An Outline of the Influenza A Virus

Table of Contents

• Influenza A Virus Structure
• Influenza Virus Genome 
• Influenza A Virus Epidemiology 
• Influenza A Viral Replication in the Nucleus
• Pathogenesis of Influenza A Virus
• Influenza A Virus Clinical Manifestations
• Complications of Influenza A Virus
• Laboratory diagnosis of Influenza A Virus
• Treatment of Influenza A Virus
• Prevention of Influenza A Virus
• Types of vaccine of Influenza A Virus
• Control of Influenza A Virus

Influenza A Virus Structure

• Influenza The Orthomyxoviridae family includes viruses.
• Influenza Virus particles are typically cylindrical and range in size from 80 to 120 nanometer.
• It is an enclosed virus, and the envelope is bordered internally by the matrix protein (M1) and externally by the glycoproteins hemagglutinin (HA) and neuraminidase (NA).
• The HA is made up of trimers of identical glycoprotein subunits that each consist of two polypeptide chains, HA1 and HA2, joined by a linkage site that can either be a single basic amino acid, typically arginine, or a string of basic amino acids. The HA is named because the virus agglutinates specific species of erythrocyte. It has a length of about 10 nm.
• The Neuramindase (NA), on the other hand, is a tetramer that makes it easier for virus particles to be released from infected cell surfaces during the budding process and aids in preventing viral virions from self-aggregating by removing sialic acid residues from viral gycloproteins via the sialidase enzyme.
• They have a spiral nucleocapsid made up of eight pieces of negative sense single-stranded RNA.
• Nine distinct structural proteins can be found in influenza viral particles.
• A ribonucleoprotein (RNP) structure with a width of 9 nm is created when the nucleoprotein (NP) and viral RNA bind together to make the viral nucleocapsid.

Influenza virus's structure the infection the influenza antibody reaction The T-cell driven reaction typically targets the comparatively conserved internal proteins such as NP, M1, and PB1, whereas a virus typically targets the surface glycoproteins HA and NA.

• For RNA transcription and replication, three big proteins—PB2, PB1, and PA—are attached to the virus RNP.
• The virion's main structural protein, the matrix (M1) protein, creates a shell beneath the viral lipid membrane and is crucial for particle morphogenesis.
• On the other hand, M2 produces the protein for membrane channels and aids in uncoating.
• The non-structural protein NS is further subdivided into NS1 and NS2, which are each responsible for blocking the translation of cellular mRNA and nucleus exporting virus ribonuclear protein, respectively.

Influenza Virus Genome 

• The influenza DNA A virus has a linear, segmented ssRNA-free DNA that is encapsulated by nucleoprotein (NP).
• There are 8 regions that code for proteins.
• The genomic measure is 13.5Kb, and segments vary in size from 890 to 2,341nt.
• Except for the nonstructural proteins (NS1 and NS2) and the M1 and M2 proteins, which are transcribed from one segment each, each protein is stored on a distinct segment.
• PB2, PB1, PA, HA, NP, NA, matrix proteins (M1 and M2), and non structural proteins make up the eight regions of the DNA (NS1 and NS2).
• The polymerase proteins, PB2, PB1, and PA, are transcription-active and change negative sense mRNA into positive sense.
• The three factors that contribute to viral adhesion are hemagglutination (HA), nucleoprotein (NP), and neuraminidase (NA), which cleaves sialic acid and encourages viral dissemination.
• The matrix proteins M1 and M2 create the membrane channel proteins that facilitate uncoating and the interior layer of the envelope, respectively.
• NS1, a non-structural protein, suppresses RNA splicing and decreases interferon response.
• On the other hand, infectious RNP cannot be exported from the nucleus without NS2.

