Emerging Viral Infections

Emerging Viral Infections

Overview of Emerging Viral Infections

  • Emerging infections are infections that:
    • Newly appeared,
    • Appeared previously but expanding in incidence and geographic range,
    • Threatened to increase in the near future.

Concepts of Enzootic and Epizootic Infections

  • Enzootic Infections:
    • Definition: Refers to a pathogen present in an animal community at all times but affecting only small numbers of animals.
    • Environmental conditions can shift the situation to epizootic proportions.
  • Epizootic Infections:
    • Definition: Affecting many animals in a region at the same time.
  • Host Switching: The emergence of viruses and other human diseases occurs when an established animal virus switches to humans and is then transmitted within human populations.
  • Analogy: Transfer between different animal hosts leads to analogous emergence of epizootic diseases.

Threats to Public Health from Newly Emerging Viral Diseases

  • Human Health Threats:
    • Newly emerging viral diseases pose significant threats.
    • Viruses from wildlife hosts caused high-impact diseases including:
    • Severe acute respiratory syndrome (SARS),
    • Ebola fever,
    • West Nile virus,
    • Yellow fever,
    • Influenza virus in humans.
  • Additional Information:
    • Syphilis is an emerging bacterial pathogen.

Factors Influencing Pathogen Spread

  • Human Modification of Nature:
    • Major factor in the spread of viruses.
    • Example: Emergence of hantaviruses and West Nile virus related to human use of unvisited habitats.
    • Accidental importation led to the introduction of monkeypox and West Nile virus into the U.S.
  • Air Travel and Globalization:
    • Major contributors to the SARS epidemic spread from China to Asia, Europe, and North America.

Ecological Factors Influencing the Spread of Emerging Viruses

  • Key Factors include:
    • Deforestation
    • Irrigation
    • Uncontrolled Urbanization
    • Population Movements and Intrusions
    • Increased Long-Distance Air Travel
    • Livestock Transportation
    • New Routing of Long-Distance Bird Migration

Zoonoses

  • Definition: Transmission between animals and humans.
  • Statistics: More than 75% of emerging viruses over the last two decades have been zoonoses with natural animal reservoirs.
  • Common Zoonotic Vectors:
    • Mosquitoes for West Nile virus.
    • Bats as important sources of new viruses to humans such as Ebola, Hendra, Nipah viruses, and likely SARS-CoV.
    • Note: No arthropod vector known for any coronavirus.

Stages of Viral Disease Emergence Leading to Successful Host Switching

  1. Initial single infection of a new host with no onward transmission (spillovers into “dead-end” hosts).
  2. Spillovers that cause local chains of transmission in the new host population before epidemic fade-out (outbreaks).
  3. Epidemic or sustained endemic host-to-host transmission in the new host population.
  • Viruses as Intracellular Parasites:
    • Two critical feats for emergence in human populations:
    1. Replication in human cells.
    2. Human-to-human transmission.

Replication Process of Viruses

  • Five Steps Required for Replication:
    1. Contact with a human host.
    2. Entry into the appropriate cell type.
    3. Production of more virus copies.
    4. Overcoming any intermediate host response.
    5. Exiting from the cell and transmitting to another host.

Understanding Human-to-Human Transmission

  • Important Note: Little is known about factors enabling successful human-to-human transmission.
  • Adaptive Change Necessity: Changes required for replication in a foreign host are independent but necessary for transmission.
    • Example: During SARS epidemics in 2003:
    • Identification of “super-spreaders” among infected individuals who infected a significant number of people.
  • Transmission Mechanics of SARS:
    • Typical spread via large droplets (less efficient) but clusters also suggested aerosol spread (efficient).
  • Historical Context: Super-spreaders documented in outbreaks of rubella, tuberculosis, and Ebola.

Importance of Understanding Virus Entry and Spread

  • Critical Considerations: Demographic factors, host properties, cellular properties, and control of virus transmission.
  • Steps Important for Host-Switching Viruses:
    1. Exposure between virus and host.
    2. Productive infection in host cells.
    3. Spread between infected and non-infected hosts.
    4. Adaptation of the virus to the new host.
  • Virus Infection Considerations:
    • Infection efficiency can be restricted by receptor binding, entry, trafficking, genome replication, and gene expression.
    • Production and shedding of infectious virus may be host-specific.

Obstacles for Virus Adaptation

  • Numerous obstacles exist for a virus to infect new hosts successfully.
  • Evolution of Viruses:
    • Evolution allowing adaptation to new hosts is not well understood.
    • Level of genetic variation is crucial for adaptation.
    • Most viruses transferred to new hosts are poorly adapted, replicate poorly, and are inefficiently transmitted.
    • Cross-species transmission more common in rapidly evolving viruses.
  • RNA Viruses:
    • Characteristics: Error-prone replication, lack a proofreading mechanism, rapid replication, short generation times, and large populations.

Human-to-Human Transmission Status of Select Viruses

  • Not Yet Achieved:
    • Nipah, Hendra, and H5N1 influenza.
  • Achieved Transmission:
    • Influenza strains H3N2/68, H1N1/09, Human Immunodeficiency Virus, Ebola virus.

Conclusion

  • Segment 2 summary: Focused on basic concepts of emerging viral infections and their public health implications.