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Orthomyxoviruses

 اﻟﻤﺮﺣﻠﮫ اﻟﺜﺎﻟﺜﮫ /ﻓﺎﯾﺮوﺳﺎت

د. اﻧﺘﻈﺎر ﻋﻼوي ﺟﻌﻔﺮ / ﻓﺮع اﻻﺣﯿﺎء اﻟﻤﺠﮭﺮﯾﮫ / ﻛﻠﯿﮫ اﻟﻄﺐ / ﺟﺎﻣﻌﮫ ذي ﻗﺎر

PhD. M.Sc. Microbiology

Introduction

These viruses are classified under two families
These are:

• 

Orthomyxoviridae, consisting of influenza viruses.

• 

Paramyxoviridae, consisting of parainfluenza, mumps, measles,
respiratory syncytial.

• 

The genus Orthomyxovirus includes influenza viruses, the causative
agents of worldwide epidemics of influenza.


Influenza Viruses

Influenza viruses belong to the family of Orthomyxoviridae and are the causative

agents  of  influenza,  a  respiratory  disease  in  humans  (Flu)with  well-defined
systemic  symptoms  that  occurs  in  sporadic,  epidemic,  and  pandemic  forms.
Influenza A and B viruses cause substantial morbidity and mortality in humans

and  a  considerable  financial  burden  worldwide,  whereas  influenza  C  viruses
cause sporadic outbreaks of mild respiratory disease, mainly in children.

N.B

v

 

Epidemic refers to an increase, often sudden, in the number of cases of a disease
above what is normally expected in a population within a geographic area

v

 

Pandemic refers to an epidemic that has spread over several countries or
continents, usually affecting a large number of people.

Properties of the Virus

Morphology

Influenza viruses are spherical or filamentous, enveloped particles 80–120 nm in

diameter.
It is composed of a characteristic segmented negative sense, single-stranded
RNA genome, a nucleocapsid, and an envelope (See-Fig).

Source: ViralZone:www.expasy.org/viralzone, SIB Swiss Institute of Bioinformatics)


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The viral genome is a single-stranded antisense RNA. The genome consists of an
RNA-dependent  RNA  polymerase,  which  transcribes  the  negative-polarity

genome into mRNA.

•  The  RNA  genome  is  segmented  and  consists  of  eight  segments  in

Influenza  A  &  B  and  seven  segments  in  influenza  C  viruses.  These
segments code for different proteins, which are NS1, NS2, NP, M1, M2, M3,

HA, and NA.

The  genome  is  present  in  a  helically  symmetric  nucleocapsid  surrounded  by  a
lipid envelope. The envelope has an inner membrane protein layer and an outer

lipid  layer.  The  membrane  proteins  are  known  as  matrix  or  M  protein  and  are
composed of two components M1 and M2.

• 

Two types of spikes or peplomers project from the envelope:

(a) The triangular hemagglutinin (HA) peplomers and
(b) The mushroom-shaped neuraminidase (NA) peplomers.


Classification and Nomenclature

Ø

 

Genus Influenza virus A contains human and animal strains of influenza
type A

Ø

 

Influenza virus B contains human strains of type B,

Ø

 

Influenza virus C contains influenza type C viruses of humans and swine.

Ø

 

Antigenic differences exhibited by two of the internal structural proteins,
the nucleocapsid (NP) and matrix (M) proteins, are used to divide
influenza viruses into types A, B, and C.

Ø

 

Antigenic differences exhibited by two of the internal structural proteins,
the nucleocapsid (NP) and matrix (M) proteins, are used to divide
influenza viruses into types A, B, and C.

Ø

 

Antigenic variations in the surface glycoproteins, HA and NA, are used to
subtype type A viruses

Ø

 

So far, 18 subtypes of HA (H1–H18) , 11 subtypes of NA (N1–N11), in
many different combinations, have been recovered from humans and
animals. e.g of current subtypes of Influenza A viruses H1N1 , H2N3

• 

The standard nomenclature system for influenza virus isolates includes
the following information: type, host of origin, geographic origin, strain
number, and year of isolation.

• 

Antigenic descriptions of the HA and the NA are given in parentheses for
type A. e.g

A/Hong Kong/03/68(H3N2)

Hemagglutinin (HA)

Ø

 

The HA protein of influenza virus binds virus particles to susceptible cells
and  is  the  major  antigen  against  which  neutralizing  (protective)

antibodies are directed.

Ø

 

Variability  in  HA  is  primarily  responsible  for  the  continual  evolution  of
new strains and subsequent influenza epidemics.


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Ø

 

Hemagglutinin  binds  with  the  sialic  acid  cell  receptor,  and  initiates  the
infection in the host cell.

Ø

 

HA  derives  its  name  from  its  ability  to  agglutinate  erythrocytes  under
certain conditions.

Ø

 

The HA agglutinates certain RBC , which is inhibited by the neutralizing
Abs. This forms the basis of the hemagglutination inhibition test used in
the serodiagnosis of influenza.

Ø

 

Hemagglutinin has potency to undergo antigenic variations.