Influenza A Virus Epidemiology 

• Influenza Based on the epitope differences between the two main membrane glycoproteins, HA and NA, A viruses are divided into subgroups.
• There are currently 18 HA and 11 NA variants identified.
• The various combos have been found in people, animals, and birds.
• Two NA (N1, N2) and four HA (H1, H2, H3, H5) variants have been found in human tissue.
• Birds serve as the virus' main reservoir, infecting people, pigs, horses, seals, mink, and whales among other animals.
• Worldwide, influenza viruses produce yearly epidemics of varying severity.
• Worldwide, seasonal influenza epidemics are thought to result in 3-5 million instances of severe sickness and 250,000–500,000 fatalities each year.
• Because of the mortality linked to infections, influenza A epidemics have a major fiscal effect.
• H1N1 Spanish influenza, H2N2, and H3N2 respectively caused pandemics in 1918, 1957, and 1968, respectively. Most recently, the human population was exposed to H1N1 from pigs (H1N1 2009pdm) in 2009.
• The deadly great pandemic of 1918–1919 was especially intense, murdering 20–40 million people over a short period of time.
• Influenza Smith Andrews and Laidlaw were the first to extract a virus from a patient's washed pharynx in 1933.
• Antigenic changes, which occur when the virus HA or NA (or both) are altered, are linked to major pandemics.
• When an entirely novel RNA section 4 or 6 is acquired, either through reassortment or human infection with an animal virus, antigenic shift occurs.
• There are presently two strains of influenza A, influenza A and influenza A H3N2.
• Regular wintertime epidemics that happen between pandemics are linked to genetic drift in the HA protein.

Influenza A Viral Replication in the Nucleus

• The virus binds to the sialic acid receptor via the HA protein and is endocytosed into the target cell via clathrin-mediated endocytosis.
• The virus ribonucleoprotein complexes are released into the cytoplasm after receptor-mediated endocytosis and transported to the nucleus, where reproduction and transcription occur.
• Messenger RNAs are transported to the cytosol where they are translated.
• Early virus proteins needed for reproduction and transcription, such as nucleoprotein (NP) and polymerase protein (PB), are returned to the nucleus.
• The PB1 protein's RNA polymerase function generates positive single-stranded RNA (ssRNA) from chromosomal negative single-stranded RNA (-ssRNA) molecules.
• The PB1 protein's RNA polymerase action copies these +ssRNA substrates.
• Some of these novel genome regions act as templates for the production of additional viral mRNA.
• The structural proteins hemagglutinin (HA) and neuraminidase are encoded by viral mRNA molecules produced from specific genome regions (NA).
• Endoplasmic reticulum-associated ribosomes translate these signals and transport them to the cell membrane.
• As progeny virions, viral genome fragments are packed and emerge from the host cell.

Influenza A Viral Pathogenesis

A schematic representation of the suggested paradigm for Plasminogen-mediated influenza virus pathogenesis. Plasminogen is transformed into plasmin during IAV infection. On the one hand, plasmin cleaves and triggers viral hemagglutinin, allowing some influenza types to replicate IAV. Plasmin, on the other hand, encourages inflammation through fibrinolysis and increases porosity.

