- •Global Impact
- •Epidemics and Pandemics
- •Current Situation
- •Individual Impact
- •The Virus
- •Requirements for Success
- •Virology
- •Natural Reservoir + Survival
- •Transmission
- •H5N1: Making Progress
- •Individual Management
- •Epidemic Prophylaxis
- •Exposure Prophylaxis
- •Vaccination
- •Antiviral Drugs
- •Epidemic Treatment
- •Pandemic Prophylaxis
- •Pandemic Treatment
- •Global Management
- •Epidemic Management
- •Pandemic Management
- •Containment
- •Drugs
- •Vaccines
- •Distribution
- •Conclusion
- •Golden Links
- •Interviews
- •References
- •Avian Influenza
- •The Viruses
- •Natural hosts
- •Clinical Presentation
- •Pathology
- •LPAI
- •HPAI
- •Differential Diagnosis
- •Laboratory Diagnosis
- •Collection of Specimens
- •Transport of Specimens
- •Diagnostic Cascades
- •Direct Detection of AIV Infections
- •Indirect Detection of AIV Infections
- •Transmission
- •Transmission between Birds
- •Poultry
- •Humans
- •Economic Consequences
- •Control Measures against HPAI
- •Vaccination
- •Pandemic Risk
- •Conclusion
- •References
- •Structure
- •Haemagglutinin
- •Neuraminidase
- •M2 protein
- •Possible function of NS1
- •Possible function of NS2
- •Replication cycle
- •Adsorption of the virus
- •Entry of the virus
- •Uncoating of the virus
- •Synthesis of viral RNA and viral proteins
- •Shedding of the virus and infectivity
- •References
- •Pathogenesis and Immunology
- •Introduction
- •Pathogenesis
- •Viral entry: How does the virion enter the host?
- •Binding to the host cells
- •Where does the primary replication occur?
- •How does the infection spread in the host?
- •What is the initial host response?
- •Cytokines and fever
- •Respiratory symptoms
- •Cytopathic effects
- •Symptoms of H5N1 infections
- •How is influenza transmitted to others?
- •Immunology
- •The humoral immune response
- •The cellular immune response
- •Conclusion
- •References
- •Pandemic Preparedness
- •Introduction
- •Previous Influenza Pandemics
- •H5N1 Pandemic Threat
- •Influenza Pandemic Preparedness
- •Pandemic Phases
- •Inter-Pandemic Period and Pandemic Alert Period
- •Surveillance
- •Implementation of Laboratory Diagnostic Services
- •Vaccines
- •Antiviral Drugs
- •Drug Stockpiling
- •General Measures
- •Seasonal Influenza Vaccination
- •Political Commitment
- •Legal and Ethical Issues
- •Funding
- •Global Strategy for the Progressive Control of Highly Pathogenic Avian Influenza
- •Pandemic Period
- •Surveillance
- •Treatment and Hospitalisation
- •Human Resources: Healthcare Personnel
- •Geographically Targeted Prophylaxis and Social Distancing Measures
- •Tracing of Symptomatic Cases
- •Border Control
- •Hygiene and Disinfection
- •Risk Communication
- •Conclusions
- •References
- •Introduction
- •Vaccine Development
- •History
- •Yearly Vaccine Production
- •Selection of the yearly vaccine strain
- •Processes involved in vaccine manufacture
- •Production capacity
- •Types of Influenza Vaccine
- •Killed vaccines
- •Live vaccines
- •Vaccines and technology in development
- •Efficacy and Effectiveness
- •Side Effects
- •Recommendation for Use
- •Indications
- •Groups to target
- •Guidelines
- •Contraindications
- •Dosage / use
- •Inactivated vaccine
- •Live attenuated vaccine
- •Companies and Products
- •Strategies for Use of a Limited Influenza Vaccine Supply
- •Antigen sparing methods
- •Rationing methods and controversies
- •Pandemic Vaccine
- •Development
- •Mock vaccines
- •Production capacity
- •Transition
- •Solutions
- •Strategies for expediting the development of a pandemic vaccine
- •Enhance vaccine efficacy
- •Controversies
- •Organising
- •The Ideal World – 2025
- •References
- •Useful reading and listening material
- •Audio
- •Online reading sources
- •Sources
- •Laboratory Findings
- •Introduction
- •Laboratory Diagnosis of Human Influenza
