10 Questions
What is unique about the spike protein genome sequence of SARS CoV-2?
It has a furin cleavage site, not seen in SARS CoV-1
What is the result of viral tropism in SARS CoV-2?
Viral replication is not restricted to the respiratory tract
What is one mechanism by which viruses can cause immunosuppression?
By replicating in one or more cells of the immune system
What is the result of viroporins in viral cell injury?
Leakage of enzymes from lysosomes
Why are very young humans more susceptible to disease?
Due to immaturity of immune response
Why are males slightly more susceptible to viral infections than females?
Due to unknown reasons
What effect does malnutrition have on susceptibility to disease?
It compromises physical barriers and immune response
What is the result of cigarette smoking on susceptibility to disease?
It increases the risk of disease, especially in the respiratory system
Why is measles more lethal in developing countries than in Europe and North America?
Due to malnutrition, which compromises physical barriers and immune response
What is the effect of stress on the body's susceptibility to disease?
It increases the risk of HSV
Study Notes
Viral Virulence
- R0 (reproduction number) measures how contagious an infectious disease is, representing the number of new cases generated by a single infected person in a completely vulnerable population.
- R0 only applies when everyone in the population is completely vulnerable, meaning no one has been vaccinated, had the disease before, or has a way to control the spread of the disease.
Virulence
- Virulence is the capacity of a virus to cause disease in a host, ranging from mild to lethal effects.
- Virulence can be quantitated by measuring:
- Mean time of death
- Mean time to appearance of symptoms
- Fever, weight loss, and pathological lesions
- Reduction in blood CD4 cells (e.g., HIV)
- Virulence is influenced by factors such as dose, route of infection, species, age, gender, and host susceptibility.
- Virulence genes are usually identified by mutation, where a virus causes reduced or no disease in a specified system.
Tropism
- Tropism refers to the spectrum of tissues infected by a virus, ranging from limited to pantropic.
- Determinants of tropism include susceptibility, permissivity, accessibility, and defense.
- Examples of tissue tropism:
- Human influenza virus: infects respiratory tract
- Avian H5N1 virus: infects many cells other than respiratory tract
- SARS CoV-2: not restricted to respiratory tract, with TMPRSS2 and furin cleavage site
Immunodeficiency
- Viruses can cause immunosuppression, globally reducing the immune response during infection.
- Mechanisms of immunosuppression include:
- Replication in immune system cells (e.g., HIV)
- Disturbing cytokine production and intracellular signaling
- Viral proteins acting as viroceptors or virokines (immune modulators)
Cell Injury
- Mechanisms of cell injury by viruses:
- Cytolytic viruses: cytopathic effects (apoptosis, necrosis, pyroptosis)
- Viroporins
- Viral inhibition of host proteins and RNA synthesis, leading to loss of membrane integrity and cytoplasmic degradation
- Syncytium formation by enveloped viruses (e.g., parainfluenza, HIV)
Other Determinants of Virulence
- Age: very young and very old individuals are most susceptible to disease
- Gender: males are slightly more susceptible to viral infections than females
- Pregnancy: hep A, B, E, influenza are more lethal, and polio is more common
- Malnutrition increases susceptibility due to compromised physical barriers and immune response
- Cigarette smoking and stress (e.g., HSV) can also affect virulence
This quiz covers the concept of virus virulence, reproduction number (R0), and how it measures the contagiousness of infectious diseases.
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