• The influenza virus moves from person to person through droplets in the air or through touch with contaminated palms or surfaces.
• When a person inhales virus-containing microdroplets, they become affected.
• The upper and lower nasal tracts contain sialic acid, to which the HA component of the virus binds.
• If deposited viral particles evade elimination by the cough reflex and prevent neutralisation by preexisting specific immunoglobulin A (IgA) antibodies or inactivation by generic inhibitors in mucous fluids, a few cells of respiratory epithelium become infected.
• Virus reproduction occurs in the nucleus, and progeny virions are quickly created and disseminate to neighbouring cells.
• Viral NA reduces the viscosity of the mucous layer in the respiratory tract, exposing the cellular surface receptors and allowing virus-containing fluid to disseminate to lower sections of the tract.
• Many cells in the respiratory system are infected and ultimately destroyed within a brief period of time.
• Sneezing, fever, chills, muscular pain, headache, and exhaustion are common flu-like symptoms.
• The incubation time between virus exposure and disease start ranges from 1 to 4 days, based on the amount of the viral dose and the host's immune state.
• Viral shedding begins the day before symptoms appear, increases within 24 hours, stays high for 1-2 days, and then begins to diminish over the next 5 days.
• Interferon levels in respiratory secretions are measurable about 1 day after viral shedding starts.
• If the virus spreads to the lower respiratory system, significant desquamation (shedding) of bronchial or alveolar epithelium down to a single-cell basal layer or the basement membrane can occur.
• Viral damage to the respiratory tract epithelium reduces resilience to secondary bacterial intruders, particularly staphylococci, streptococci, and Haemophilus influenzae.
• Infection with influenza causes an inflammatory cell reaction of the mucosal membrane composed mainly of monocytes, lymphocytes, and a few neutrophils.
• There is submucosal swelling.
• Lung tissue examination may show hyaline membrane disease, alveolar fibrosis, and alveolar wall disintegration.
• T-cell reactions are essential in parts of recovery and immunopathogenesis, but antibodies, including vaccine-induced antibodies, have the ability to avoid illness.
• The development of antibodies to HA is mainly linked with protection against reinfection, but antibodies to NA are also protective.
• The antibody reaction is unique to each influenza strain, whereas the cell-mediated immune response is more universal and can respond to influenza strains of the same type.

Influenza A Virus Clinical Manifestations

• The typical incubation time for influenza is one to four days (average: 2 days).
• The start of constitutional and respiratory signs and symptoms is rapid in uncomplicated influenza disease (e.g., fever, myalgia, headache, malaise, nonproductive cough, sore throat, and rhinitis).
• Otitis media, nausea, and vomiting are also typical symptoms of influenza sickness in toddlers.
• The majority of people recover from uncomplicated influenza sickness in 3—7 days, though cough and lethargy can last up to 2 weeks.
• Infections with influenza viruses can induce primary influenza, viral pneumonia, aggravate preexisting medical conditions (e.g., pulmonary or cardiac illness), lead to secondary bacterial pneumonia, sinusitis, or otitis media, and contribute to coinfections with other viral or bacterial pathogens.
• Influenza virus infection has also been linked to dementia, transverse myelitis, myositis, myocarditis, pericarditis, and Reye's syndrome.

The Influenza A Infection and Its Complications

• Tracheobronchitis and bronchiolitis– 

        - When rales and rhonchi are heard but the chest is radiologically clean, a small percentage of patients acquire more serious respiratory symptoms.

• Pneumonia-

        - It is possible to acquire either primary virus pneumonia or secondary bacterial pneumonia.

        - Although primary viral pneumonia is rare, instances have been reported in many influenza epidemics.

        - Primary virus pneumonia is less prevalent than secondary bacterial pneumonia.

• Secondary bacterial pneumonia-

        - It typically happens late in the course of the disease, after the acute disease has shown signs of improvement.

        - The symptoms and indications are consistent with bacterial pneumonia.

        - S. aureus is the most prevalent pathogen, but S. pneumoniae and H. influenzae can also be detected.

        - Influenza infection of cells A requires proteases to cleave the viral haemagglutinin, and some types of aureus generate such enzymes, promoting transmission by damaging the healthy respiratory mucosa.

• Myositis and myoglobinuria-

        - Clinical myositis and myoglobinuria may develop in addition to myalgia, which is typical of acute influenza illness.

• Reye’s syndrome-

        - Encephalopathy and fatty liver deterioration define Reye's syndrome.

        - The illness has a 50% fatality rate among hospitalised patients and has been linked to a number of viruses, including influenza A and B, Coxsackie B5, echovirus, HSV, VZV, CMV, and adenovirus.

• Other complications-

        - Infections with influenza have been linked to severe viral encephalopathy and Guillain-Barre syndrome.

        - The infant mortality syndrome was also linked to influenza A.

