Henipavirus


Henipavirus is a genus of RNA viruses in the family Paramyxoviridae, order Mononegavirales containing five established species. Henipaviruses are naturally harboured by pteropid fruit bats and microbats of several species. Henipaviruses are characterised by long genomes and a wide host range. Their recent emergence as zoonotic pathogens capable of causing illness and death in domestic animals and humans is a cause of concern.
In 2009, RNA sequences of three novel viruses in phylogenetic relationship to known henipaviruses were detected in African straw-colored fruit bats in Ghana. The finding of these novel henipaviruses outside Australia and Asia indicates that the region of potential endemicity of henipaviruses may be worldwide. These African henipaviruses are slowly being characterised.

Taxonomy

Virus structure

Henipavirions are pleomorphic, ranging in size from 40 to 600 nm in diameter. They possess a lipid membrane overlying a shell of viral matrix protein. At the core is a single helical strand of genomic RNA tightly bound to N protein and associated with the L and P proteins, which provide RNA polymerase activity during replication.
Embedded within the lipid membrane are spikes of F protein trimers and G protein tetramers. The function of the G protein is to attach the virus to the surface of a host cell via EFNB2, a highly conserved protein present in many mammals. The structure of the attachment glycoprotein has been determined by X-ray crystallography. The F protein fuses the viral membrane with the host cell membrane, releasing the virion contents into the cell. It also causes infected cells to fuse with neighbouring cells to form large, multinucleated syncytia.

Genome structure

As all mononegaviral genomes, Hendra virus and Nipah virus genomes are non-segmented, single-stranded negative-sense RNA. Both genomes are 18.2 kb in length and contain six genes corresponding to six structural proteins.
In common with other members of the Paramyxoviridae family, the number of nucleotides in the henipavirus genome is a multiple of six, consistent with what is known as the 'rule of six'. Deviation from the rule of six, through mutation or incomplete genome synthesis, leads to inefficient viral replication, probably due to structural constraints imposed by the binding between the RNA and the N protein.
Henipaviruses employ an unusual process called RNA editing to generate multiple proteins from a single gene. The specific process in henipaviruses involves the insertion of extra guanosine residues into the P gene mRNA prior to translation. The number of residues added determines whether the P, V or W proteins are synthesised. The functions of the V and W proteins are unknown, but they may be involved in disrupting host antiviral mechanisms.

Causes of emergence

The emergence of henipaviruses parallels the emergence of other zoonotic viruses in recent decades. SARS coronavirus, Australian bat lyssavirus, Menangle virus and probably Ebola virus and Marburg virus are also harbored by bats and are capable of infecting a variety of other species. The emergence of each of these viruses has been linked to an increase in contact between bats and humans, sometimes involving an intermediate domestic animal host. The increased contact is driven both by human encroachment into the bats' territory and by movement of bats towards human populations due to changes in food distribution and loss of habitat.
There is evidence that habitat loss for flying foxes, both in South Asia and Australia as well as encroachment of human dwellings and agriculture into the remaining habitats, is creating greater overlap of human and flying fox distributions.