Analytical validation of two RT-qPCR tests and detection of spring viremia of carp virus (SVCV) in persistently infected koi Cyprinus carpio

Abstract

Spring viremia of carp virus (SVCV) ia a carp sprivivirus and a member of the genus Sprivivirus within the family Rhabdoviridae. The virus is the etiological agent of spring viremia of carp, a disease of cyprinid species including koi Cyprinus carpio L. and notifiable to the World Organisation for Animal Health. The goal of this study was to explore hypotheses regarding inter-genogroup (Ia to Id) SVCV infection dynamics in juvenile koi and contemporaneously create new reverse-transcription quantitative PCR (RT-qPCR) assays and validate their analytical sensitivity, specificity (ASp) and repeatability for diagnostic detection of SVCV. RT-qPCR diagnostic tests targeting the SVCV nucleoprotein (Q2N) or glycoprotein (Q1G) nucleotides were pan-specific for isolates typed to SVCV genogroups Ia to Id. The Q2N test had broader ASp than Q1G because Q1G did not detect SVCV isolate 20120450 and Q2N displayed occasional detection of pike fry sprivivirus isolate V76. Neither test cross-reacted with other rhabdoviruses, infectious pancreatic necrosis virus or co-localizing cyprinid herpesvirus 3. Both tests were sensitive with observed 50% limits of detection of 3 plasmid copies and high repeatability. Test analysis of koi immersed in SVCV showed that the virus could be detected for at least 167 d following exposure and that titer, prevalence, replicative rate and persistence in koi were correlated significantly with virus virulence. In this context, high virulence SVCV isolates were more prevalent, reached higher titers quicker and persisted in koi for longer periods of time relative to moderate and low virulence isolates.

Isolations of the Spring Viremia of Carp Virus in the Upper Mississippi River (USA), Including a New Host, the Quillback

“…2016), and the virus has been detected by quantitative RT‐PCR in koi survivors for 167 d (Clouthier et al. 2021), collectively indicating the potential for long‐term detection of SVCV from infected fish survivors. Another potential manner whereby SVCV could persist among wild populations, which appears to have received little attention, is for invertebrates to serve as reservoirs and even vectors for this virus.…”

“…The factors that would allow a rhabdovirus like SVCV, which produces acute infections with high levels of mortality that also leaves survivors immune, to persist in populations for extended periods are not well understood (Purcell et al 2012). Experimental SVCV infections of native North American fish demonstrated that survivors could maintain infections with the potential to shed virus for at least 28 d (Emmenegger et al 2016), and the virus has been detected by quantitative RT-PCR in koi survivors for 167 d (Clouthier et al 2021), collectively indicating the potential for long-term detection of SVCV from infected fish survivors. Another potential manner whereby SVCV could persist among wild populations, which appears to have received little attention, is for invertebrates to serve as reservoirs and even vectors for this virus.…”

Assessment of listing and categorisation of animal diseases within the framework of the Animal Health Law (Regulation (EU) 2016/429): Spring Viraemia of Carp (SVC)

“…It is common that not all the fish in a population that is positive for SVCV become infected or show clinical signs of infection, further complicating parameter estimates (Ahne et al., 2002 ; WOAH, 2021 ). The incubation period of SVCV in common carp is between 7 and 14 days (Ahne et al., 2002 ), the virus can be detected using RT‐PCR for at least 167 days post infection in koi; however, the maximum length of the viraemia is unknown (Clouthier et al., 2021b ). It is unclear how factors such as host species or temperature affect the transmission rate and infectious period of SVCV.…”

“…within 30 days (Ahne et al., 2002 ). SVCV can be detected in koi carp for at least 167 days after infection (Clouthier et al., 2021b ). The maximum infectious period is unknown and could potentially be the lifetime of the animal (typically 13 to 20 years in the wild) as in the case for koi herpes virus (EFSA AHAW Panel, 2017b ).…”

“…Parameter 7 -Presence and duration of latent infection and/or carrier status As discussed previously, it is unclear how long the virus can remain latent within the fish that survive infection, but potentially up to the lifetime of the fish (Clouthier et al, 2021b). It is unclear whether the virus can become infectious after latency.…”