Abstract

West Nile virus (WNV) is a single-stranded RNA virus that causes West Nile fever. It is a member of the family Flaviviridae, specifically from the genus Flavivirus which also contain the Zika virus, dengue virus, and the yellow fever virus. The West Nile virus is primarily transmitted through mosquitoes, mostly by the Culex species. However, ticks have been found to carry the virus. The primary hosts of WNV are birds, so that the virus remains within a "bird-mosquito-bird" transmission cycle.

Structure

WNV, like most other flaviviruses, are enveloped viruses, but have icosahedral symmetry. Their envelopes consist of a protein shell and a lipid membrane. The protein shell is made up of two structural proteins: the glycoprotein E and the small membrane protein M. Protein E serve numerous functions, several of which include receptor binding, viral attachment, and entry into the cell through membrane fusion.

The flavivirus lipid membrane has been found to contain cholesterol and phosphatidylserine, but other elements of the membrane have yet to be identified. The lipid membrane has many roles in viral infection including acting as signaling molecules and enhancing entrance into the cell. Cholesterol, in particular, plays an integral part to WNV entering a host cell.

Within the viral envelope, the genome is contained within the capsid. The capsid is one of the first proteins created in an infected cell and has been found that that the capsid prevents apoptosis by affecting the Akt pathway. The capsid is a structural protein and its main purpose is to package RNA into the developing viruses.

Genome

WNV is a single stranded, positive sense RNA virus. Its genome is approximately 11,000 nucleotides long and is flanked by a 5' and 3' non-coding stem loop structures. The coding region of the genome encodes for seven nonstructural (NS) proteins , proteins that aren't incorporated into the structure of new viruses, and three structural proteins, proteins that are a part of the virus structure. The WNV genome is first translated into a polyprotein and later cleaved by virus and host proteases into separate proteins (i.e. NS1, C, E).

Genome | Structural proteins

Structural proteins (C, prM/M, E) are capsid, precursor membrane proteins, and envelope proteins respectively. The structural proteins are located at the 5' end of the genome and are cleaved into mature proteins by proteases.

Genome | Nonstructural proteins

Nonstructural proteins consist of NS1, NS2A, NS2B, NS3, NS4A, NS4B, and NS5. These proteins are mainly in viral replication or act as proteases. The nonstructural proteins are located near the 3' end of the genome.

Life cycle

Once WNV has successfully entered the bloodstream of its host, the envelope protein, E, binds to attachment factors on the host cell, glycosaminoglycans. These attachment factors aid entry into the cell, however, binding to primary receptors are also necessary. Primary receptors include DC-SIGN, DC-SIGN-R, and the integrin αβ. By binding to these primary receptors, WNV enters the cell through clathrin-mediated endocytosis. As a result of endocytosis, WNV enters the cell in an endosome.

The acidity of the endosome catalyzes the fusion of the endosomal and viral membranes, allowing the genome to be released into the cytoplasm. Translation of the positive single stranded RNA occurs at the endoplasmic reticulum; the RNA is translated into a polyprotein which is then cleaved by NS2B-N23, viral proteases, to produce mature proteins.

In order to replicate its DNA, NS5, a RNA polymerase, forms a replication complex with other nonstructural proteins to produce an intermediary negative sense single stranded RNA; the negative sense strand serves as a template for synthesis of the final positive sense RNA. Once the positive sense RNA has been synthesized, the capsid protein, C, encloses the RNA strands into immature virions. The rest of the virus is assembled along the endoplasmic reticulum and through the Golgi apparatus, and results in non-infectious immature virions. The E protein is then glycosylated and prM is cleaved by furin, a host cell protease, into the M protein, thereby producing an infectious mature virion. The mature viruses are then secreted out of the cell.

Phylogeny

Studies of phylogenetic lineages determined WNV emerged as a distinct virus around 1000 years ago. This initial virus developed into two distinct lineages. Lineage 1 and its multiple profiles is the source of the epidemic transmission in Africa and throughout the world. Lineage 2 was considered an African zoonosis. However, in 2008, lineage 2, previously only seen in horses in sub-Saharan Africa and Madagascar, began to appear in horses in Europe, where the first known outbreak affected 18 animals in Hungary in 2008. Lineage 1 West Nile virus was detected in South Africa in 2010 in a mare and her aborted fetus; previously, only lineage 2 West Nile virus had been detected in horses and humans in South Africa. A 2007 fatal case in a killer whale in Texas broadened the known host range of West Nile virus to include cetaceans.

The United States virus was very closely related to a lineage 1 strain found in Israel in 1998. Since the first North American cases in 1999, the virus has been reported throughout the United States, Canada, Mexico, the Caribbean, and Central America. There have been human cases and equine cases, and many birds are infected. The Barbary macaque, "Macaca sylvanus", was the first nonhuman primate to contract WNV. Both the United States and Israeli strains are marked by high mortality rates in infected avian populations; the presence of dead birds—especially Corvidae—can be an early indicator of the arrival of the virus.