How do viruses cause disease in humans Flashcards
(51 cards)
What is the basic reproduction number (R0)
- Is an approximate measure of how many new infections one person will generate during their infectious period
- Note that R0 values are approximate, and can vary by outbreak, mode of transmission and location
What is the duration of infection period divided into
- Incubation period and symptoms
What is the serial interval
- Time between the onset of symptoms in the primary cause and onset of symptoms in secondary cause
What is zika virus associated with in pregnant women
- Virus causes birth defects in babies born to some infected pregnant women including microcephaly
- Microcephaly is when babies are born with underdeveloped heads and brain damage
- Zika has also been linked to guillian-barre syndrome, a condition in which the immune system attacks the nerves
What is zika mainly transmitted via
mosquitoes
What is a virus
- Particle made of nucleic acid and a protein coat
- Obligate intracellular - only replicates inside living cells
How can viruses infiltrate skin
- Abrasions
- Insect/animal bites
- Needle punctures
How can viruses enter alimentary tract
- Gastroenteritis viruses
- Movement facilitate viral entry
How is the alimentary tract protective against viruses
Hostile environment
- Extreme acidity/alkalinity
- Digestive enzymes
How can viruses enter the urogenital tract
Abrasions facilitate viral entry
- HPV - local lesions
- HIV - viral spread
Urogenital tract - protective factors
- Mucus membranes
- Low pH
Eye - viral entry
- Localised infection - conjunctivitis
- Viral spread - eye blindness/CNS
Baltimore system for viral classification
I dsDNA II ssDNA III dsRNA IV +ssRNA V -ssRNA VI +ssRNA with DNA intermediate VII gapped dsDNA
Viral spread
After replication at the site of infection
- Some remain localised within epithelium or within one system
- Some cause disseminated or systemic infection (inflammation compromises integrity of cell basement membrane)
Viral release
- Apical e.g. flu - facilitate viral dispersal, but virus does not invade underlying tissues
- Basolateral e.g. rabies - provides access to underlying tissues and may facilitate systemic spread
Haematogenous spread - ways of viral entry to blood
- Directly through capillaries
- By replicating in endothelial cells
- Through vector bite
- By lymphatic capillaries
Once in the blood, virus has access to almost every tissue
Viraemia
Presence of infectious virus in the blood
- Passive/active viraemia
- Primary/secondary viraemia
Diagnostic value - measuring viral replication
Practical problem - need to screen blood donors
Neural spread
Less common than haematogenous spread
Viruses can go either way
- From peripheral sites to CNS
- From CNS to peripheral sites
Infection of the CNS
Neurotropic - virus can infect neural cells
Neuroinvasive - virus can enter CNS following infection of a peripheral site
Neurovirulent - Virus can cause disease of nervous tissue
Comparison of viruses - Neuroinvasiveness vs neurovirulence
NI NV
HSV + ++++
Mumps ++++ +
Rabies ++++ ++++
Tissue tropism - determinants
Limited or pantropic
- Cell receptors - HIV/CD4+
- Cellular proteins that regulate viral transcription - JC/viral enhancers in oligodendrocytes
- Cell proteases - flu/serine protease
What makes virus virulent
Viral genes affecting virulence
- Those that affect the ability of virus to replicate
- Those that modify host’s defense mechanisms (eg. virokines/viroreceptors)
- Those that enable virus to spread
- Those that have intrinsic cell killing effects
How do cytolytic viruses injure cells
- Inhibition of host protein and RNA synthesis - leads to loss of membrane integrity
- Syncytium formation
- Induction of apoptosis
How do non-cytolytic viruses injure cells
- CD8+ mediated
- CD4+ mediated
- B cell mediated
