Influenza Virus

Introduction

The orthomyxoviridae (influenza virus) are a major determinant of morbidity and mortality caused by respiratory disease and outbreaks of infection sometimes occur in worldwide epidemic influenza virus has been responsible for millions of death worldwide.

Three immunologic types of influenza virus are known that is type A, B and C. influenza type A is antigenically highly variable and is responsible for most cases or epidemic influenza. Influenza type B may exhibit antigenic changes and sometimes causes an epidemic. Influenza type C is antigenically stable and causes only mild illness in immunocompromised individuals.

Microbiology of influenza virus

Morphology

Shape and size

The virus is a spherical particle with a diameter of 80-120 nm and with helical nucleocapsids, although virion may display great variation in size.

Envelope

A lipid envelopes derived from the cell surround the virus articles. Two virus-encoded glycoproteins i.e. hemagglutinin (HA) and the neuraminidase (NA) are inserted into the envelope and are exposed as spikes (about 10nm long) on the surface of the particles and is internally lined by matrix (M) proteins. These two surface glycoproteins are the important antigens that determine an antigenic variation of influenza virus and host immunity. The HA represent about 25% of the viral proteins and the NA about 5%. The hemagglutinin is so named because the viral hemagglutinin attacks to specific receptors at certain species of erythrocytes and cause agglutination. The HA proteins of influenza virus bind particles to susceptible host cells are the major antigen against which antibodies are directed.

The NA functions at the end of the viral replication cycle. It facilitates at the end of the viral replication cycle. It facilitates a release of the virus [articles from infected cell surface during the budding process.

Genome

The single stranded negative sense RNA genome of influenza A and B virus occur at 8 separate segments and the influenza C viruses contain 7 segments of RNA. Because of the segmented nature of the genome when the cell is co-infected by two different viruses of a given type, mixtures of parental gene segments may be assembled into progeny virion. This phenomenon called genetic reassortment may result in sudden changes sudden changes in the viral surface antigen.Most of segment code for a single proteins. The complete nucleotide sequence is known for many infectious viruses. The first 12-13 nucleotides at each end of viral genomic segment are conserved among all 8 RNA segment. These sequences are important in viral transcription.

Antigenic drift and antigenic shift

Influenza viruses are remarkable because of the frequent antigenic changes that occur in hemagglutinin and neuraminidase. The variation is highest in type A, low in type B, none in type C. all isolates from influenza virus from non-human (i.e. birds and animals) belongs to be exclusively human viruses except one report from china where type C virus was isolated from pig.

Antigenic drift

It refers to the major antigenic changes in either the hemagglutinin or neuraminidase or both. This drift results from the point mutation in the HA and NA genes. Periodic epidemic (every 2-3 years) of influenza is associated with antigenic drift causing lical outbreaks of influenza A and B infection.

Antigenic shift

It refers to the major antigenic changes in HA and NA or both as a result of the reassortment of the 8 nucleic acid segments of genome between different strains including animal and bird strains but thus occurs only with influenza A virus. Such changes are often associated with pandemic. With a major antigenic shift, the new virus infect a primarily susceptible population, one that has never been exposed to that particle strains of influenza A virus has occurred several times through infrequently taking place on an average every 10 years. 3 trains have emerged as a cuase of pandemic in the last 30 years.

H1N1—Swine flu

H2N2—Asian flu

H3N2—Honkong flu

Pathogenesis

Influenza virus spread from person to person by air borne droplets or by contact with contaminated hands and surfaces. When virus is introduced into the respiratory tract by aerosols or by contact with infected individuals, it attaches to and replicates in epithelial cells or respiratory tract. The virus replicates in the cells of both upper and lower respiratory tract. Virus replication leads to destruction and loss of cells lining the respiratory tract. As infection subsides, the epithelium is regenerated; a process that can take up to a mouth cough and weakness may persist for up to two weeks after infection. Viral damage to the respiratory tract epithelium lowers its resistance to secondary bacterial invaders specially Staphylococci, Streptococci and Haemophilus influenzae.

Clinical disease

Incubation period varies from one day to 4 days depending on the size of viral dose and immune status of the host.

Although influenza type A and B causes virtually an identical disease, infections due to B tend to be milder than A. typical symptoms include:

• Fever, chills, headache, dry cough, generalized mylgia (pan in muscles), malaise and anorexia.
• Fever last for about 3 days and respiratory symptoms last for another 3-4 days.
• Normally, influenza is a self limiting disease. In about 10% cases, there occurs secondary pneumonia due to Staphylococci, Streptococci and Haemophilus influenzae.
• Occasionally extra pulmonary lesions such as Gullian-Barre Syndrome (ascending mycelitis) and Raye’s syndrome (encephalopathy) may develop.

Lab diagnosis

Diagnosis of influenza relies on identification of viral antigens or antibody or viral /nucleic acid in specimen and the isolation of virus.

Specimen

• Nasal washings, gargles and throat swabs are the best specimen for diagnostic testing and should be obtained within 3 days after the onset of symptoms.

Isolation of virus

• Specimen is inoculated onto primary monkey kidney or embryo kidney cell, baboon kidney cells etc. Non specific cytopathological effects (CPE) may be observed within two days.
• Hemagglutination results following the addition of erythrocytes to media containing influenza virus.

Serology

• It is essential to examined paired i.e. acute and convalescent sera to demonstrate a 4 fold or greater increase of antibody titre. Since, normal individual already may have influenza virus antibody. Antibody may be detected by ELISA.
• Viral antigens in clinical specimens can be detected by immunofluorescent technique and ELISA.

PCR

• Rapid test based on detection of influenza RNA in clinical specimens using reverse transcription polymerase chain reaction (RT-PCR) are preferred for diagnosis of influenza, RT-PCR is rapid, sensitive and specific.

Types of Influenza virus

There are three types of influenza viruses:

1. Influenza virus A,
2. Influenza virus B and
3. Influenza virus C.

• Human influenza A and B viruses cause seasonal epidemics of disease.
• The emergence of a very different and new influenza virus to infect people can cause an influenza pandemic.
• An influenza type C infection is not thought to cause epidemics and causes a mild respiratory illness.
• Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus i.e. the hemagglutinin (HA) and the neuraminidase (NA).
• There are 11 different neuraminidase subtypes and18 different hemagglutinin subtypes.
• Influenza A viruses can be further broken down into different strains. Current subtypes of influenza A viruses found in people are influenza A (H3N2) and influenza A (H1N1) viruses.
• Influenza B viruses are not divided into subtypes, but can be further broken down into lineages and strains.

REFERENCE

Cheesbrough, M.Medical Laboratory Manual for Tropical Countries. Vol 2. ELBS London, 2007.

Tille, P.Diagnostic Microbiology.13th. Elsevier, 2014.

D Grenwood, Slack RCB, and Peutherer J.Medical Microbiology.Dunclude Livingstone: ELBS, 2001.