- •Appropriate specimen collection
- •Respiratory specimens
- •Blood specimens
- •Clinical role and value of laboratory diagnosis
- •Patient management
- •Surveillance
- •Laboratory Tests
- •Direct methods
- •Immunofluorescence
- •Enzyme immuno assays or Immunochromatography assays
- •Reverse transcription polymerase chain reaction (RT-PCR)
- •Isolation methods
- •Embryonated egg culture
- •Cell culture
- •Laboratory animals
- •Serology
- •Haemagglutination inhibition (HI)
- •Complement fixation (CF)
- •Ezyme immuno assays (EIA)
- •Indirect immunofluorescence
- •Rapid tests
- •Differential diagnosis of flu-like illness
- •Diagnosis of suspected human infection with an avian influenza virus
- •Introduction
- •Specimen collection
- •Virological diagnostic modalities
- •Other laboratory findings
- •New developments and the future of influenza diagnostics
- •Conclusion
- •Useful Internet sources relating to Influenza Diagnosis
- •References
- •Clinical Presentation
- •Uncomplicated Human Influenza
- •Complications of Human Influenza
- •Secondary Bacterial Pneumonia
- •Primary Viral Pneumonia
- •Mixed Viral and Bacterial Pneumonia
- •Exacerbation of Chronic Pulmonary Disease
- •Croup
- •Failure of Recovery
- •Myositis
- •Cardiac Complications
- •Toxic Shock Syndrome
- •Reye’s Syndrome
- •Complications in HIV-infected patients
- •Avian Influenza Virus Infections in Humans
- •Presentation
- •Clinical Course
- •References
- •Treatment and Prophylaxis
- •Introduction
- •Antiviral Drugs
- •Neuraminidase Inhibitors
- •Indications for the Use of Neuraminidase Inhibitors
- •M2 Ion Channel Inhibitors
- •Indications for the Use of M2 Inhibitors
- •Treatment of “Classic” Human Influenza
- •Antiviral Treatment
- •Antiviral Prophylaxis
- •Special Situations
- •Children
- •Impaired Renal Function
- •Impaired Liver Function
- •Seizure Disorders
- •Pregnancy
- •Treatment of Human H5N1 Influenza
- •Transmission Prophylaxis
- •General Infection Control Measures
- •Special Infection Control Measures
- •Contact Tracing
- •Discharge policy
- •Global Pandemic Prophylaxis
- •Conclusion
- •References
- •Drug Profiles
- •Amantadine
- •Pharmacokinetics
- •Toxicity
- •Efficacy
- •Resistance
- •Drug Interactions
- •Recommendations for Use
- •Warnings
- •Summary
- •References
- •Oseltamivir
- •Introduction
- •Structure
- •Pharmacokinetics
- •Toxicity
- •Efficacy
- •Treatment
- •Prophylaxis
- •Selected Patient Populations
- •Efficacy against Avian Influenza H5N1
- •Efficacy against the 1918 Influenza Strain
- •Resistance
- •Drug Interactions
- •Recommendations for Use
- •Summary
- •References
- •Rimantadine
- •Introduction
- •Structure
- •Pharmacokinetics
- •Toxicity
- •Efficacy
- •Treatment
- •Prophylaxis
- •Resistance
- •Drug Interactions
- •Recommendations for Use
- •Adults
- •Children
- •Warnings
- •Summary
- •References
- •Zanamivir
- •Introduction
- •Structure
- •Pharmacokinetics
- •Toxicity
- •Efficacy
- •Treatment
- •Prophylaxis
- •Children
- •Special Situations
- •Avian Influenza Strains
- •Resistance
- •Drug Interactions
- •Recommendations for Use
- •Dosage
- •Summary
- •References
92 Pathogenesis and Immunology
Chapter 4: Pathogenesis and Immunology
Georg Behrens and Matthias Stoll
Introduction
The influenza virus is notoriously known for its unique ability to cause recurrent epidemics and global pandemics during which acute febrile respiratory illness occurs explosively in all age groups. Two qualities of influenza account for much of the epidemiological spread of the virus. First, is the ability to emerge and circulate in avian or porcine reservoirs by either genetic reassortment or direct transmission and subsequently spread to humans at irregular intervals. Second, is the fast and unpredictable antigenic change of important immune targets once the virus has become established in a human.
