| Characteristic: | Orthomyxoviridae: | Paramyxoviridae: |
|---|---|---|
| Representatives: | One genus: Influenzavirus: Influenza virus A, B, C |
Three genera: Paramyxovirus: Parainfluenzavirus 1-4, Senai virus (mouse), mumps, Newcastle disease virus Morbillivirus: Measles, rinderpest, canine distemper Pneumovirus: Respiratory syncytial virus (RSV) |
| Size: | Particle: 80-120nm (highly pleiomorphic) Core diameter: 9nm |
Particle: 125-250nm (somewhat pleiomorphic) Core diameter: 18nm |
| Replication: | Nuclear | Cytoplasmic |
| Genome: | Segmented (-)sense RNA | Non-segmented (-)sense RNA |
It is believed that all (-)sense RNA viruses may have evolved from a common ancestor - all replicate their genomes by a common mechanism; Paramyxoviruses and Rhabdoviruses have non-segmented genomes with similar organization and control of gene expression. Although unlike these other families, Orthomyxovirus genomes are segmented, but a detailed study of one of these types explains much about the others.
| Segment: | Size(nt) | Polypeptide(s) | Function |
|---|---|---|---|
| 1 | 2341 | PB2 | Transcriptase: cap binding |
| 2 | 2341 | PB1 | Transcriptase: elongation |
| 3 | 2233 | PA | Transcriptase: protease activity (?) |
| 4 | 1778 | HA | Haemagglutinin |
| 5 | 1565 | NP | Nucleoprotein: RNA binding; part of transcriptase complex; nuclear/cytoplasmic transport of vRNA |
| 6 | 1413 | NA | Neuraminidase: release of virus |
| 7 | 1027 | M1 | Matrix protein: major component of virion |
| M2 | Integral membrane protein - ion channel | ||
| 8 | 890 | NS1 | Non-structural: nucleus; effects on cellular RNA transport, splicing, translation |
| NS2 | Non-structural: nucleus+cytoplasm, function unknown |
i) During the initial phase of infection (~2h), active host cell DNA synthesis is required and replication is prevented by U.V, mitomycin C, etc, but not thereafter. The reason is that the initial step in replication is that PB2 attaches to the m7G cap of host mRNAs. This structure is cleaved from the mRNA by PB1, remaining attached to PB2. The cap serves as a primer for RNA synthesis and 11-15 nucleotides (complementary to the conserved sequence at the 3' end of the vRNA) are added by PB1, after which PB2 dissociates from the growing strand (these structures can be isolated from infected cells). PB1 + PA then complete the synthesis of the (+)sense strand.
ii) Two classes of (+)sense RNA are made in infected cells:
Most of the proteins made (e.g. HA, NA) remain in the cytoplasm or become associated with the cell membrane. However, the NP protein migrates back into the nucleus, where it associates with newly-synthesized vRNA to form new nucleocapsids. These migrate back out into the cytoplasm and towards the cell membrane (mechanism unclear). The level of free nucleoprotein (NP) is thought to control whether mRNA or cRNA is produced, i.e. later in infection there is lots of NP, mRNA synthesis stops but cRNA synthesis continues. NP is thus a crucial switch in the replication cycle between expression and assembly.
~4h after infection, patches of M1 protein form on the cell membrane, which appears to thicken, incorporating HA and NA on the outside of the membrane. The nucleocapsid segments are incorporated into the particle as it buds out through the membrane. NA is thought to have a role in release of budding particles (inhibited by anti-NA Abs).
The packaging mechanism responsible for sorting 8 distinct genome segments into each particle is not known. However, it has recently been found that there >8 RNA segments/particle - the exact number is not known and may vary.
Virus particles are gradually released from the surface of the cell over a period of several hours. The cell does not lyse, but eventually dies (due to disturbance of normal cellular macromolecular synthesis?).
Pathogenesis:
Spread is by aerosols - very efficient (occasionally fomites). Even in epidemics, there are 3:1 - 9:1 infections:clinical case - very infectious. Primary infection involves the cilliated epithelial cells of the U.R.T. Necrosis of these cells results in the usual symptoms of the acute respiratory infection (fever, chills, muscular aching. headache, prostration, anorexia). Normally self-limited infection usually lasts 3-7 days (+convalescence). Death from primary Influenza infection is very rare and appears to be determined by host factors rather than 'virulence' of virus. Damage to respiratory epithelium predisposes to secondary bacterial infections which accounts for most deaths.
