnostic level according to the OIE standard. In the present work, we estimated diagnostic test sensitivity (DSe) and diagnostic test specificity (DSp) for five laboratory tests used for SAV detection. As there is no gold standard, the study was designed using Bayesian latent class analysis. Real-time RT-PCR, cell culture, histopathology, virus neutralization test, and immunohistochemistry were compared using samples taken from three different farmed Atlantic salmon populations with different infection status; one population regarded negative, one in an early stage of infection, and one in a later stage of infection. The average fish weight in the three populations was 2.0, 1.6, and 1.5 kg, respectively. The DSe and DSp of real-time RT-PCR is of particular interest due to its common use as a screening tool. The method showed high DSe (≥0.977) and moderate DSp (0.831) in all 3-populations models. The results further suggest that a follow-up test of serum samples in real-time RT-PCR negative populations may be prudent in cases where epidemiological information suggest a high risk of infection and where a false negative result is of high consequence. This study underlines the need to choose a test appropriate for the purpose of the testing. In the case of a weak positive PCR-result, a follow-up test should be conducted to verify the presence of SAV. Cell culture showed high DSe and DSp and may be used to verify viral presence.
Highlights
The ability to reliably detect the presence of important infectious disease agents in aquatic animal populations is essential for such diverse reasons as disease surveillance and international trade
This study evaluated the performance of five different diagnostic tests for detection of SAV infection and histopathological changes consistent with SAV infection in farmed Atlantic salmon, as listed in the chapter on infection with SAV in the Organization for Animal Health (OIE) Manual of Diagnostic Tests for Aquatic Animals [20]
In the current study the tests were applied to three different salmon farming sites, of which Site 1 was known to be free from SAV infection
Summary
The ability to reliably detect the presence of important infectious disease agents in aquatic animal populations is essential for such diverse reasons as disease surveillance and international trade. An important stage in OIE’s test development and validation pathway is the validation of a test’s diagnostic sensitivity (DSe) and diagnostic specificity (DSp) under field conditions [2], using a reference or gold standard test i.e., a diagnostic test which correctly identifies all true positive animals as positive and all true negative animals as negative. The Bayesian approach for estimating diagnostic test accuracy does not require knowledge of true infection status and allows for the incorporation of prior scientific knowledge, such as disease prevalence in study populations, when specifying model parameters. Both prior information and new, collected data can be used to assess diagnostic test performance. The models provide posterior distributions of the parameters of interest e.g., the median and 95% probability interval for test DSe and DSp
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