Higher order taxa
Virus; ssRNA negative-strand viruses (group v); Mononegavirales
Families; Bornaviridae, Filoviridae, Paramyxoviridae, Rhabdoviridae
Description and Significance
The Midway virus (MIDWV) was first isolated in 1966 from seabird ticks of two species Ornithodoros and Alectorobius, collected on the Midway, Kure, and Manana islands in the Central Pacific and from northern Honshu, Japan(1). Concurrently, on Aomatsushima Island, nestling seabirds of two seabird species, Larus crassirostris and Nycticorax, were found to make antibodies to MIDWV(1). The virus is defined as being cytopathic, which causes degenerative changes in cells. The current efforts to classify MIDWV have produced futile results. A number of important human diseases including; mumps, measles, rabies, bonra disease, and ebola haemorrhagic fever can be linked to the order of Mononegavirales. The creation of the genus Nyavirus was suggested in 2009 for two viruses, Nyamanini virus (NYMV) and Midway virus (MIDWV). According to the suggestion, both viruses would be assigned to species of the same name (Nyamanini virus and Midway virus, respectively) that would constitute the genus. No formal proposal for the creation of the genus or the two species has yet been forwarded to the International Committee on Taxonomy of Viruses (ICTV)(1).
The Midway virus genome is non-segmented, and contains six open reading frames with only two open reading frames similar to known viral protiens. It is a negative-stranded-RNA in the order of Mononegavirales. The genome size is roughly 11.8 kb. Since the Midway virus was found to be highly divergent from known viruses, it has been classified as being a virus of the novel genus, Nyavirus.
Midway virions are spherical and measured to be 100-130 nm in diameter.(4) The virus forms enveloped virions, which contain an RNA-dependent RNA polymerase (RDRP) and genomic RNA with a process known as RNA editing to allow for greater number of gene products. (source: ICTVdB)
Reproductive Cycle in a Host Cell
The Midway virions enter a cell by binding to a cell receptor and inducing fusion between the viral envelope and the cell membrane. Since the genomic sequence is negative sense, it does not code for proteins. This forces complementary sequences to be first transcribed by an RNA-dependent RNA polymerase (RDRP)(2). The RDRP released from the viral particle binds to a promoter site at 3’ end of the genome and begins transcription to produce a positive-sense RNA strand that can serve directly as mRNA to code for protiens (2). The RDRP will delay in between each gene, thus releasing the completed mRNA. Transcription can then either terminate or continue transcribing the next gene. A polarity of transcription is formed, where genes close to 3’ end of the genome are transcribed in greatest abundance, while the 5’ end are less likely to be transcribed. The midway virus is able to use this polarity as a form of transcriptional regulation(4). Once mRNA production has begun, the cellular protein translation system is stimulated to produce viral proteins, which aggregate in the cytoplasm. At some point, the process is not fully understood; the RDRP molecule transcribing the viral genome produces full-length, positive-strand anti-genomes, which are used in the transcription into negative strand viral genome copies. The newly synthesized viral genomes aggregate near the inside of the cell membrane, and the virions bud off from the cell, removing a piece of the cell membrane as the virus exits. The new viral particle infects another cell to repeat the cycle(3). The evolutionary aspect of the Midway virus allows for adaptation at a rapid rate due to the absence of proof-reading ability in the RDRP enzyme.
Viral Ecology & Pathology
The natural reservoirs for Midway viruses are unknown. Furthermore, it is currently unknown if the Midway virus can be transmitted to humans via animals. However, it is conceivable that transmission could occur due to the similarities between the genomic and virion structures of other known viruses within the Mononegavirales, family. In a study, the virus was found to be pathogenic to newborn Swiss mice, but not to four-week old Swiss mice injected intracranially(1). Since the virus was first isolated from ticks that had infected seabirds via direct transmission, it is possible that the parasitic seabird tick, Ornithodoros, is a prominent carrier of the virus. Currently no outbreaks among humans have been reported from the Midway Virus(5).
1 Bock, J. O., T. Lundsgaard, P. A. Pedersen, and L. S. Christensen. 2004. Identification and partial characterization of Taastrup virus: a newly identified member species of the Mononegavirales. Virology 31949-59. [PubMed]
2 Mihindukulasuriya, Kathie A., Nang Nguyen, Guang Wu, Henry V Huang, Vsevolod Popov, and David Wang. "Nyamanini and Midway Viruses Define a Novel Taxon of RNA Viruses in the Order Mononegavirales[down-pointing Small Open Triangle] †." Journal of Virology 83.10 (2009): 5109-116. US National Library of Medicine National Institutes of Health. Web. 2 Feb. 2013. <http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2682064/>.
3 "Http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=644609." Midway Virus (n.d.): n. pag. Web. 13 Mar. 2013.
4 Wang, D., A. Urisman, Y. T. Liu, M. Springer, T. G. Ksiazek, D. D. Erdman, E. R. Mardis, M. Hickenbotham, V. Magrini, J. Eldred, J. P. Latreille, R. K. Wilson, D. Ganem, and J. L. DeRisi. 2003. Viral discovery and sequence recovery using DNA microarrays. PLoS Biol. 1:E2.Medline
5Center for Disease Control. N.p., n.d. Web. 13 May 2013. <http://wwwn.cdc.gov/arbocat/catalog-listing.asp?VirusID=340&SI=5>.