Lecture 16 Flashcards
(13 cards)
Virus entry mechanisms
Virus attaches to host cell receptors, though the receptors are normally used for other host cell functions (e.g., immunity or cell signaling).
Membrane fusion - Viral envelope fuses directly with host membrane (common for enveloped viruses like HIV).
Endocytosis - Virus is internalized into endosome, followed by membrane fusion/release inside the cell (used by both enveloped and non-enveloped viruses).
Virus exit mechanisms
Lysis - Host cell bursts, releasing new virions; often lethal to host cell (common in non-enveloped viruses and bacteriophages).
Budding - Viral particles push through host membrane and bud off with a portion of membrane as an envelope (e.g., HIV).
What are some emerging viral diseases
HSV-2 (Herpes Simplex Virus 2)
SARS-CoV (Severe Acute Respiratory Syndrome Coronavirus)
Zika Virus
MERS-CoV (Middle East Respiratory Syndrome Coronavirus)
Hepatitis A Virus (HAV) Infection Course
Incubation: Long period (~10 weeks)
Clinical illness onset: Begins ~2 weeks after initial liver damage
Markers:
ALT/AST rise: Indicates liver damage
Virus peak: In both blood and faeces around start of clinical symptoms
Antibodies:
IgM: Rises early with liver damage, declines at end of clinical phase
IgG: Rises slightly later but persists as memory immunity
Case Fatality Rate (CFR)
Definition: Proportion of deaths among identified confirmed cases
Highly variable:
Depends on age, health care access, comorbidities, and accurate case identification
Asymptomatic Carriers and Herd Immunity: COVID-19 Insight
For effective herd immunity, high rates of asymptomatic infection would be beneficial.
Reality: COVID-19 caused symptoms in most infected individuals, some severe.
Implication: Higher medical burden and greater need for vaccines to achieve control.
Zoonotic Transmission and Viral Evolution Toward Pandemics
Wild birds infect domestic poultry (e.g., chickens)
Infected chickens transmit to pigs (pigs serve as mixing vessels for viruses)
Pigs transmit virus to humans
In humans, the virus may:
Reassort genes with existing human influenza viruses (genetic shift)
Adapt during repeated human-to-human transmission
The adapted virus spreads efficiently between people → pandemic potential
Antigenic drift
Slow and steady evolution
Definition: Gradual accumulation of point mutations in viral genes
Genetic mechanism: Due to errors in RNA replication (e.g. in HA or NA genes)
Virus type: Occurs in both influenza A and B
Effect on antigens: Small changes in surface proteins (e.g. HA, NA)
Frequency: Happens constantly, year-to-year
Antigenic shift:
Sudden transformation
Definition: Abrupt, major genetic change in the virus
Genetic mechanism: Due to reassortment of gene segments between viruses
Virus type: Occurs only in influenza A
Effect on antigens: New combinations of surface proteins
Frequency: Rare, every 10–50 years
Viruses: Main Routes of Direct Contact Transmission
Blood and Body Fluid Exchange
Includes sexual contact, sharing needles, transfusions
Kissing
Exchange of saliva (e.g., EBV/mononucleosis)
Vector Bites
Transfer of virus from animal to human via insects (e.g., mosquitoes, ticks)
Viruses: 3 main initial sites of infection
Bloodborne or Sexual Contact (Systemic Access)
Virus enters via bloodstream or sexual fluids
Common in viruses targeting internal organs (e.g., liver)
Example: Hepatitis B virus (HBV)
Aerosol/Droplet Inhalation (Respiratory-Limited Infection)
Entry via nasal or oral inhalation of droplets
Localized mainly in the lungs
Example: Influenza virus
Respiratory Entry + Systemic Spread
Infection begins in the lungs, then spreads to other sites like the skin or central nervous system (CNS)
Example: Measles virus
Viruses: Main indirect transmission routes
Aerosolised Particles
Inhaled droplets from coughing/sneezing
Skin Contact
Via contaminated surfaces or broken skin
Urine/Faeces
Ingested or contacted (faecal-oral route, e.g., hepatitis A
