INTRODUCTION TO INFLUENZA VIRUS
⇒ Influenza virus is an Orthomyxovirus, belongs to family orthomyxoviridae; causes influenza, an Acute Respiratory illness.
⇒ Orthomyxoviruses are spherical or filamentous, enveloped viruses with single-stranded RNA genome, that causes Respiratory illness.
⇒ Orthomyxoviruses have the ability to adsorb onto mucoprotein receptors on RBCs, causing haemagglutination.
MORPHOLOGY OF INFLUENZA VIRUS
⇒ Influenza virus is a Spherical or Pleomorphic or Filamentous, Enveloped virus having a helical nucleocapsid.
⇒ Size – 80 – 120 nm in diameter.
⇒ Genetic material – (-) ssRNA, segmented & exists in eight pieces.
⇒ In the envelope, two types of glycoprotein spikes are present – the Haemagglutinin (HA) and the Neuraminidase (NA).
RESISTANCE SHOWN BY INFLUENZA VIRUS
⇒ Influenza virus is inactivated by heating at 50oC for 30 minutes, remains viable at 0-4oC for about a week, can be preserved for years at -70oC or by freeze drying.
⇒ It is sensitive to Ether, Formaldehyde, Phenol, Salts of Heavy metals and particularly to Iodine.
ANTIGENIC PROPERTIES OF INFLUENZA VIRUS
⇒ The influenza virus has three serotypes type A, type B and type C, of which Influenza A virus causes widespread epidemics & pandemics.
⇒ The characteristic feature of this virus is its ability to undergo antigenic variation due to frequent changes in the antigenicity of HA & NA.
⇒ Hemagglutinin (HA) is a strain-specific glycoprotein composed of two polypeptides – HA1 & HA2, Responsible for the Hemagglutination & Hemadsorption.
⇒ Neuraminidase (NA) is also a strain-specific glycoprotein, present in neuraminidase spikes, facilitates infection.
HOST RANGE & CULTIVATION OF INFLUENZA VIRUS
⇒ The human influenza virus, especially type A influenza, can cause infection in a large number of animal species, due to its high Antigenic variability & adaptability.
⇒ It can grow in the Amniotic cavity, after few egg passages, influenza virus type A & B can grow well in the Allantoic cavity, growth is detected by Hemagglutination test performed with Amniotic & Allantoic fluid.
⇒ It can also be isolated n primary monkey kidney or the human embryonic kidney cell.
PATHOGENICITY OF INFLUENZA VIRUS
⇒ Influenza virus causes influenza, is an acute respiratory disease.
The virus enters the host body via respiratory tract (RT)
Viral neuraminidase facilitates infection by reducing
the viscosity of mucus lining the RT, exposing the
cell surface receptors for virus adsorption.
Ciliated cells of RT are the main sites of viral infection,
damages the cells, laying bare the basal cells in
trachea and bronchi.
This renders the RT highly vulnerable to bacterial invasion.
CLINICAL FEATURES OF INFLUENZA VIRUS INFECTION
⇒ The incubation period of influenza virus varies from 1-4 days.
⇒ The clinical features vary from mild coryza to fulminating & rapidly fatal pneumonia.
⇒ Initial symptoms include fever, headache, and generalized Myalgia.
⇒ The uncomplicated disease usually resolves within 7 days.
⇒ In severe cases, viral pneumonia is seen, associated with –
- Alveolar wall thickening
- Intestinal infiltration with leucocytes
- Capillary thrombosis
- Leucocytic exudation
⇒ It especially affects young children & is characterized by acute degenerative changes in Brain, Liver, and Kidney.
LABORATORY DIAGNOSIS OF INFLUENZA VIRUS
⇒ Specimen: Nasal secretions, Nasal or Throat swab & throat Garglings can be collected using buffered salt solution.
⇒ Demonstration of viral Antigens: Rapid detection of viral antigen can be done by Immunofluorescence; detection of viral RNA by RT PCR is highly sensitive.
⇒ Isolation of the virus: virus isolation is obtained readily during the first 2-3 days of the illness. The specimens should be treated with antibiotics to destroy bacteria. Isolation may be made in eggs or in monkey kidney cell culture.
⇒ Serology: routinely employed serological tests includes – Complement fixation Test (CFT), Hemagglutination Inhibition Test (HAI test) & Radial Immunodiffusion Test.
PROPHYLAXIS OF INFLUENZA VIRUS
⇒ Vaccination is the primary means of prevention of influenza but is quite difficult due to the frequent change in the antigenic stricture of influenza.
⇒ Two types of vaccines are available for influenza virus – Inactivated vaccine-administered Parenterally; Live attenuated vaccine-administered intranasally or by aerosol spray.
⇒ Rimantadine or Amantadine may be given orally as chemoprophylaxis, to unimmunized people during a major epidemic of Influenza A virus.
TREATMENT OF INFLUENZA INFECTION
⇒ Amantadine & the Rimantadine are useful in the treatment of influenza.
⇒ They cause symptomatic improvement but virus shedding and antibody response are unaffected.
⇒ Zanamivir & Oseltamivir, new drugs designed to block viral neuraminidase, have been found effective in the treatment & prevention of influenza when administered as a nasal spray.