24 Questions
Through which route is the Hepatitis A virus excreted?
Gastro-intestinal tract
What is the term for the ability of a virus to infect specific cells or tissues?
Tropism
Which of the following is a host factor that determines the severity of a viral disease?
Genetic predisposition
What is the term for the presence of a virus in the blood?
Viraemia
Which of the following is NOT a route of viral excretion?
Musculoskeletal system
What is the term for the ability of a virus to cause disease?
Pathogenicity
Which of the following is a factor that can influence the immune response to a viral infection?
All of the above
What is the term for the initial symptoms that occur before the onset of a specific disease?
Prodrome
What is pathogenicity?
The capacity of a virus to cause disease in a susceptible host
What is meant by viral tropism?
The specificity of a virus for a particular host, tissue, or cell
What is an example of tissue tropism?
Influenza infecting lung tissue
What is an example of host tropism?
Myxoma virus infecting rabbits but not humans
What is viral virulence?
The capacity of a virus to cause disease
What determines the ability of a virus to cause disease?
The virus's pathogenicity
What is an example of cellular tropism?
HIV infecting macrophages but not neurons
What is the relationship between viral virulence and pathogenicity?
Viral virulence is a measure of pathogenicity
What determines the specific target tissues that a virus can infect?
The presence of specific receptors on the cells
What is the term for the period between exposure to an infection and the appearance of the first symptoms?
Incubation period
What is the primary route of transmission for rhinoviruses?
Inhalation
What is the term for the early symptom(s) that might indicate the start of a disease before specific symptoms occur?
Prodrome
What is the term for the spread of a virus from the initial site of infection to other parts of the body?
Systemic spread
What factor can influence the length of the incubation period?
Route of inoculation
What is the term for the process by which a virus enters the body and initiates infection at a primary site?
Acquisition
What type of cells does HIV infect?
CD4 cells
Study Notes
Severity of Disease
- Host factors that determine severity of disease include age, genetic predisposition, malnutrition, immunosuppression, other underlying illnesses, and psychological factors.
Excretion
- The virus is excreted via various routes after replication and spread, including:
- Skin via vesicles/lesions (e.g. HSV 1 and 2)
- Respiratory tract (e.g. RSV or influenza)
- Genito-urinary tract (GUT) (e.g. measles or CMV - in urine, e.g. HIV or HBV - in semen)
- Gastro-intestinal tract (GIT) (e.g. rotavirus or Hepatitis A)
- Body fluids (breast milk) (e.g. HIV or HBV)
Viral Tropism
- Viral replication at specific target tissues often defines the symptoms of the disease
- Examples of viral tropism include:
- Hepatitis viruses infecting hepatocytes
- Varicella-zoster virus infecting neurons and epithelial cells
- HIV infecting CD4 cells
- Determined by the presence of specific receptors on cells that can be used by the virus
Spread in the Host
- Local spread: restricted to cells where the virus entered, replicates, and is released from infected cells, then spreads to neighboring cells
- Systemic spread: virus breaches epithelial barrier, invades sub-epithelial tissues, breaks through basement membrane, and allows spreading to other parts of the body via blood, lymphatics, or nerves
- Examples of localized infections:
- Rhinoviruses: upper respiratory tract (URT)
- Rotaviruses: intestinal epithelium
- Papillomaviruses/poxvirus: epidermis
- Examples of systemic infections:
- Enteroviruses: intestinal epithelium, lymphoid tissues, CNS
- Herpesviruses: oropharynx or GUT, lymphoid cells, CNS
Systemic Infection
- Acquisition: virus enters the body
- Primary replication at the site of entry
- Incubation period: virus is amplified and may spread to a secondary site
- Primary viraemia: spread via neurons/lymph
- Secondary site of replication
- Secondary viraemia
- Prodrome: early symptom(s) that might indicate the start of a disease before specific symptoms occur
Incubation Period (IP)
- Period between exposure to an infection and the appearance of the first symptoms
- Factors that influence the length of the IP include:
- Dose or inoculum of infectious agent
- Route of inoculation
- Rate of replication of infectious agent
- Host susceptibility and immune response
Pathogenicity and Viral Virulence
- Pathogenicity: ability of an organism (e.g. virus) to cause disease in a susceptible host
- Viral virulence: capacity of a virus to cause disease, often used as a measure of pathogenicity
- Viral tropism: specificity of a virus for a particular host, tissue, or cell, i.e. where the virus can infect and replicate
This quiz covers factors that determine the severity of disease caused by viruses, including host factors and modes of viral excretion. Test your knowledge of virology!
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