Higher order taxa
Virus; Bunyaviridae (Family); Hantavirus
Description and Significance
Hantaviruses are responsible for causing a range of clinical manifestations collectively known as haemorrhagic fever with renal syndrome (HFRS). More recently, they are known simply as hantavirus disease (HVD). 10000 year old Chinese medical records show evidence of the prevalence of this disease even back then.
Hantaviruses live in rodents such as rats and mice without causing any symptoms. Urine, faeces or saliva fromt he rodents can transmit the viruses to humans. The five different kinds of hantaviruses discovered so far are found in different geographical regions and in different kinds of rodents.
Hantavirus is the causative agent of two serious diseases: hantaan virus causes HFRS while Sin Nombre, another hantavirus, causes severe cases of Hantavirus Pulmonary Syndrome (HPS). (source :Virology-Online)
The hantavirus genome is monomeric, segmented and consists of three segments of circular, supercoiled, negative-sense and ambisense, single-stranded RNA that forms a non-covalently closed ring. Minor species of non-genomic nucleic acids are lacking in virions. The complete genome is 11800-13800 nucleotides long. About 6500-6550 nucleotide long, the RNA-L is completely sequenced while although RNA-S is sequenced too, only an estimate of its length is available, a complete sequence being 1700 nucleotides long. The genome has terminally redundant sequences that have reiterated terminal sequences. The genome sequence is repeated at both ends. Terminal nucleotides are base-paired, forming non-covalently closed, circular RNAs. Nucleotide sequences at the 3'-terminus are complementary to the 5' end and form a panhandle. The 5'-terminal sequence has conserved regions and repeats complementary to the 3'-terminus; terminal repeats at the 5'-end are 8 nucleotides long; at the 5'-end UAGUAUA...The 5'-terminus has no poly (C) tract. The 3'-terminus has conserved nucleotide sequences, of 11 nucleotides in length; at the 3' end AUCAUCAUCUG...The multipartite genome is found in one type of particle only. Each virion contains a single copy of the genome. (source: ICTV dB Descriptions)
Virion Structure of a Hantavirus
The virions of a hantavirus consist of an envelope and a nucleocapsid. The life cycle of a viron has an extracellular phase that occurs in one phenotype only and is encapsidated during this phase. Inclusion bodies that are not occluded may sequester a virus which typically contains one nucleocapsid. The shape of the virion varies from spherical to pleomorphic and has no protrusions. Virions measure 100-120-270nm in diameter and the virus capsid is enveloped by a single layer envelope. The envelope surrounds three nucleocapsids and has surface projections. Surface projections are distinctive spikes which are surrounded by a prominent fringe embedded in a lipid bilayer that is 5 nm thick. These projections are 5-10 nm long and they produce a grid-like structure. While host ribosomes are not seen inside the envelope, a regular capsid structure is present. The capsid or nucleocapsid is elongated with helical symetry. The ribonucleocapsid is filamentous and 200-300nm long, depending on arrangement, and 2-2.5 nm wide. Nucleocapsids are circular but not segmented.
The virion population is comprised of particles of uniform size and only one species is recovered in preparations. (source: ICTV dB Descriptions)
Reproductive Cycle of a Hantavirus in a Host Cell
During the repication, the L protein, the virus polymerase, copies the genome to form an mRNA that encodes a N protein. A 5' m7G cap is obtained by cannibalizing host cell m RNAs. A (+)sense intermediate is also formed. This intermediate is copied by L protein to form new genomic RNA. The virus buds into Golgi vacuoles and is released when cell lyses. (source: Microbiology@Leicester: Bunyaviruses)
Viral Ecology & Pathology
The hantavirus has been reported within many species of rodents and man worldwide. It has been confirmed that an antigenically-related group of viruses is causing the HVDs that are being reported in different parts of the world. These agents also share similarities in their epidemiological and ecological characteristics. Adult rodents show persistent infection but there are no clinical manifestations and they secrete virus for prolonged periods. A viraemia develops in the rodent after innoculation and it is then that the virus is disseminated throughout the body. The virus is found in the lungs, spleen and kidneys for long periods in the rodent, if not its' whole life. Horizantal transmission between rodents is greatly facilitated by saliva.
It is expected that man gets infected through the respiratory route via the aerosols of virus particles excreted by rodents in their lungs, saliva, urine and faeces. Rodent bites have also led to reported cases of transmission. Horizontal transmission among humans has not been documented even though blood and urine are infectious for the first five days of illness. Although hantaviruses have been reported to have been found in some species of mites, arthropods are not necessary for the transmission of the viruses between rodents and from rodents to man. There are at least 14 subtypes of hantaviruses that have been isolated from humans and rodent hosts and have been typed according to their serological cross-reactivity.
The multisystem pathology of hantavirus disease is characterized by damage to capillaries and small vessel walls, resulting in vasodilation and congestion with hemorrhages. The damage is most severe hypotensive and the oliguric phases as this is when widespread hemorrhages occur. Nephritis is also a possibility because of the involvement of the kidney. There is a limited amount of immune complexes in the kidney and it is thought that immunopathological mechanisms play a less important role in inducing nephritis than viral cellular disease.