L16 - GL Smith - Virus persistence and transmission Flashcards
Measles virus, rubella virus, varicella-zoster virus (VZV) and foot and mouth disease virus (FMDV) enter via the respiratory route, but are not be described as,.,….
Measles virus, rubella virus, varicella-zoster virus (VZV) and foot and mouth disease virus (FMDV) enter via the respiratory route, but are not be described as ‘respiratory infections’.
Virus infection may remain local (_____) or become ______
Virus infection may remain local (superficial) or become systemic.
describe local infections
describe systemic infections
describe how ectromelia virus spready systemically
After entry into the respiratory track epithelium the virus replicates and, via the lymphatics, enters the local lymph node. From here it enters the bloodstream (primary viraemia) and then infects many internal organs such as liver, spleen and the vascular endothelium. Here the virus replicates extensively and is released in much higher titres into the bloodstream (secondary viraemia). Finally, the virus replicates in the lungs and skin (the portals of exit) for transmission to new hosts.
paths for viruses to spread to systemic levels
blood, lymph = most common
some, eg rabies virus can spread via nerves
give some factors governing virus spread
describe apical vs basal budding
influenza buds from apical surface of respiratory epithelial cells and so virions are released into the airways (local).
This is determined by trafficking of the virus glycoproteins to that cell surface.
In contrast, a virus that budded from a basal layer (into tissue) might have greater chance of establishing a systemic infection.
viruses can be divided into three groups based on their ability to persist in the host.
what are they
acute infections
persistent infections
latent infections
label
describe acute viral infections
The virus is cleared after an acute infection.
This is the normal outcome in the normal host, but clearance is not inevitable. Measles virus mutants can persist in the CNS and cause a chronic demyelinating disease (SSPE) with a frequency of about one per million infections.
describe chronic infections
- In these cases the virus is detectable in the host for years, or even lifelong.
- HIV and hepatitis C virus almost always establish persistence.
- Hepatitis B virus is cleared after the acute phase in about 90% of normal adults, the remaining 10% become persistently infected. In contrast, 90% of infected male neonates become infected chronically.
describe latent infections
Infectious virus disappears after a primary infection but reappears (reactivates), sometimes many years later, to cause a recurrence of disease.
All herpesviruses establish latency and are not cleared.
All ____ establish latency and are not cleared.
All herpesviruses establish latency and are not cleared.
do herpes virus lie dormant?
Following acute infection the virus genome lies dormant (few, if any, virus proteins are made and the infected cell is not an immune target).
describe the nature of latency in the Varicella-zoster virus
Varicella-zoster virus is a good example. The acute disease is chickenpox (varicella) and, during the subsequent latent infection, the virus genome resides in neurones of sensory ganglia for the lifetime of the host. After many decades the virus may re-appear to cause shingles (zoster).
give some features of herpes virus latency
D: Latency
Virus in a quiescent state
D: Reactivation
Virus replication after change to latently infected cell (cell is now permissive)
D: Recurrence
Disease after reactivation, seeding of permissive cells and virus multiplication
Sites of herpes virus replication / latency
where does a virus noramlly reach the highest titre?
in the organ its shed from
T or F
Often, the portal or entry and exit are the same.
T
list 8 portals of exit
- Blood
- Skin
- Alimentary canal
- Respiratory system
- Saliva
- Genital tract
- Breast milk
- Placenta
give some viruses which exit via the blood
which viruses exit via the skin
human papilloma virus (6 & 11) herpes simplex virus varicella-zoster virus
measles virus
variola virus (formerly)
viruses which exit through te alimentary canal
poliovirus, hepatitis A virus, rotavirus, calicivirus
probs dont need to learn
4 factor affecting transmission
- Virion stability
- Durationofshedding
- Concentrationofvirus
- Availabilityofsusceptiblehosts
describehow virion stability can affect transmission
- Enveloped viruses are less stable than non-enveloped ones
Enveloped viruses are less stable than non-enveloped ones.
why?
