Chapter 6: Viruses and Prions

Introduction

This chapter introduces the fundamental terminology and concepts related to viruses and prions. Understanding these terms is essential for studying viral structure, replication, pathogenesis, and the clinical implications of viral infections.

Key Definitions and Concepts

• Virology: The scientific study of viruses and virus-like agents, including their structure, classification, evolution, methods of infection, and interaction with host organisms.

• Bacteriophage: A virus that infects and replicates within bacteria. Also known as a phage.

• Animal virus: A virus that infects animal cells, including those of humans.

• Virion: The complete, infectious form of a virus outside a host cell, consisting of the viral genome enclosed in a protein coat (capsid) and, in some cases, an envelope.

• Capsid: The protein shell that surrounds and protects the viral genome.

• Capsomeres: The individual protein subunits that make up the capsid.

• HA and NA spikes: Surface glycoproteins found on the envelope of influenza viruses. HA (hemagglutinin) facilitates viral entry into host cells, while NA (neuraminidase) assists in viral release from infected cells.

• Attenuated strain: A weakened form of a virus that is less virulent and often used in vaccines to stimulate immunity without causing disease.

• Tropism: The specificity of a virus for a particular host tissue, cell type, or species, determined by the interaction between viral surface proteins and host cell receptors.

• Antigenic shift: A major change in the antigenic structure of a virus, often due to genetic reassortment, leading to new viral subtypes. Common in influenza viruses and can result in pandemics.

• Antigenic drift: Minor, gradual changes in viral antigens due to point mutations during replication. This leads to the emergence of new viral strains and can reduce vaccine effectiveness.

• Pandemic: An epidemic that spreads across multiple countries or continents, affecting a large number of people.

• Host range: The spectrum of host species, tissues, or cell types that a virus can infect.

• Prophage: The latent form of a bacteriophage genome that is integrated into the host bacterial chromosome during lysogeny.

• Phage conversion: The alteration of a bacterial cell's phenotype due to the expression of prophage genes.

• Acute infection: A viral infection characterized by rapid onset of disease, short duration, and often complete recovery or death of the host cell.

• Persistent infection: A viral infection in which the virus remains in the host for long periods, often with ongoing replication or latency.

• Provirus: The viral genome integrated into the DNA of a eukaryotic host cell, especially in retroviruses like HIV.

• Latent infection: A type of persistent infection where the virus remains dormant within the host cell and can reactivate later.

• Episomal: Refers to viral genetic material that exists as an independent DNA molecule in the host cell nucleus, not integrated into the host genome.

• Oncogenic virus: A virus capable of causing cancer by transforming normal cells into cancerous cells. Examples include human papillomavirus (HPV) and Epstein-Barr virus (EBV).

• Antigen: Any substance (often a protein) that is recognized by the immune system and can elicit an immune response.

• Agglutinate: The clumping together of particles, such as cells or viruses, typically due to an antigen-antibody interaction.

• Latex agglutination test: A laboratory test that uses latex beads coated with antibodies or antigens to detect the presence of specific antigens or antibodies through visible clumping (agglutination).

• Seroconversion window: The period between initial infection and the appearance of detectable antibodies in the blood.

• Post exposure prophylaxis: Preventive medical treatment started immediately after exposure to a pathogen (such as HIV or rabies) to prevent infection.

• Nucleoside analog: Synthetic molecules structurally similar to natural nucleosides, used as antiviral drugs to inhibit viral replication by interfering with nucleic acid synthesis. Example: Acyclovir for herpesvirus infections.

• Interferons: A group of signaling proteins produced by host cells in response to viral infection. They help inhibit viral replication and modulate the immune response.

Examples and Applications

• Example of antigenic shift: The emergence of the H1N1 influenza virus in 2009 was due to antigenic shift, resulting in a global pandemic.

• Example of nucleoside analog: Zidovudine (AZT) is a nucleoside analog used in the treatment of HIV infection.

• Application of latex agglutination test: Rapid diagnosis of bacterial infections such as Streptococcus using latex agglutination assays.

Table: Comparison of Acute and Persistent Viral Infections

Additional info:

• Prions are infectious proteins that lack nucleic acids and cause neurodegenerative diseases such as Creutzfeldt-Jakob disease.

• Interferons are classified into Type I (e.g., IFN-α, IFN-β) and Type II (IFN-γ), each with distinct roles in antiviral defense.