Emerging and Reemerging Viral Diseases

Emerging and Reemerging Viral Diseases

Loubna Tazi, Ph.D
Department of Medical Microbiology & Immunology
UC Davis, School of Medicine
MMI 177 Winter 2026

Outline

  • Factors involved in the emergence of viral infectious diseases.

  • Notable emerging viral diseases.

Learning Objectives

Following completion of this chapter, students should be able to:

  • Define emerging and reemerging infectious diseases.

  • Identify the human, ecological/environmental, and viral factors that are involved in the emergence or reemergence of viral diseases, providing examples of each.

  • Explain arbovirus transmission cycles and outline Dengue virus transmission and course of infection.

  • Compare emerging viruses transmitted by rodents and bats, relating the transmission of arenaviruses from rodents to humans.

  • Identify the geographic history of existing Ebolavirus species and summarize past outbreaks, including the 2014 – 2016 Ebola virus epidemic in West Africa.

Emerging and Reemerging Diseases

  • Emerging Infectious Disease (EID):

    • Definition: Disease with rapidly increasing incidence caused by a pathogen that has not been observed before in a population or geographic location.

    • Note: Emerging viruses are not entirely new; they benefit from better detection efforts (often referred to as virus hunters).

    • Causes of emergence include the rise of agriculture, leading to the invasion of new infectious pathogens among a growing human population.

  • Reemerging Infectious Disease:

    • Definition: Disease caused by an established pathogen in a new geographical location that was previously controlled but is now returning with a higher incidence.

    • Significance: Represent a major public health threat worldwide.

  • Examples of Notable Emerging and Reemerging Diseases:

    • Newly Emerging: Cryptosporidiosis, Heartland virus, Enterovirus D68, Human monkeypox, H3N2V Influenza, Hepatitis C, E. coli O157:H7, etc.

    • Re-emerging/Resurging: Powassan virus, Drug-resistant tuberculosis, Lyme disease, West Nile virus, Ebola virus, COVID-19, etc.

Factors in the Emergence and Reemergence of Viral Diseases

1. Human Factors

  • Population Growth:

    • Major driver of spillover of viruses from animals to humans.

    • Result of urbanization, conflicts, inadequate healthcare systems, and anthropogenic activities (e.g., deforestation).

  • Globalization:

    • Facilitates rapid disease spread through travel, trade, and economic exchanges.

    • Risks include imported infected products and increased tourism, enhancing exposure to emerging zoonoses.

  • Sociological Factors:

    • Inadequate public health infrastructures, access to health care, misinformation, and vaccination hesitancy worsen viral disease prevalence.

  • Human Behaviors:

    • Activities that promote viral transmission include handling of food, sexual activities, and body art practices.

  • Susceptibility of Human Hosts:

    • Populations at higher risk include the elderly, pregnant women, immunocompromised individuals, and malnourished populations.

2. Environmental and Ecological Factors

  • Urbanization:

    • Leads to habitat destruction and increased human-wildlife interactions, resulting in zoonotic virus spillover.

  • Weather and Climate Change:

    • Influences vector populations and transmission potential; effects observed in outbreaks of zoonotic diseases linked to climatic events.

  • Biodiversity:

    • Regions with high wildlife host species richness are hotspots for zoonotic transmissions, as observed in areas of Brazil.

3. Viral Factors

  • Genomic Changes:

    • Mechanisms like reassortment, recombination, and mutations contribute to viral adaptability and the emergence of new virus strains.

    • RNA viruses are the most common cause of emerging diseases in humans due to their high mutation rates.

Transmission Dynamics

Arboviruses

  • Definition:

    • Arboviruses (arthropod-borne viruses) are transmitted to humans via vectors, primarily mosquitoes and ticks.

    • Over 500 recognized arboviruses exist, 150 of which can cause human diseases.

    • Notable examples include Dengue virus, Zika virus, and Yellow fever virus.

  • Transmission Cycles:

    • Enzootic cycle: Between arthropods and natural reservoirs.

    • Epizootic cycle: Widespread transmission among domestic animals.

    • Urban cycle: Direct transmission between vectors and humans.

Summary of Notable Viral Diseases

  • Dengue Virus (DENV):

    • Most rapidly spreading mosquito-borne disease globally, with half the world's population at risk.

    • Transmission primarily through Aedes mosquitoes, with complications including severe dengue (DHF) upon successive infections.

  • Yellow Fever Virus:

    • Characterized by jaundice and associated with high mortality rates in Africa and South America.

    • Prevention relies heavily on vaccination.

  • Zika Virus:

    • Initially isolated in 1947, primarily transmitted by Aedes mosquitoes.

    • The virus can cause serious congenital malformations in fetuses (e.g., microcephaly) and is notable for its sexual transmission potential.

  • West Nile Virus (WNV):

    • The leading cause of neuro-invasive arboviral disease in the US, with asymptomatic infections prevalent but serious complications possible, especially in vulnerable populations.

  • Ebola Virus:

    • First identified in 1976, resulting in high mortality rates and marked by frequent outbreaks in Africa.

    • Initial infection often occurs through contact with infected animals, followed by rapid human-to-human transmission.

Conclusion

  • Emerging and reemerging viral diseases pose significant challenges globally. Understanding the interplay of human, environmental, and viral factors is essential for effective public health responses and disease prevention strategies.

  • The necessity for ongoing research into the dynamics of these diseases and their transmission patterns is critical in preventing future outbreaks and mitigating impacts on public health.