Othomyxoviridae Documents

ORTHOMYXOVIRIDAE (Influenza Viruses) Members:  Influenza A: infects Humans, other mammals, birds  Influenza B: infects Humans only  Influenza C: infects Humans only *most significant human infections: Influenza A & B Classification and Nomenclature  Antigenic differences exhibited by two of the internal structural proteins (nucleocapsid & matrix) are used to divide the influenza viruses into types A, B, & C  Antigenic Variations in the surfaces glycoproteins (HA & NA) are used to subtype the viruses.  Standard nomenclature includes the following information: o Type o Host origin o Geographic origin o Strain number o Year of isolation  The host origin is only indicated for non-human isolates. Ex. (H1, N1 or Swine flu)  So far, 15 subtypes of HA (H1–H15) and nine subtypes of NA (N1–N9), in many different combinations, have been recovered from birds, animals, or humans.  Four HA (H1–H3, H5) and two NA (N1, N2) subtypes have been recovered from humans. Important Properties:  80 – 120 nm  Usually spherical shape viral molecule  8 different helical nucleocapsid (NP) segments (9 nm) for Influenza A & B); 7 segments for Influenza C & lacking a neuraminidase gene.  Single-stranded RNA, segmented, negative-sense  Proteins: Nine structural proteins, one nonstructural  Envelope: surface glycoproteins exposed as spikes o Hemagglutinin o Neuraminidase  Outstanding characteristics: Genetic Reassortment; causes worldwide epidemics Structural Proteins:  Nucleoprotein (NP) – associates with the viral RNA to form ribonucleoprotein (RNP)  Ribonucleoprotein (RNP) – assumes a helical configuration and forms the viral nucleocapsid  PB1, PB2, PA – bound to RNP; responsible for RNA transcription and replication  Matrix (M1) Protein o forms a shell underneath the viral lipid envelope; o important particle in morphogenesis;

o a major component (40% of viral protein) of the virion.  Hemagglutinin (HA) o binds virus particles to susceptible cells; o the major antigen against protective (neutralizing) antibodies; o variability in HA is responsible for continual evolution of new strains and influenza epidemics; o has the ability to agglutinate erythrocytes.  Neuraminidse (NA) o Removes sialic acid from glycoconjugates. o Facilitates release of virus particles from infected cell (budding). o Helps prevent self-aggregation of virions by removing sialic acid residues. o Possibly helps the virus to negotiate through the mucin layer in the respiratory tract to reach the target epithelial cells.

Antigenic Drift  Minor antigenic changes.  Accumulation of point mutations in the HA & NA gene, resulting amino acid changes.  Produces new virus strains that may not be recognized by the immune system. Antigenic Shift  Complete antigenic changes of the HA, NA or both.  Drastic changes in the sequence of a viral surface protein, changes too extreme to be explained by mutation.  The segmented genomes of influenza viruses reassort readily in doubly infected cells.  Antigenic shift is genetic reassortment between human and avian influenza viruses.  Influenza B & C viruses do not exhibit antigenic shift. Virus Replication 1. After receptor-mediated endocytosis, the viral ribonucleoprotein complexes are released into the cytoplasm and transported to the nucleus, where replication and transcription take place. 2. Messenger RNAs are exported to the cytoplasm for translation. 3. Early viral proteins required for replication and transcription, including nucleoprotein (NP) and a polymerase protein (PB1), are transported back to the nucleus. RNA polymerase activity of the PB1 protein synthesizes positive single-stranded RNA (+ssRNA) from genomic negative single-stranded RNA (– ssRNA) molecules. 4. These +ssRNA templates are copied by the RNA polymerase activity of the PB1 protein. 5. Some of these new genome segments serve as templates for the synthesis of more viral mRNA. Later in the infection, they become progeny genomes. Viral mRNA molecules transcribed from some genome segments encode structural proteins such

