Non-Living Infectious Agents

Influenza Virus Structure and Glycoproteins

The influenza virus is an enveloped RNA virus notable for its surface glycoproteins, which play critical roles in infection and immune evasion. Understanding these glycoproteins is essential for grasping the mechanisms of viral entry and release.

• Glycoprotein Spikes: The viral envelope contains two major glycoproteins: Hemagglutinin (H) and Neuraminidase (N).

• Hemagglutinin (H): Exists in 18 subtypes; responsible for binding the virus to host cell receptors, facilitating entry.

• Neuraminidase (N): Exists in 11 subtypes; hydrolyzes sialic acid and assists in viral budding and release from host cells.

• Both glycoproteins frequently undergo genetic changes, reducing the effectiveness of host immune responses.

Example: The H1N1 subtype refers to a virus with hemagglutinin type 1 and neuraminidase type 1.

Influenza Virus Genome and Mutation

The influenza virus genome is segmented and highly mutable, contributing to its epidemiological significance and challenges in vaccine development.

• Genome: Composed of 10 genes encoded on 8 separate RNA strands.

• Antigenic Drift: Gradual accumulation of mutations in viral genes, leading to changes in amino acid composition of glycoproteins. This process causes seasonal variation in influenza strains.

• Antigenic Shift: Abrupt genetic reassortment where one gene or RNA strand is replaced by that from another influenza virus, often from a different animal host. This can result in new, potentially pandemic strains.

Example: The 2009 H1N1 pandemic resulted from antigenic shift involving swine, avian, and human influenza viruses.

Classification and Nomenclature of Influenza Strains

Influenza viruses are classified based on their type, host of origin, geographic location, and year of isolation. The three main types differ in epidemiological and clinical significance.

• Influenza A: Most virulent; infects humans and animals (especially birds). Capable of both antigenic drift and shift, leading to pandemics.

• Influenza B: Infects humans; undergoes only antigenic drift, not shift. Associated with seasonal epidemics but not pandemics.

• Influenza C: Causes only minor respiratory illness; not involved in epidemics.

Example: Influenza A (H5N1) originated in birds and underwent antigenic shift to infect humans.

Pathogenesis and Clinical Features of Influenza A

Influenza A is an acute, highly contagious respiratory illness responsible for seasonal epidemics and pandemics. It is a leading cause of morbidity and mortality, especially among vulnerable populations.

• Transmission: Primarily via respiratory droplets.

• Target Cells: Binds to ciliated cells of the respiratory mucosa.

• Pathology: Causes destruction of epithelial cells and severe inflammation.

• Symptoms: Fever, headache, myalgia (muscle pain), pharyngeal pain, shortness of breath, coughing.

• Complications: Weakened host defenses predispose to secondary bacterial infections, especially pneumonia.

Example: Elderly and young children are at highest risk for severe disease and complications.

Diagnosis, Treatment, and Prevention of Influenza

Accurate diagnosis and effective management are crucial for controlling influenza outbreaks and reducing morbidity.

• Diagnosis:

  • Rapid immunofluorescence tests to detect viral antigens in pharyngeal specimens.

  • Serological testing to screen for antibody titers.

• Treatment:

  • Symptom control with antiviral drugs: amantadine, rimantadine, zanamivir (Relenza), and oseltamivir (Tamiflu).

  • High rates of resistance have developed to amantadine and rimantadine.

• Prevention: Annual vaccination is recommended to reduce risk and severity of infection.

Example: The annual flu vaccine is updated to match circulating strains, primarily targeting hemagglutinin and neuraminidase antigens.

Table: Comparison of Influenza Virus Types

Key Equations

• Antigenic Drift Rate: Mutation rate per nucleotide per replication cycle (varies by virus, but for influenza A, approximately mutations per nucleotide per cycle).

Additional info: The above mutation rate is inferred from virology literature and is not explicitly stated in the provided notes.