ORTHOMYXOVIRIDAE Fahareen-Binta-Mosharraf MNS

What is Orthomyxoviridae? The Orthomyxoviridae are a family of RNA viruses that includes five genera: 1. Influenzavirus A, 2. Influenzavirus B, 3. Influenzavirus C, 4. Isavirus and 5. Thogotovirus. The first three genera contain viruses that cause influenza in vertebrates, including birds,humans, and other mammals.

The Biology of Orthomyxoviruses: Influenza pleomorphic ssRNA consists of 10 genes encoded on 8 separate RNA segments. 3 distinct influenza virus types: A, B, C; Type A causes most infections 11 hemagglutinin antigenic subtypes and 8 neuraminidase antigenic subtypes are designated 3

 Influenza (the flu) is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness, and at times can lead to death.  In virus classification influenza viruses are RNA viruses/ Subfamily: Orthomyxoviridae.  Influenza virus A  Influenza virus B  Influenza virus C Influenza (The flu)

 the most virulent human pathogens among the three influenza types.  capable of infecting human as well as animals (ducks, chickens, pigs, whales, horses and seals). Wild aquatic birds are the natural hosts for a large variety of influenza A.  Influenza A viruses subtypes e.g., (H1N1)….(H5N1) Influenza A virus

Influenza B virus it almost exclusively infects humans. less common than influenza A. Influenza B virus; mutates at a rate 2–3 times lower than type A. This reduced rate of antigenic change, combined with its limited host range ensures that pandemics of influenza B do not occur.

Influenza C virus  infects humans.  Influenza C virus; less common than the other types and usually only causes mild disease in children.

8 TYPE A ++++ yes shift, drift Yes 2 severity of illness animal reservoir human pandemics human epidemics antigenic changes segmented genome surface glycoproteins TYPE B ++ no yes drift Yes 2 TYPE C + no no (sporadic) drift Yes 1

Influenza A Virus Structure  The viral particles of all influenza Viruses are similar in composition. These are made of a viral envelope containing two main types of glycoproteins, wrapped around a central core.  The central core contains the viral RNA genome and other viral proteins that package and protect this RNA.

Structure approximately spherical particles virus particle has a lipid envelope that is derived from the host cell membrane. Three envelope proteins — haemagglutinin (HA), neuraminidase (NA) and an ion channel protein (matrix protein 2- M2) — are embedded in the lipid bilayer of the viral envelope. The ratio of HA to NA molecules in the viral envelope usually ranges from 4:1 to 5:1. Matrix protein 1 (M1), which is the most abundant protein in the virion, underlies the viral envelope and associates with the ribonucleoprotein (RNP) complex The single-stranded, negative-sense RNA genomes of Influenza A and B viruses occur as eight separate segments; influenza C viruses contain seven segments of RNA, lacking a neuraminidase gene.

Inside the M1 inner layer are eight single-stranded RNA molecules are encapsidated with nucleoprotein (NP) and associated with three RNA polymerase proteins — 1. polymerase basic protein 1 (PB1), 2. PB2 and 3. polymerase acidic protein (PA) — to form the RNP complex. The PB1, PB2 and PA proteins are responsible for the transcription and replication of viral RNA. The virus also encodes a non-structural protein (NS/NS1) that is expressed in infected cells and a nuclear export protein (NEP/NS2). The location of NEP in the virion is not known.

Influenza virus proteins and its function

Consequences of segmented RNA Viral RNA has not proved to be infectious. Because of a divided genome, viruses of this group exhibit several biologic phenomena such as 1. high recombination frequency, 2. multiplicity reactivation and 3. ability to synthesize hemagglutinin and neuraminidase after chemical inactivation of viral infectivity.

The Influenza A Genome  The influenza A genome encoding for 11 proteins: hemagglutinin (HA), neuraminidase (NA), nucleoprotein (NP), M1, M2, NS1,NS2 (NEP/nuclear export protein), PA, PB1 (polymerase basic 1), PB1-F2 and PB2.

Hemagglutinin and Neuraminidase Influenza virus particles can be classified by the serotype of two surface proteins, hemagluttinin (HA) and neuraminidase (NA). With 16 types of HA (H1-H16) and 9 of NA (N1-N9) Hemagglutinin (HA) The HA glycoprotein is synthesized as an HA0 molecule that is post-translationally cleaved into HA1 and HA2 subunits; this cleavage is essential for virus infectivity. The HA glycoprotein is responsible for binding of the virus to sialic- acid residues on the host cell surface and for fusion of the viral envelope with the endosomal membrane during virus uncoating Neuraminidase (NA). The NA glycoprotein cleaves sialic-acid receptors from the cell membrane and thereby releases new virions from the cell surface

The Influenza A Subtypes  Type A subtypes of the influenza virus are classified by a naming system that includes:  The place the strain was first found  A lab identification number  The year of discovery  The type of HA and NA it possesses.

