7.2 Host range and tissue tropism

Viruses are picky eaters, choosing specific hosts and tissues to infect. This selectivity, known as host range and tissue tropism, shapes how viruses spread and cause disease. Understanding these preferences is key to grasping viral behavior and impact.

From rabies targeting mammals to influenza infecting birds and humans, viral host choices vary widely. These preferences are influenced by viral features, host factors, and environmental conditions. Knowing a virus's dining habits helps us predict its spread and develop targeted treatments.

Host range and tissue tropism

Definitions and significance

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• Host range encompasses the spectrum of host species a virus infects and replicates within, varying from narrow (one or few closely related species) to broad (multiple diverse species)
• Tissue tropism determines virus specificity for certain cell types or tissues within a host organism, affecting primarily infected organs or systems
• Interconnected concepts influence virus capacity to infect different host species and specific tissues
• Fundamental properties help classify and characterize viruses, providing insights into evolutionary history and cross-species transmission potential
• Critical factors determine epidemiology, pathogenesis, and zoonotic risk of animal viruses

Classification and characterization

• Narrow host range viruses infect limited species (rabies virus primarily infects mammals)
• Broad host range viruses infect diverse species (influenza A viruses infect birds, humans, pigs)
• Tissue tropism examples include hepatotropic viruses (hepatitis B virus) and neurotropic viruses (rabies virus)
• Host range and tissue tropism aid in virus taxonomy and classification systems
• Provide insights into virus evolution and adaptation to new hosts or tissues

Factors influencing host range

Viral factors

• Viral surface proteins, particularly receptor-binding proteins, mediate attachment to specific host cell receptors
• Genome composition impacts host range by requiring specific host factors for replication or gene expression
• Replication strategies vary among viruses, affecting their ability to utilize host cell machinery
• Genetic mutations in viral genomes lead to host range alterations, potentially causing emerging infectious diseases
• Viral stability under different environmental conditions (temperature, pH) affects infectivity across hosts

Host factors

• Presence and distribution of cellular receptors in different species and tissues determine viral tropism
• Intracellular factors availability (host cell machinery, restriction factors) affect viral replication
• Host immune response (innate and adaptive immunity) selectively controls viral replication in tissues or species
• Species-specific differences in cellular metabolism or gene expression influence virus ability to complete life cycle
• Host genetics play a role in susceptibility or resistance to viral infections

Environmental factors

• Temperature affects viral stability and replication efficiency in different host species (avian influenza viruses)
• pH conditions in various host environments impact viral particle stability and entry mechanisms
• Humidity levels influence viral transmission and survival outside the host (respiratory viruses)
• Seasonality affects prevalence and transmission of certain viruses (influenza viruses in winter months)
• Geographical distribution of host species impacts viral ecology and evolution

Viral recognition of host cells

Attachment and entry mechanisms

• Viral attachment proteins (spike proteins, fiber proteins) recognize and bind specific cellular receptors
• Multiple receptors or co-receptors utilized by some viruses for cell entry (HIV uses CD4 and chemokine receptors)
• Receptor-mediated endocytosis common for viral entry, exploiting various pathways (clathrin-mediated, caveolin-mediated)
• Membrane fusion facilitated by viral fusion proteins, triggered by specific cellular conditions (low pH in endosomes)
• Alternative entry mechanisms include direct penetration or cell-to-cell spread

Post-entry events

• Uncoating and genome release often dependent on cell-type specific factors
• Initial replication steps require host cell machinery and may be cell-type specific
• Host cell factors (transcription factors, proteases) contribute to tropism and successful infection establishment
• Viral genome replication and protein synthesis utilize host cell resources differently across cell types
• Assembly and release of viral particles may depend on cell-type specific mechanisms

Implications of host range and tissue tropism

Impact on viral pathogenesis

• Host range directly affects zoonotic transmission potential and emergence of new human pathogens (SARS-CoV-2)
• Tissue tropism determines primary viral replication sites, influencing clinical manifestations and disease severity
• Changes in viral tropism lead to altered pathogenesis and potentially more severe or novel clinical presentations
• Infection of immune cells affects host's ability to mount effective immune responses (HIV targeting CD4+ T cells)
• Organ-specific tropism results in targeted damage or dysfunction, contributing to overall infection pathology

Public health and medical implications

• Understanding host range and tissue tropism crucial for developing targeted antiviral therapies
• Influences effective vaccination strategies by impacting drug delivery and immune response induction
• Evolution of viral host range and tropism leads to new strains with altered virulence or transmission patterns
• Challenges disease control efforts and requires continuous surveillance for emerging threats
• Informs risk assessment and preparedness planning for potential pandemics