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Virology > Virus Transmission and Host Outcomes > Flashcards

Virus Transmission and Host Outcomes Flashcards

1
Q

Viral host catagories

A

1 prok hosts

2 multicellular hosts

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2
Q
  1. Prokaryotic (Bacteria) host
A

t: virus infecting bacteria and other microbial host are released from infected cells into the environment of host, where further susceptible cells are likely to be encountered

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3
Q
  1. Multicellular hosts
A

s: Viruses of these hosts must invade a susceptible host cell to ensure the replication of its genes.
i. Animal host: Virus infecting an animal may spread to adjacent cells, or to cells in a distant part of the host after transport in the blood of an animal.
ii. Plant host: Virus infecting plants may spread to adjacent cells or to distant cells after transport in phloem of the plant

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4
Q

Strategies for Virus transmission

ubiquitous

A

Virionscan be found anywhere in our environment ranging from soil, air or water.

•Viruses have capacity to spread across wide areas in a variety of ways:

a. River and wind: the foot and mouth disease outbreak in the Isle of Wright in 1981 was initiated by virus that had spread through the air from Brittany (France), more than 250km away.
b. Bird migration: (avian influenza viruses)
c. Animal export: Eg. Monkeypoxvirus, Ebola virus, Lassa fever virus
d. Human travel: SARS COV-1, SARS COV-2, Ebola viru

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5
Q

Strategies for Virus transmission

host manipulation

A

Viruses modify the behaviourof their hosts in order to increase the probability of transmission.

  • Example, mammals infected with rabies virus often become aggressive which increases the chances of biting a new host and transmitting the virus in saliva.
  • Some plants infected with baculovirusesbecome more mobile in the late stages of infection, thus aiding virus dispersal.
  • Some insects infected with virus migrate towards the top of the plants, under the leaves, where other larvae are likely to feed, and become intoxicated when the host (insect) dies and putrefies.
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6
Q

Vertical Transmission

A

: Viruses are transmitted without exposure to the external environment. Can occur in several ways:

i. Transmission of viruses from parent to members of the next generation (offspring).
ii. Contacts between hosts (hosts to hosts) with fluids containing viruses, such as kissing, sexual intercourse.
iii. Vectors: Insects such as aphids, mosquitoes, e\

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7
Q

Horizontal Transmission

A

: Viruses present in infected body surfaces such as epithelial cells and/or lymphoid of the skin, respiratory tract are spread to new host through external channels such as wind, fomites (door handles, furniture, clothes, utensils, syringes, vehicles etc.)

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8
Q

Vertical transmission of Viruses using vectors

A

•Most are biological agents that transmit viruses from one hosts to another mostly through feeding.

•Majority of the vectors of viruses are arthropods (mites, insects, ticks) and are known as arboviruses(arthropod-borne viruses)
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vertical transmission from mother to egg

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9
Q

General mechanisms of vector transmission

A

THe general principle of vector transmission is that a vector aquires a virus from an infected host though feeding and further transmits it to a new host through the same process

direct transmission

circulative transmission

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10
Q

Direct Transmission

A

most viruses are located only in the mouthparts of the vector for easy transmission. The aquired virus c could be transmitted within minutes to hours

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11
Q

Circulative Transmission

A

SOme ciruces migrate in the cevotr from the gut wall to the circulatory system, to the salivary glands before transmission into a new host. This could take a few hours or days to be transmitted

most viruses undergoing circulatory transmission may replicate in one or more tissues or organs of both th`eir invertabrate or vertebrate vectors while a few can replicate in both invertebrate and plant

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12
Q

General Mechanism of Vector Transmission

A

Invertebrate vectors suffer little to no harm to the presence of a virus compared to plants and vertebrates

Thus, it has been hypothesized that modern viruses infecting vertebrates and plants are descendents of invertebrate viruses which later evolved to infect the former

vector to vector transmission

Transovarial Transmission

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13
Q

Vector to Vector

Transmission

A

This is possible when teh reproductive organ of a vector is infected which may be transmitted to other vectors though sexual intercourse (male-female)

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14
Q

Transovarial Transmission

A

Viruses are transmitted to the next generation through contamination of the egg

VIruses exhibit a high degree of vector specificity which is greater among vectors of plant viruses than vectors of animal viruses

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15
Q

Categories of vector VIruses

3

A

plant-virus vectors

animal-virus vectors

inanimate vectors

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16
Q

Plant Virus vectors

A
  • A wide range of organisms use plants as sources of nutrient, especially invertebrates (insects).
  • Plants are surrounded by thick cell walls that present significant barrier to viral entry.
  • Virus transmission into plant cell occurs through the process of feeding by the vector by biting or piercing through cell walls

piercing vectors

biting vectors

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17
Q

Piercing Vectors

pl0nt

A

Invertebrates such as aphids, nematodes, whiteflies feed by piercing through plant cell walls to acquire nutrients

18
Q

Biting vectors

pl0nt

A

Insects such as mites, beetles, feed by biting through plant cell walls and eating the tissue

19
Q

Aphids

A

Most common vector of plant viruses.

•They are effective in transmitting viruses to plants through the probing of various tissues in search of suitable cell to feed on.