H1N1, which caused Spanish flu in 1918, and the 2009 flu pandemic H2N2, which caused Asian Flu in 1957 H3N2, which caused Hong Kong Flu in 1968 H5N1, a current pandemic threat H7N7, which has unusual zoonotic potential H1N2, endemic in humans and pigs H9N2 H7N2 H7N3 H10N7

The Influenza A

Trimmers of HA on influenza virus membrane

Method of Infection and Replication:

1. The viruses bind to a cell through interactions between its hemagglutinin glycoprotein and sialic acid sugars on the surfaces of epithelial cells in the lung and throat 2. The cell imports the virus by endocytosis. In the acidic endosome, part of the haemagglutinin protein fuses the viral envelope with the vacuole's membrane, releasing the viral RNA (vRNA) molecules, accessory proteins and RNA-dependent RNA polymerase into the cytoplasm 3. These proteins and vRNA form a complex that is transported into the cell nucleus, where the RNA- dependent RNA polymerase begins transcribing complementary positive-sense cRNA (Steps 3a and b)

4. The cRNA is either exported into the cytoplasm and translated 5. Newly-synthesised viral proteins are either secreted through the Golgi apparatus onto the cell surface (in the case of neuraminidase and hemagglutinin, step 5b) or transported back into the nucleus to bind vRNA and form new viral genome particles (step 5a) 6. Negative-sense vRNAs that form the genomes of future viruses, RNA-dependent RNA transcriptase, and other viral proteins are assembled into a virion. Hemagglutinin and neuraminidase molecules cluster into a bulge in the cell membrane. The vRNA and viral core proteins leave the nucleus and enter this membrane protrusion

7. The mature virus buds off from the cell in a sphere of host phospholipid membrane, acquiring hemagglutinin and neuraminidase with this membrane coat After the release of new influenza virus, the host cell dies.

Cause of Genetic Reassortment RNA proofreading enzymes are absent, the RNA-dependent RNA transcriptase makes a single nucleotide insertion error roughly every 10 thousand nucleotides, which is the approximate length of the influenza vRNA. Nearly every newly-manufactured influenza virus will contain a mutation in its genome. The separation of the genome into eight separate segments of vRNA allows mixing (reassortment) of the genes if more than one variety of influenza virus has infected the same cell (superinfection).reassortmentsuperinfection The resulting alteration in the genome segments packaged into viral progeny confers new behavior, sometimes the ability to infect new host species or to overcome protective immunity of host populations to its old genome (in which case it is called an antigenic shift)antigenic shift

Antigenic drift A process by which circulating influenza viruses are constantly changing, which allows the virus to cause annual epidemics of illness. Antigenic drift occurs when mutations accumulate in the haemagglutinin and neuraminidase genes that alter the antigenicity of these proteins such that the 'drifted' strains are no longer neutralized by antibodies that were specific for previously circulating strains.

ANTIGENIC DRIFT HA and NA accumulate mutations immune response no longer protects fully sporadic outbreaks, limited epidemics 29

Antigenic shift A process by which a new influenza A virus haemagglutinin subtype (with or without an accompanying new neuraminidase subtype) is introduced into the human population, which lacks prior experience of and immunity to the subtype. Antigenic shift can occur as a result of the direct introduction of an influenza virus from an animal or avian host into humans or by the exchange or reassortment of gene segments between human and non-human influenza viruses when they co-infect animals or humans.

ANTIGENIC SHIFT “new” HA or NA proteins pre-existing antibodies do not protect may get pandemics 31

Importance of Evolutionary Reassortment The evolutionary importance of reassortment is the exchange of RNA segments between mammalian and avian influenza viruses that give rise to pandemic influenza. For example, the 2009 H1N1 pandemic strain is a reassortant of avian, human, and swine influenza viruses, as illustrated.

Influenza Virus Transmission Three ways:  Direct contact with infected individuals;  Contact with contaminated objects (called fomites, such as toys, doorknobs); and  Inhalation of virus-laden aerosols.

Avian Influenza  Avian influenza is an infectious disease of birds caused by type A strains of the influenza virus.  These viruses occur naturally among wild aquatic birds worldwide and can infect domestic poultry and other bird and animal species. The disease, which was first identified in Italy more than 100 years ago.

Avian Influenza  Fifteen subtypes of influenza virus are known to infect birds, thus providing an extensive reservoir of influenza viruses potentially circulating in bird populations.  H5N1; the strain of avian flu known as has been behind outbreaks of deadly avian flu.

Avian Influenza  Avian influenza transmitted by birds usually through feces or saliva.  Avian influenza is not usually passed on to humans, although it has been contracted by people who have handled infected birds or touched surfaces contaminated by the birds.

Swine Flu  Swine influenza (swine flu) is a respiratory disease of pigs caused by type A influenza virus that regularly cause outbreaks of influenza in pigs.  Like human influenza viruses, there are different subtypes and strains of swine influenza viruses. The main swine influenza viruses circulating in U.S. pigs in recent years are: H1N1 influenza virus, H3N2 virus, H1N2 virus.

Swine Flu Symptoms and Signs/In Human  Systemic: fever  Nasopharynx: Runny nose; sore throat  Respiratory: Coughing  Gastric: Nausea; Vomiting  Intestinal: Diarrhea  Psychological: Lethargy; Lack of appetite

Seasonal flu/ Pandemic flu  Epidemic (seasonal) influenza which occurs annually and is attributable to minor changes in genes that encode proteins on the surface of circulating influenza viruses. These are known as interpandemic epidemics.  Pandemic influenza which occurs when more significant changes in the influenza A virus arises when human virus strains acquire genes from influenza viruses of other animal species. When this happens, everyone in the world is susceptible to the new virus, and a worldwide epidemic or pandemic can result.