Lec 4: Transmission Flashcards
if the virus does not produce enough visions it will
die out
another possibility for viral survival…
which are…
to be maintained in cells as nucleic acids
which are replicated and passed on to daughter cells when they divide
Viruses of bacteria and other microbial hosts are…
…released from infected cells into the environment
- dependent upon chance encounters with susceptible cells.
Viruses of multicellular animals and plants — may…
…spread to adjacent cells, or to cells in a distant part of the host.
Some virus infections modify the…
& ex:
…behaviour of their hosts in order to increase the probability of transmission.
& ex: Mammals infected with rabies virus often become aggressive or rabid.
some viruses can be transmitted to new hosts without…
…seeing the light of day , (without exposure to the outside environment) — transmitted by kissing, sex, birth, vectors, etc.
Classical route of journey between hosts:
Cell free virions bind to permissive host cell with receptor-ligand interactions, followed by fusion to enter the cytoplasm and then replicative events.
Cell to cell transmission:
An actively infected cell can directly infect a second cell, without requirement for releasing cell-free virions into the local environment.
Why is cell to cell transmission advantageous? (2)
- Bypasses the need for virus diffusion into the local enviroment.
- Minimizes exposure to harsh environment, and viral neutralization defenses such as antibodies and complement.
some viruses travel ____ _________, through: (3)
long distances, through:
- ) bird migration
- ) human travel
- ) animal export
In a multicellular organism, viruses must survive…
In theory,…
…host defense mechanisms before suitable host cells are encountered.
…a single virion can initiate an infection, but in practice a host must be inoculated with a minimum number of virions in order for that host to become infected.
most vectors are
and sometimes referred to as
arthropods
“arboviruses”
Some viruses replicate in…
- There are viruses that can…
…one or more tissues and organs of their vectors.
- …replicate in both invertebrates and plants, and viruses that can replicate in both invertebrates and vertebrates.
Many (but not all) of the invertebrates suffer…
little or no harm when they are infected.
It’s possible that many of the modern plant viruses and vertebrate viruses are descended from…
…viruses of invertebrates that later extended their host ranges to plants or vertebrates.
high degree of specificity between…
most vectors and the viruses that they transmit
Plant cells have thick cell walls =
tf most viruses are carried…
significant barriers to virus entry.
…across these barriers by vectors
Some plant virus-vector specificity depends on…
specific amino acid sequences in capsid proteins.
Some plant-parasitic fungi can also act as
virus vectors.
~20% of plant viruses can be
many can be transmitted by…
vertically via seeds
…artificial means (ex: grafting)
Transmission of vertebrate viruses: (3)
- Non-vector transmission of vertebrate viruses
- Vector transmission of vertebrate viruses
- Virus survival in the vertebrate host
(Non-vector transmission of vertebrate viruses) can happen via: (3)
- via mucous membranes of upper respiratory tract, (e.g. flu virus)
- Release from lesions and by direct contact, (e.g. foot and mouth disease virus, herpes simplex virus (lips))
- Some infections (e.g. common cold, rotavirus) remain limited to these tissues, while others (e.g. measles, polio, HIV) cross the epithelial surface and spread to other organs and tissues.
(Vector transmission of vertebrate viruses) can happen via: (2)
- Mainly blood-feeding arthropods that acquire their viruses when they take blood meals from infected animals.
- Transmitted during medical procedures by inanimate vectors (needles)
(Virus survival in the vertebrate host)
Viruses must…
& ex.’s:
…evade several defense systems
- removal by the mucociliary escalator in airways.
- Inactivation by extremes of pH in the gastro-intestinal tract .
- The surfaces fluids of respiratory, intestinal and urinogenital tracts which contain antiviral substances.
- The antiviral substances such as complement proteins, which can undergo modifications and damage enveloped virions by insertion into their membranes.
- Viruses may be phagocytosed by a range of cells such as macrophages.
- Antibodies
- Vaccines
(Transmission of INVERTEBRATE viruses)
• Invertebrates are hosts…
• Many of these viruses have evolved…
- …to many other viruses that replicate only in invertebrates.
- …occlusion bodies, (large protein structures in which virions become embedded in the infected cell)
(Transmission of INVERTEBRATE viruses)
Putrefaction of insects that have been killed by virus infection results in…
…the deposit of occlusion bodies on leaves. Occlusion bodies ingested by insects are dissolved in the midgut, releasing virions.
Permissive cells support…,
- leading to…
…complete DNA and protein synthesis,
- …the production of infectious progeny and lysis of the host cell.
Semipermissive cells support…
limited viral replication.
Nonpermissive cells survive…
…viral challenge because if the virus enters the host, viral replication does not occur.
Permissive cells must have…
…appropriate receptors that the virus can bind to.
Must be inducible; cell proteins, such as transcription factors and enzymes.
(Permissive cells)
Some viruses of eukaryotes need…
…the host cell to be in a particular phase of the cell cycle
Permissive cells must lack…
…defenses against the virus, or the virus must have the ability to overcome the defences of the cell.