Neuraminidase (NA)

The  NA  is  a  glycoprotein  and  tetramer.  It  consists  of  100  mushroom-shaped

spikes.

v

 

The NA functions at the end of the viral replication cycle. It cleaves the

neuraminic acid and to release progeny virions from the infected host
cells.

v

 

It is a sialidase enzyme that removes sialic acid from glycoconjugates.

v

 

NA facilitates release of virus particles from infected cell surfaces during
the  budding  process  and  helps  prevent  self-aggregation  of  virions  by
removing sialic acid residues from viral glycoproteins.

v

 

It is possible that NA helps the virus negotiate through the mucin layer in
the respiratory tract to reach the target epithelial cells.

Antigenic variations

Antigenic variation is a unique feature of influenza virus.
The surface antigens HA and NA show variations and are primarily responsible
for antigenic variations exhibited by influenza viruses. The internal RNP antigen

and  M  protein  are  stable,  hence  do  not  contribute  to  the  antigenic  variations.
Antigenic variations are of two types: antigenic shift and antigenic drift.


Antigenic shift

²

 

Antigenic  shift:  Occurs  due  to  major  antigenic  changes  in  HA  or  NA
antigens

²

 

Caused by replacement of the gene for HA by one coding for a completely
different amino acid sequence.

²

 

The  antigenic  shift  is  characterized  by  alteration  of  virtually  all  the

antigenic sites of the HA.

²

 

Demonstrated in type A influenza virus only.

²

 

Antigenic  shift  variants  appear  less  frequently,  about  every  10  or  11
years.

• 

Antigenic shift reflects drastic changes in the sequence of a viral surface
protein,  caused  by  genetic  reassortment  between  human,  swine,  and
avian influenza viruses.

• 

The  completely  novel  antigens  that  appear  during  antigenic  shift  are
acquired by genetic reassortment.

• 

Influenza  B  and  C  viruses  do  not  exhibit  antigenic  shift  because  few

related viruses exist in animals.


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Antigenic drift

• 

Minor antigenic changes are termed antigenic drift

• 

Occurs  due  to  minor  antigenic  changes  in  the  HA  or  NA  occurring  at

frequent intervals.

• 

Antigenic  drift  is  caused  by  the  accumulation  of  point  mutations  in  the

gene,  resulting  in  amino  acid  changes  in  the  protein.  Sequence  changes
can  alter  antigenic  sites  on  the  molecule  such  that  a  virion  can  escape
recognition by the host’s immune system.

• 

Antigenic drift variants occur very frequently, virtually every year.

Key Points

• 

Influenza A virus shows maximum antigenic variations.

• 

Influenza  B  virus  does  not  undergo  antigenic  shift  because  influenza  B

virus is the only human virus for which there is no animal source of new
RNA segments. However, influenza B virus undergoes antigenic drift.

• 

Antigenic variation never occurs in type C influenza virus beacuse its lack

of NA.

Gene reassortment

Because  the  influenza  virus  genome  is  segmented,  genetic  reassortment  can
occur when a host cell is infected simultaneously with viruses of two  different

parent  strains. This process of genetic reassortment accounts for the periodic
appearance  of  the  novel  types  of  influenza  A  strains  that  cause  influenza
pandemics.

Influenza viruses of animals, such as aquatic birds, chickens, swine, and horses
show  high  host  specificity.  These  animal  viruses  are  the  source  of  the  RNA

segments  that  encode  the  antigenic  shift  variants  that  cause  epidemics  among
humans.  For  example,  if  a  person  is  infected  simultaneously  by  an  avian  and
human  influenza  strains,  then  it  is  possible  that  a  genetic  reassortment  could

occur in infected cells in humans. The reassortment could lead to emergence of a
new  human  influenza  A  virus,  the  progeny  of  which  will  contain  a  mixture  of
genome segments from the two strains (e.g., a new variant of human influenza A

virus bearing the avian virus HA).


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REPLICATION of Influenza virus

• 

Viral  infection  initiates  with  the  binding  of  a  virion  to  cell  surface

receptors containing sialic acid, followed by the endocytosis of the virion.

• 

After  fusion  of  the  viral  and  endosomal  membranes,  the  viral

ribonucleoproteins  (vRNPs)  are  released  into  the  cytoplasm  and  then
transported into the nucleus.

• 

In the nucleus the viral RNA polymerase transcribes the vRNA segments

into mRNAs.

• 

Viral  mRNA  is  exported  to  the  cytoplasm  for  translation  by  cellular
mechanisms.  The  viral  RNA  polymerase  also  performs  replication  of

vRNA by copying it into complementary RNA (cRNA), which serves as a
template  for  the  production  of  more  vRNA.  Newly  synthesised  viral

polymerase and nucleoprotein are imported into the nucleus and bind to
cRNA and vRNA to assemble vRNPs and cRNPs, respectively.

• 

Following  nuclear  export,  progeny  vRNPs  are  transported  across  the

cytoplasm on recycling to the cell membrane, where assembly of progeny
virions takes place.

• 

Mature virions are released by budding.

  Infleunza A virus replication cycle.

