Lecture 16 - Viruses 2024

Viruses (Chapter 33 – Introduction and Sections 33.1-33.4)

Overview

• By the end of this discussion, you should be able to answer the following questions:

  1. Pathogens and ecological/social factors causing significant historical events:

     • Human population collapse in 16th century Mexico.

     • Influenza pandemic of 1918-1920.

     • Coronavirus-19 pandemic of 2020-2023.

Human Population Collapse in 16th Century Mexico

Key Events and Statistics

• 1520: Smallpox outbreak; ~8 million deaths.

• 1545: Cocoliztli outbreak; 10-12 million deaths.

• 1576: Second Cocoliztli outbreak; ~2 million deaths.

Phase 1 of Epidemic

• Pathogen: Smallpox virus.

• Ecological/Social Causes:

  • New virus introduced by Spanish colonists.

  • High population densities and poverty among indigenous people leading to poor living conditions.

Phase 2 of Epidemic

• Pathogen: Hemorrhagic fever virus.

• Ecological/Social Causes:

  • Cycles of wet years followed by drought years led to spikes in deer mouse populations, increasing virus transmission to humans.

  • Continuing poor living conditions exacerbated exposure to the virus.

Hantavirus Example

• Infection Method: Inhalation; harmful to lungs.

• Ecosystem Dynamics: More rainfall leads to increased vegetation and food resources, supporting larger mouse populations that spread hantavirus.

Influenza Pandemic of 1918-1920

Impact and Statistics

• Death toll estimated between 50-100 million (3-5% of global population).

Pathogen and Causes

• Pathogen: Influenza virus with avian origins, later evolving to infect pigs and then humans, creating a highly virulent strain.

• Ecological/Social Causes:

  • Conditions stemming from World War I: high population density, increased global travel, and psychological stress.

Coronavirus-19 Pandemic of 2020-2023

Impact and Statistics

• Death toll of at least 7 million reported globally.

Pathogen and Causes

• Pathogen: Coronavirus with origins in non-human mammals capable of infecting humans.

• Ecological/Social Causes:

  • High population densities and extensive global travel.

  • Factors of poverty, leading to reduced access to medical care and occupation conditions promoting virus exposure.

Basic Characteristics of Viruses

1. Genetic Information: Stored as DNA or RNA.

2. Capsid Structure: Made of protein.

Virus Reproduction

1. Obligate Intracellular Parasites:

   • Rely on host cellular machinery (ATP, amino acids, ribosomes, enzymes) for reproductive purposes.

Processes of Virus Reproduction

Process 1: Replicative Growth

1. Virus attaches to host cell, injecting genetic material.

2. Viral genome is transcribed; proteins are produced.

3. Viral genome replicates.

4. Particles assemble inside the host.

5. New virions exit to infect additional host cells.

Process 2: Lysogeny

1. Virus inserts its genome into the host genome, remaining dormant as the host divides.

2. Reactivation leads to replicative growth, following similar steps as above.

Importance of Viruses

In Ecosystems

1. Virtually all species are parasitized by viruses.

2. By impacting host populations, viruses promote biodiversity and can drive evolutionary changes.

In Human Welfare

1. Agricultural Impact: Viruses can cause significant disease in livestock affecting industries (e.g. avian influenza).

2. Human Diseases: RNA viruses associated with common illnesses like colds and emerging diseases (e.g. COVID-19); DNA viruses linked to diseases like smallpox and hepatitis.

3. Mechanisms of Impact: Illness from viruses arises through:

   • Host cell energy exploitation.

   • Damage to host cells during reproduction.

   • Severe immune responses (cytokine storms).

Challenges in Eradicating Pathogenic Viruses

1. High Reproductive Rates: Rapid virion production complicates eradication efforts.

2. High Mutation Rates: Particularly in RNA viruses lacking effective error correction mechanisms.

3. Genomic Reassortment: Mechanism leading to new virus strains further complicating eradication efforts.

4. Emerging Viral Diseases: Continuous threat from RNA viruses like Influenza and COVID-19, stemming from non-human species.