as hemagglutinin (HA) and neuraminidase (NA). These messages are translated by endoplasmic reticulum associated ribosomes and delivered to the cell membrane. 6. Viral genome segments are packaged as progeny virions bud from the host cell. Pathologenesis and Pathology  Mode of Transmission: airborne droplets or contact with contaminated hands or surfaces.  Viral Neuraminidase lowers viscosity of the mucus film in the respiratory tract, making bare the cellular surface receptors.  Incubation period: 1-4 days after exposure  Viral shedding: o starts at the day before the onset of symptoms; o peaks within 24 hours o remains elevated for 1-2 days o declines over the next 5 days  Interferon detectable about 1 day after the beginning of viral shedding  Influenza viruses are sensitive to the antiviral effects of interferon, innate immunity response contributes to host recovery  Specific antibody and cell-mediated response are detectable after 1-2 weeks  Influenza infections can cause cellular destruction and desquamation of superficial mucosa of the respiratory tract thus lowering its resistance to secondary bacterial invaders.  Local symptoms: edema and mononuclear infiltrations in response to cell death  Systemic symptoms: production of cytokines.  At risk: o Early adults o Very young children o People with underlying medical conditions Clinical Findings 1. Uncomplicated Influenza (flu) o Central Nervous System: headache o Nasopharynx: runny or stuffy nose, sore throat, aches o Respiratory: coughing o Gastric: vomiting o Mascular: tiredness & aches o Joints: aches o Systemic: fever (usually high) - may be induced by any strain of influenza A or B; influenza C causes common cold illness - children have higher fever and higher incidences of gastrointestinal manifestations (vomiting) - febrile convulsions can occur - otitis media may develop 2. Pneumonia o Usually occur in elderly adults & individuals with underlying chronic disease o Pregnant women are also at risk

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Contributes to the mortality of the influenza epidemic including cardiopulmonary diseases. Can be viral, secondary bacterial or a combination of the two Pneumonia enhances susceptibility of patients to bacterial superinfection due to the loss of ciliary clearance, dysfunction of phagocytic cells and with the help of the alveolar exudate as a good bacterial growth medium Combined viral-bacterial pneumonia is 3 times more common than primary influenza pneumonia. Most common pathogens:  S. aureus  S. pneumoniae  H. influenza

3. Reye Syndrome o An acute encelopathy of children and adloescents (ages 2-16) o Idiopathic o It is a rare complication of influenza B, A and herpesvirus varicella-zoster infections. o Possibly related to salicylate use (aspirin)

Immunity  Resistance to initiation of infection is related to the antibody against hemagglutinin (HA)  Severity of the disease is related to the antibody against neuraminidase (NA)  Local secretory IgA antibodies is important in preventing infection  While serum antibodies persist longer (months to years)  The three types of influenza viruses are antigenically unrelated and cannot induce cross-protection  However, when a viral type undergoes antigenic drift in a person with pre-existing antibody to the original strain may only have a mild infection with the new strain  Cell-mediated immune response is the clearance of an established infection o Cytotoxic T cells lyse infected cells o Cytotoxic T lymphocyte is cross-reactive (able to lyse infected cells with any subtype of the virus) o Cytotoxic T lymphocyte is directed against both internal (NP, M) and surface glycoproteins (NA, HA) Laboratory diagnosis  Specimens: o Nasal washing o Gargles o Throat swabs  Specimen should be obtained within 3 days after onset of symptoms A. Polymerase Chain Reaction (PCR)  Reverse transcription PCR (RT-PCR)  Rapid, sensitive, specific B. Isolation and identification of Virus  Embryonated eggs & primary monkey kidney cells

 Inoculated cell cultures are incubated in the absence of serum (may contain inhibitory factors)  Cell cultures can be tested for the presence of virus by hemagglutination after 57 days  Viral isolates can be identified by hemagglutination inhibition for a rapid determination of the influenza type and subtype C. Serology  ELISA to antibodies formed in response to viral proteins (HA, NA, NP, M)  4 fold or greater increase in titer indicate an influenza infection Epidemiology  Antigenic subtypes of Influenza A: has the ability to cause PANDEMIC o H1N1 – Spanish flu in 1918; Swine flu pandemic in 2009 o H2N2 – Asian flu in 1957 o H3N2 – Hong Kong flu in 1968 o H5N1 – current pandemic threat, 1997, 2003, China, Avian flu o H7N7 – has unusual zoonotic potential o H1N2 – endemic in humans and pigs o H9N2 o H7N2 o H7N3 o H10N7 Treatment & Prevention  M2 ion channel inihibitors (prevents uncoating): Amantadine, Rimantidine  NA inhibitors: Zanamivir & Oseltamivir o *must be administered very early in the disease for maximum effectivity  Primary prevention: Flu vaccine (either live-virus or inactivated) o *however existing vaccines are continually being rendered obsolete as the viruses undergo antigenic drift and shift