A highly contagious virus causing extensive morbidity and major case fatality rates is an archetypal anxiety. Influenza has the potential to create such a scenario. The influenza virus, as a pathogenic agent for humans, has been circulating in the human population since at least the sixteenth century (Cox & Kawaoka 1998) leading to recurrent epidemics of febrile respiratory disease every 1 to 3 years. In addition, each century has seen some pandemics rapidly progressing to involve all parts of the world due to emergence of a novel virus to which the overall population holds no immunity. The characteristics of pandemics include occurrence outside the usual season, extremely rapid transmission with concurrent outbreaks throughout the globe, and high attack rates in all age groups with high mortality rates even in healthy young adults. Given the growing world population and international travel and tourism, impending pandemic influenza outbreaks gain the potential to spread even more rapidly. In order to understand the background of this global epidemic threat more thoroughly, this chapter aims to describe both the pathogenesis of the disease and the contest between the virus and the immune system.
Pathogenesis
The pathogenicity and virulence of the influenza virus is determined by several interacting factors:
a)Host factors:
•Presence of target receptors on host cells
•Availability of enzymes in host cells which are essential for viral entry and replication
•State of immunocompetence of the individual host
•Specific immunity against certain viral epitopes in the individual host and target population
•Ability of the immune system to control the viral replication effectively without causing serious collateral damage for the host by its inflammatory response
Pathogenesis 93
b)Viral factors:
•Ability to bind to host cells
•Ability of virus shedding
•Restriction of cytopathogenic effects to allow for an appropriate balance between viral replication and control by the host
•Escape from immunosurveillance by evolution of antigenic variation driven by selective pressure of the immune response
•Escape from immunosurveillance by recombination with different virus strains from zoonotic disease
•Modulation of the immune response to attenuate effective host defense mechanisms
Viral entry: How does the virion enter the host?
The predominant way in which influenza is transmitted is from person to person by aerosols and droplets. Influenza then enters the host through the respiratory tract. In a human lung there are about 300 million terminal sacs, called alveoli, that function in gaseous exchange between inspired air and the blood. The total absorptive area of the human lungs ranges from 80-120 m2. The resting ventilation rate in humans is about 6 liters of air per minute, which introduces large numbers of foreign particles and aerosolized droplets potentially containing virus into the lungs. Deposition of foreign particles depends on their size: inhalation of very small particles does not result in absorption through the alveoli or bronchial system. Small droplets with a diameter of approximately 1 to 4 µm precipitate in the small airways. Much larger particles are either not able to enter the respiratory system or are deposited in the upper respiratory tract (Figure 1A).
Numerous host defense mechanisms including mechanical barriers block respiratory tract infection. The respiratory tract is covered with a mucociliary layer consisting of ciliated cells, mucus-secreting cells and glands (Figure 1 B). Foreign particles in the nasal cavity or upper respiratory tract are trapped in mucus, carried back to the throat, and swallowed. From the lower respiratory tract foreign particles are brought up by the ciliary action of epithelial cells. In the alveoli that lack cilia or mucus, macrophages are responsible for destroying particles (Figure 1).