Source:

Aartjan J.W. te Velthuis and Ervin Fodor, 2017.

Pathogenesis and Immunity

Influenza  virus  is  transmitted  from  person  to  person  primarily  in  droplets
released by sneezing and coughing.
Inhaled  influenza  viruses  reach  lower  respiratory  tract,  tracheobronchial  tree,

the primary site of the disease. They attach to sialic acid receptors on epithelial
cells by HA present on the viral envelope. Relatively few viruses are needed to

infect lower respiratory tract than the upper respiratory tract. Neuraminidase of
the viral envelope may act on the N -acetyl neuraminic acid residues in mucus to
produce liquefaction.


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Infection of mucosal cells results in cellular destruction and desquamation of the
superficial mucosa. The resulting edema and mononuclear cell infiltration of the

involved  areas  are  accompanied  by  symptoms  including  nonproductive  cough,
sore throat, and nasal discharge. Although the cough may be striking, the most

prominent symptoms of influenza are systemic: fever, muscle aches, and general
prostration. The virus remains localized to the respiratory tract; hence viremia
does  not  occur.  In  an  uncomplicated  case,  virus  can  be  recovered  from

respiratory secretions for 3–8 days.

Clinical Syndrome

Incubation period is short (1–3 days). The classic influenza syndrome is a febrile
illness  of  sudden  onset,  characterized  by  tracheitis  and  marked  myalgias.

Headache,  chills,  fever,  malaise,  myalgias,  anorexia,  and  sore  throat  appear
suddenly. The body temperature rapidly rises to (38.3–40.0°C) and respiratory

symptoms  ensue.  Nonproductive  cough  is  characteristic.  Sneezing,  rhinorrhea,
and nasal obstruction are common.

Patients  may  also  report  photophobia,  nausea,  vomiting,  diarrhea,  and
abdominal  pain.  They  appear  acutely  ill  and  are  usually  coughing.  Minimal  to
moderate nasal obstruction, nasal discharge, and pharyngeal inflammation may

be present.

Complications

1-Secondary bacterial infections: Life-threatening influenza is often caused by

secondary  bacterial  infections  with  staphylococci,  pneumococci,  and
Haemophilus influenzae. Pneumonia may develop as a complication and may be
fatal, particularly in

(a) Elderly persons above 60 years with underlying chronic disease.
(b) In people chronic cardiorespiratory disease, renal disease, etc.)

(c) Pregnant women.

2-Reye’s  syndrome  is  a  noted  complication  of  influenza  B  infection.  The
condition  is  seen  most  commonly  in  young  children  and  is  associated  with

degenerative changes in the brain, liver, and kidney.
3-  Central  nervous  system  complications:  Guillain–Barre  syndrome
characterized  by  encephalomyelitis  and  polyneuritis  is  a  rare  complication  of

influenza virus infection.

Reservoir, source, and transmission of infection

Infected  humans  are  the  main  reservoir  of  infections  for  influenza  A  virus.

Respiratory secretions of infected persons are the important source of infection.
The virus is excreted in respiratory secretions immediately before the onset of
illness and for 3–4 days thereafter. Wild aquatic birds are known reservoirs of

influenza  A.  They  secrete  the  viruses  in  their  feces,  which  contaminates  ponds
and  lakes.  The  virus  is  spread  from  person-to-person  primarily  by  air-borne
respiratory droplets released during the acts of sneezing and coughing.

Influenza  B  virus  only  causes  epidemics.  Infection  is  from  humans-to-humans.
No animal reservoir hosts are known.


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Laboratory Diagnosis

During  an  epidemic  of  influenza,  the  clinical  diagnosis  can  be  made,  but
definitive diagnosis depends on the laboratory methods.
Specimens:

• 

Nasal or throat washings or sputum for viral antigen and viral RNA.

• 

Throat gargles for isolation of viruses.

• 

Serum for viral antibodies.

Direct antigen detection

Is  made  by  demonstrating  viral  antigens  directly  on  cells  obtained  from  the
nasopharynx. Immunofluorescence (IF) or enzyme-linked immunosorbent assay

using specific monoclonal antibodies are used to detect viral antigen.
The results of the rapid tests are useful to start treatment with the NA inhibitors
within 48 hours of the onset of symptoms.

Detection of antigens by ELISA

Isolation of the virus

Throat  gargles  are  the  specimen  of  choice.  The  specimen  is  collected  in  saline

broth or a buffered salt solution and is sent immediately to the laboratory, or if
delayed is stored at 4°C.
The virus is isolated from the specimen by inoculation into embryonated eggs or

into certain cell cultures.

Treatment

Amantadine and Rimantadine are the specific antiviral agents available for

treatment of influenza. Tamiflu ((Oseltamivir phosphate)

Prevention

This is based on the following:

Immunoprophylaxis by vaccines: Influenza A subtypes H1N1 and H3N2 are most
common predominate human influenza viruses.




رفعت المحاضرة من قبل: Naba Hussein
المشاهدات: لقد قام عضو واحد فقط و 124 زائراً بقراءة هذه المحاضرة








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