. The lipid envelope is fragile and can easily be disrupted. Since the envelope contains the virus attachment protein(s), loss of the envelope results in loss of infectivity. So enveloped viruses are spread by close contact; measles virus, influenza virus, SARS-CoV-2 and HSV.
is the flu seasonal?
yep
are non elveloped viruses more stable?
small non-enveloped viruses are very stable outside the host and can be transmitted over long distances. Example: the 1981 FMDV epidemic in southern England was caused by virus being blown across the English Channel from France.
Picornaviruses that are spread by the oro-faecal route (poliovirus, hepatitis A virus) are stable in _____.
Picornaviruses that are spread by the oro-faecal route (poliovirus, hepatitis A virus) are stable in water.
describe how the duration of shedding affects transmission
acute: high levels short in short time
chronic: lower levels shed over long time
Poliovirus is stable for long
periods in _____
Poliovirus is stable for long
periods in water
describe how concentration of virus affects distribution
Acute viruses shed virions at high concentrations. A good example is
rotavirus that is shed in watery diarrhoea at concentrations of 109 p.f.u. / mL.
how does Availability of susceptible hosts affect transmission
measles virus suceptible hosts:
In isolated populations of less than ____ measles is eliminated,
In isolated populations of less than 100,000 measles is eliminated
e.g. the intermittent epidemics in the Faroe Islands. After an epidemic the virus disappears. A new epidemic can occur only when: i) there are a sufficient number of new susceptible hosts, and ii) the virus is introduced from an external source.
do you understand animal reservois
what are zoonotic infections
infections from an animal or insect reservoir
are zoonotic infections difficult to eliminate/
very
describe verticle transmission types (3)
Congenital infection
Perinatal
Germ line transmission
describe Congenital infection
Across the placenta, infects foetus in
utero. E.g. HCMV, rubella, Zika
describe perinatal infection
– During birth or from breast milk
– Herpes simplex virus (genital herpes present in birth canal). Human cytomegalovirus (present in birth canal or in breast milk). Hepatitis B virus (probably from blood contact at birth).
describe germ line transmission in viruses?
The presence of a retrovirus, as the provirus, in the germ line enables direct transmission to offspring.
The virus is transmitted as an inheritable genetic element.
These are called endogenous retroviruses.
Most species carry endogenous retroviruses in the germ line.
They are usually silent, but may reactivate under some circumstances
describe what is meant by multiple entry routes
many entry and exit points:
eg - HCMV
are viruses transmitted yb. respiratory aerosols highly contagious?
yep
do you know about the Myxoma virus in Australia
Myxomatosis is a caused by a myxoma virus, a poxvirus whose natural host is the South American rabbit in which it causes relatively little disease. But infection of the European rabbit is fatal. Myxoma virus was introduced into Australia to control the European rabbit which, in this foreign environment, had replicated out of control (500m rabbits = consumption of 1.8b kg of vegetation / yr). But myxoma virus quickly mutated to become less virulent. Now infection killed fewer rabbits and more slowly, so the probability of the virus being transmitted increased because the host survived for longer.
do you know about the Yellow fever virus (YFV)
Yellow fever is caused by a flavivirus that originated in Africa and was taken to the Americas via European colonisation and the slave trade. YFV infects primates, mosquitoes and humans and causes yellow fever (yellow jack). Transmission by the female mosquito Aedes aegypti. Greatly feared by European settlers: very susceptible with high mortality rates.
Life cycle worked out by Walter Reed and co-workers. A live attenuated vaccine (17D) produced by Max Theiler in 1937: still in use today.
Epidemic in Saint Domingue (now Haiti) in 1802: Saint Domingue was a French colony in the late 18th century. After the French Revolution, slavery was abolished in France and all its colonies (1795). Napoleon wished to re-introduce slavery in Saint Dominigue, which had been highly lucrative. Resisted by the emancipated slaves. So in 1802 Napoleon sent ~30,000 soldiers led by General Charles Leclerc (brother-in-law). Yellow fever killed ~85% of them.
fat
mamba