An extra enzyme helps make some strains of the flu virus especially deadly, researchers reported on Monday. They said knowing about the enzyme might help doctors find people infected with the most dangerous strains, and might provide a target for drugs. Yoshihiro Kawaoka and colleagues at the University of Wisconsin said the most dangerous strain of influenza A makes use of an extra protein that helps it infect cells throughout the body. The influenza A virus has two surface proteins -- hemagglutinin (HA) and neuraminidase (NA). To infect cells, the HA protein has to be cut into two pieces by enzymes known as proteases. The proteases that can do this are normally common in the lungs and throat but not elsewhere -- which is why flu usually is just a respiratory illness. Writing in the Proceedings of the National Academy of Sciences, Kawaoka's team said the worst strain of flu uses a different enzyme, plasmin, to help chop up HA. Plasmin is common throughout the body, meaning this strain can infect all sorts of tissues. They looked at a virus descended from the strain that caused the 1918 pandemic of flu, blamed for killing more than 20 million people around the world. "This is a mechanism that we never knew existed in human viruses," Kawaoka said in a statement. His group tested 10 other strains of flu and did not find the same enzyme being used. He said perhaps doctors could look for this enzyme in a flu outbreak to determine how likely it is to be dangerous. "Now we have additional markers that we can look for when a peculiar outbreak of human or animal flu occurs," he said. "Their findings point us in a direction to better understand the pathology of these more virulent influenza viruses," Dominick Iacuzio, programme officer for influenza at the National Institute for Allergies and Infectious Diseases (NIAID), which funded the study, said. |
 A novel mechanism for the acquisition of virulence by a human influenza A virus.Hideo Goto & Yoshihiro Kawaoka. PNAS USA 95: 10224-10228, 1998: Cleavage of the hemagglutinin (HA) molecule by proteases is a prerequisite for the infectivity of influenza A viruses. Here, we describe a novel mechanism of HA cleavage for a descendant of the 1918 pandemic strain of human influenza virus. We demonstrate that neuraminidase, the second major protein on the virion surface, binds and sequesters plasminogen, leading to higher local concentrations of this ubiquitous protease precursor and thus to increased cleavage of the HA. The structural basis of this unusual function of the neuraminidase molecule appears to be the presence of a carboxyl-terminal lysine and the absence of an oligosaccharide side chain at position 146 (N2 numbering). These findings suggest a means by which influenza A viruses, and perhaps other viruses as well, could become highly pathogenic in humans. |
Due to the important role played by neuraminidase in transmission of the virus throughout the host, it has recently been seen as a prospective target for anti-influenza drugs. The active site of neuraminidase is made up of 11 universally conserved amino acid residues.
Synthetic sialic acid analogues work well against neuraminidase at lower levels than amantadine and rimantadine, but need to be given intranasally, and are more effective as a prophylactics, with little effect after infection.
GS4071 is a novel orally administered inhibitor of neuraminidase called which has been shown to confer decreased severity and duration of symptoms in clinical trials.
Vaccines: Isolated HA gives good serological protection (estimated at 60-80%). Vaccines are produced by reassortment of egg-adapted strains with strains with the required HA type. Large amounts of virus are then grown in embryonated eggs (cheap and efficient), purified and formalin inactivated. Vaccine is given sub-cutaneously: if of a new antigenic type, 2 doses are necessary for adequate protection (i.e. depends on age of patient).
The Snag: In order to give time for adequate vaccine stocks to be produced, a decision must be made, usually in about August, as to which HA type to use for this years vaccine (for the winter season). There is an elaborate and sophisticated epidemiological monitoring system worldwide, which helps these decisions:
STOP PRESS:
Killer 'flu is coming - or is it ???
In May 1997, a 3-year-old boy died of complications of influenza in the intensive care unit of a Hong Kong hospital. Note that the child did not die from influenza directly, but rather from Reye's syndrome, a rare neurological complication of certain virus infections caused when young people with fevers take aspirin. This case was the first isolation of an influenza type-A subtype H5N1 in a human. Subtype H5 influenza viruses can cause lethal avian influenza, a disease that may decimate flocks of domestic poultry. It is not known how the boy became infected with the virus, however there had been an avian flu epidemic in the region a month earlier.
Dr. Robert G.Webster, Chairman of the Department of Virology and Molecular Biology at St. Jude Children's Research Hospital, Memphis, Tennessee, said:
"Predictions we've been making, is that there will be another pandemic - probably before the end of the century. We've been preparing pandemic manuals, what to do when the pandemic comes. And this is the first example of what could develop into it. We don't think it will. But how well have we performed our duties? If we go back and rate ourselves, we haven't done all that well, because the virus occurred in Hong Kong in May, and it was months before we realized it was there, so we have to get our act together. We will learn from this how to perform better when the real one comes ... You've got a very large, virgin population out there - large enough to start a pandemic."
Is flu on the warpath once again?
Probably not yet, says Nancy Arden, a CDC epidemiologist who's followed the Hong Kong incident. Scientists have not found H5N1 in more than 4,000 throat samples taken from Chinese respiratory-disease patients since the outbreak. "It appears not to have been spreading among humans", she says. But she notes that "this is the first time that a purely avian strain of influenza has been found to cause the flu in humans, and that's why there was such an intensive investigation." Viruses can't cause pandemics unless they have two qualities, Arden notes. First, they must be able to infect humans, as H5N1 apparently can. Second, they must be able to spread easily among the human population. For unknown reasons, H5N1 appears to lack this vital talent.
But could H5N1 mutate so that it can spread rapidly through the human population?
To date (June 1998) H5N1 has been linked to the deaths of six people including two children, and a dozen others have been infected.