Binding to the host cells
The main targets of the influenza virus are the columnar epithelial cells of the respiratory tract. These cells may be susceptible to infection if the viral receptor is present and functional. Thus, viral receptors are determinants of tropism. However, this simplified model is often insufficient to explain viral tropism since the receptor distribution in the host is generally more widespread than the observed virus tropism.
94 Pathogenesis and Immunology
Figure 1. Sites of influenza entry in the respiratory tract. (A) The anatomical and functional structures of the human airways are shown. Influenza first infects the upper airway and the ciliated cells in the bronchus and bronchioli. Resulting clinical syndromes include tracheitis, bronchitis, bronchiolitis, and bronchopneumonia. The adaptive immune response is initiated in lymph nodes along the airways. (B) The respiratory epithelia is especially equipped to defend from incoming pathogens by a layer of mucus (bronchus), ciliated cells (bronchus and bronchioli), and alveolar macrophages (alveoli).
In influenza infection, the receptor binding site of viral hemagglutinin (HA) is required for binding to galactose bound sialic acid on the surface of host cells (Weis 1988). Certain areas of the binding site of HA are highly conserved between subtypes of the influenza virus (Daniels 1984). Hosts may prevent the attachment by several mechanisms: (1) specific immune response and secretion of specific IgA antibodies, (2) unspecific mechanisms, such as mucociliary clearance or production of mucoproteins that able to bind to viral hemagglutinin, and (3) genetic diversification of the host receptor (sialic acid), which is highly conserved in the same species, but differs between avian and human receptors (Matrosovich 2000). As a result, the avian virus needs to undergo mutations at the receptor binding site of hemagglutinin to cross the interspecies barrier between birds and humans. In pigs, polymorphisms of sialic acid species and linkage to galactose of both humans and birds are co-expressed in the tissue. Therefore, co-infection with avian and human influenza can occur in pigs and allow genetic reassortment of antigenic properties of both species in the co-infected cells. Recently, it has been shown that certain avian influenza viruses in human and birds are able to bind to different target cells (Matrosovich 2004). This could explain the observation of several cases since the end of the 1990s with transmission of avian influenza directly from poultry to humans. H5N1 and some other subtypes of influenza A virus are able to bind to receptors in the human eye (Olofson 2005).
Pathogenesis 95
As essential as the binding of the influenza virus is its cleavage from the binding site at the host cell. Cleavage is the functional role of viral neuraminidase (Chen 1998). The virulence of the influenza virus depends on the compatibility of neuraminidase with hemagglutinin. A virulent virus which has undergone mutations in the hemagglutinin needs compensatory mutations in the neuraminidase to maintain its virulence (Baigent & McCauley 2003, Hulse 2004). As a consequence, viral fitness and virulence were found to be reduced in influenza viruses resistant to neuraminidase inhibitors (Yen 2005).
Once the cell membrane and the virus have been closely juxtaposed by virusreceptor interaction, the complex is endocytosed. Importing H+ ions into the late endocytic vesicles as a physiologic event then acidifies the interior. Upon acidification, the viral HA undergoes a conformational rearrangement that produces a fusiogenic protein. The loop region of the HA becomes a coiled coil eventually bringing the viral and endosomal membranes closer so that fusion can occur. To allow release of viral RNA into the cytoplasm, the H+ ions in the acidic endosome are pumped into the virion interior by the M2 ion channel. As a result, viral RNA dissociates from M1 by disrupting the low pH-sensitive interaction between the M1 and ribonuclein complex after fusion of the viral and endosomal membranes. The viral RNA is then imported in an ATP-dependent manner into the nucleus for transcription and translation (Flint 2004).
Figure 2: Replication cycle of influenza A virus. Binding and entry of the virus, fusion with endosomal membrane and release of viral RNA, replication within the nucleus, synthesis of structural and envelope proteins, budding and release of virions capable of infecting neighboring epithelial cells (Modified from Cox & Kawaoka 1997)