Rhabdoviridae to the Rescue?

As noted in my last post, Lepeophtherius salmonis is a serious problem for Atlantic salmon aquaculture and is getting progressively worse. In Norway, use of neurotoxins has led to the development of strains of multi-resistant super lice unaffected by the standard neurotoxins used to keep them under control. Given that other methods such as cleaner wrasse are relatively ineffective this is clearly a cause for concern. Viruses offer an alternative and are thought to have a significant negative impact on lice. However, substantial work is required to develop them as a realistic option. Okland et al. (2014) took the first steps by describing the genomes, target tissues and virion morphology of two (RNA) rhabdoviruses infecting L. salmonis.

Lice were collected from five farming sites in Norway. Sections for T.E.M (transition electron microscopy) were taken from abnormal areas of the cephalothorax in infected lice. This revealed large number of bascilliform or enveloped and rod-shaped virus particles budding from cellular membranes. The glandular tissues the particles were found infesting also showed necrosis and disintegration. Illumina sequencing of the RNA from the infected lice was then carried out to generate the (almost) complete genomes of two rhabdoviruses. These were named Lepeophtherius salmonis rhabdovirus No9 (LSRV-No9) and L. samonis rhabdovirus No127 (LSRV-No127). Their genomes consisted of five structural protein-encoding genes and phylogenies were generated using the sequence for the proteins L and N. These revealed the two viruses were closely related and separate from other rhabdovirus viruses suggesting they were distinct species in their own genus. Interestingly, the N-sequence of No9 sat closely with a sequence also obtained from lice in the Canadian Pacific.

Sequencing of the genomes allowed the development of primers facilitating further investigation of the prevalence of the viruses in lice but also salmon tissue. RT-PCR revealed that all lice developmental stages were positive for the viruses but the highest levels occurred in the adults. In situ hybridisation of the tissue showed the presence of both viral mRNA and DNA in the lice with all lice being positive for at least one virus. However, ovaries and eggs only contained the viral mRNA. In comparison, the viruses were absent or only occurred only at low levels in most fish tissues. In contrast high levels of e viruses occurred at the lice attachment sites. Indicating that the lice maybe injecting the virus into the salmon. The authors postulated the lice could doing this on purpose. By using the viruses in some manner to suppress the rejection of the attached parasite by the host. However, the viruses were unable to replicate in fish cell lines and are genetically distant from rhabdoviruses that can. On balance, it seems more likely the viruses just happens to circulate into the host as it reaches high levels in the lice.

Overall, I found this publication extremely interesting. I was however disappointed they did not discuss the bio-control potential of the viruses further. Clearly, a number of questions need to be asked next. Firstly, how deadly are these viruses? Secondly, how specific are they and do they hit non-target crustacean species? Most importantly, the potential role of the virus as an immunosuppressant must be investigated to rule out any danger to the fish themselves. In conclusion, lice are on the rise and viruses could play an important role in preventing an aquaculture apocalypse.

Reference: Økland, A.L., Nylund, A., Øvergard, A-C., Blindheim, S., Watanabe, K., Grotmol, S., Arnesen, C-E. and Plarre, H. (2014). Genomic Characterisation and Phylogenetic Position of Two New Species in Rhabdoviridae infecting the Parasitic Copepod, Salmon Louse (Lepeophtheirus salmonis). PLOS One, 9(11), e112517.