Influenza A Viral Diagnosis in the Labs

• Specimen– tracheal aspirate, bronchoalveolar lavage (BAL), sputum, nasopharyngeal aspirate, pharynx biopsy, nose swab.
• Virus isolation-
• Throat swabs, NPA, and nose washings can all be used to isolate viruses.
• The material could be cultured in embryonated eggs or tissue culture.
• For viral isolation, 10-12 day embryonated eggs are used.
• The sample is inserted into the intrauterine canal.
• The virus replicates in the amniotic membrane cells and significant amounts are discharged back into the amniotic fluid.
• Virus in amniotic fluid can be identified after 2-3 days incubation by introducing aliquots of harvested amniotic fluid to chick, guinea pig, or human erythrocytes.
• Pathological specimens can be transplanted onto kidney, chick, or a number of other species tissue cultures, with Rhesus monkey cells being the most responsive.
• Although no CPE is generated, freshly produced virus can be identified by haemadsorption using tissue culture cells and haemagglutination using culture medium containing free virus particles.
• CPE is occasionally produced by influenza A in MDCK (Madin Darby Canine Kidney) cells.
• Serological or molecular techniques can be used to identify influenza viruses separated from embryonated eggs or tissue culture.
• Rapid diagnosis
• Indirect Immunofluoresence can be used to test cells from clinical tissues for the presence of Influenza A protein (IFA).
• There are extremely sensitive and precise enzyme immunoassays (EIA) available for detecting viral antigen.
• In addition, RT-PCR methods for detecting influenza RNA have been created.
• Serology
• The serological test is predicated on the presence of an increase in antibody to the infectious virus.
• The most prevalent technique is the complement fixation test (CFT), which uses type specific soluble antigen, but its specificity is poor.
• In contrast to CFT, the haemagglutination inhibition (HAI) test is more precise. However, for detection, both assays require a 4-fold or higher increase in antibody titre.
• Single Radial Haemolysis (SRH) is a more accurate technique for measuring antibody that is more sensitive than the CFT and HAI tests.

Treatment of Influenza A Virus

• Amantidine and Rimantidine are M2 ion channel inhibitors, which block pH shifts that occur prior to membrane union, which is required for nucleocapsid release.
• Zanamavir is a powerful neuraminidase inhibitor that is inhaled.
• Oseltamavir is a neuraminidase inhibitor that is taken orally.

Prevention of Influenza A Virus

• Immunization is the most efficient way to reduce the severity of influenza.
• Due to the virus's shifting antigenic characteristics, new vaccines are continually needed and should include H and N components from the common strain.

Types of vaccine of Influenza A Virus

• Split virus vaccines
• Split vaccines were created by disrupting inactivated particles with surfactants.
• These vaccines have been shown to have fewer adverse effects in vaccine recipients and to be as immunogenic as whole viral vaccines.

• Live attenuated vaccines
• Normal attenuation techniques, such as multiple passages and temperature adaptation, take a long time to finish and are likely too long for the vaccine to be ready for vaccination against the current influenza strain.
• A single dosage of a trivalent live attenuated influenza vaccine is given intranasally.
• The trivalent vaccine is made up of reassortant for the HA and NA gene portions of the preferred influenza strains, as well as a master donor virus that has been cold adapted to thrive best at 25° C.

• Killed vaccines
• Vaccines are created by growing viruses in embryonated eggs and then chemically inactivating them with formalin or beta propiolactone.
• The amount of HA in each vaccine dosage is uniform (15 g of antigen), but the amount of NA is not because it is more labile under purification and storage circumstances.
• The vaccine is typically prepared in an aqueous or saltwater solution.
• The immunisation is given subcutaneously or intramuscularly.

• Subunit virus vaccines
• Subunit immunisations were created by combining HA and NA proteins.
• These can be used in aqueous suspensions or taken by transporters such as alhydrogel.

Control of Influenza A Virus

• Sheltering domestic livestock to prevent interaction with migrating birds in the sky.
• Getting rid of migratory bird marketplaces.
• Different bird types are separated in marketplaces.
• Separate housing for aquatic fowl and household livestock.
• Domestic livestock afflicted with extremely pathogenic influenza A viruses are being slaughtered.