Each plant that was pierced during the probing cells is likely to be infected with the virus, not necessarily the one that was fed on

20
Q

Nematodes

A
  • They are soil-dwellers which feed by piercing and ingesting the contents of the root cells of plants. Molting terminates virus transmission by the nematode, as it ensures the virus remains only in the gut.
  • Nematodes that act as vector for viruses show specificity for transmitting certain shapes of viruses. E.g. Nematodes in the family (Longiridae) transmit nepoviruseswith isometric shape, while nematodes in the family (Trichodoridae) transmits the tobraviruseswhich are rod-shaped virions.
  • Note that the specificity of vectors to plant viruses are controlled by specific amino acid sequences in the virus protein which bind to specific receptors in the mouth parts of their vector
21
Q

Parasitic fungi as plant virus vectors

A
  • Certain plant-parasitic fungi act as vectors of plant viruses by developing fungi spores (houses both the virus and fungus).
  • The virus in the spores can survive for months or years until the spore germinates on a new plant which is now infected with both the virus and the fungus.
  • For example, the fungus (Spongosporasubterranea)which infect potato causing the powdery scab disease, also acts as a vector for the potato mop-top virus
22
Q

Transmission of Vertebrate Viruses

A
  1. Non-vector: transmission of virus can occur through inhalationof the virus from contaminated environment or contactwith an infected host. Examples are syringes and needles.
  2. Vector: Vectors that transmit vertebrate viruses are majorly blood-feedingarthropodsthat acquire their viruses when they take blood meals from infected animals
23
Q

Non-vector Transmission of Vertebrate Viruses

A

Transmission through contact with an infected host or fomite

inhalation of viruses in contaminated environment

24
Q

Transmission through contact with an infected host or fomite

3

A

1a. Sexual contact: Viruses present in genital secretions of an infected host can be transmitted to a new host during intercourse. E.g. HIV
1b. Skin or fomite contact: Transmission occur from contact with infected body parts. E.g. foot and mouth virus of sheep, herpes simplex virus from lesions of the lips and papilovirusesfrom skin warts.
1c. Faecal-oral contact: Viruses present in the intestinal tract of an infected host can be transmitted through hand to mouth contamination with faecesor faecal-contaminated water.

25
Q

inhalation of viruses in contaminated environment

A

Viruses in released in air droplets by an infected host can be transmitted to a new host through inhalation. Viruses mainly contaminate the environment through coughing, sneezing and speaking. E.g. SAR-COV-2, common cold.

26
Q

Vector Transmitted Vertebrate Viruses

2 categories

A
  • These are mainly blood feeding arthropods which acquire the virus through feeding on an infected host and in turn transmit it onto a new host.
  • Vectors can be grouped into 2 categories:
  1. Living vectors: Examples are mosquitoes, ticks
  2. Inanimate vectors: Syringes and needle
27
Q
  1. Living vectors: Examples are mosquitoes, ticks
A
  • Most arthropod vectors of vertebrate viruses are infected with the virus for life. E.g. the tick that transmits the tick-borne encephalitis virus.
  • Also, transovarialtransmission of viruses from femaleto the egghave been observed in the mosquito that transmits the yellow fever virus
28
Q
  1. Inanimate vectors: Syringes and needle
A
  • Transmission of viruses through inanimate vectors can occur through blood transfusion equipment, syringes and needles.
  • The level of virus present in the blood of an infected animal is referred to as viremia(virus load).
29
Q

Transmission of invertebrate viruses

A
  • Some invertebrates are vectors of invertebrate viruses, just as they are for vertebrate viruses.
  • The invertebrate host implies that the virus can replicate its genome in them which most times lead to an infection.
  • Most invertebrate viruses uses insects as hosts.
  • Occlusion bodies: Most invertebrate viruses have evolved to form large protein structures known as Occlusion bodies for which the virus is embedded in the infected cells
30
Q

Invertebrate viruses in occlusion bodie

A
  • Invertebrate viruses in occlusion bodies can be transmitted to new host in several ways:
  • Predation: Invertebrates that prey on virus-infected insects can consume occlusion bodies through feeding.
  • Feaces: A virus-infected insect can pass out occlusion bodies in faecesto the external environment on plant leaves which are ingested by another insect.
  • After the deathof a virus-infected insect, occlusion bodies are resistant to putrefaction and protect the virionsin the external environment until its ingested with plant material by a new host.
  • Once ingested, occlusion bodies are broken down by enzymesand high pHin the gut setting free the virionsto infect new cells
31
Q

Vertical Transmission of Invertebrate Viruses

A
  • Virus transmission from an infected mother to the egg (embryo) is possible for invertebrate viruses.
  • Virus transmission may occur inside the eggs or on surface of the eggs.
  • Example of an invertebrate virus vector is the parasitic waspwhich transmits viruses to insects.
32
Q

Viruses and Permissive Cells

A
  • Virus transmission to a new host is incomplete until the virus finds a suitable cell to evade and replicate it genome. A host cell that support the replication of an evading virus are referred to as permissive cells.
  • Invading viruses need to first bind to specific receptorson host cell membrane/wall using its attachment structure on its surface before releasing its genome into the new cells.
  • Some viruses are limitedin their permissive cell types (E.g. Hepatitis B viruses restricted only to hepatic cells in the liver), some others have a broad rangeof cell choices while a fewcan replicate in both plant and animal cell
33
Q

Viruses and Permissive Cells

examples

A
  • Invading viruses can either code for or possess its enzymes needed for transcription (most viruses that undergo transcription in the cell cytoplasm possess transcription enzyme) or
  • Other invading viruses utilizes host cell enzymes for gene transcription (most viruses that replicate inside the nucleus utilize host transcription enzymes and proteins).
  • Viruses invading eukaryotic cells may require host cells to be in a specific stage in cell division.
  • E.g. 1: Retroviruses in the cell cytoplasm require the host cell nuclear membrane to be broken during mitosis to allow its entry.
  • E.g. 2: Parvoviruses and papillovirusesare small DNA viruses that requires host DNA replicating enzymes present in the DNA synthesis phase (S phase) of its cell cycle.
  • Also, papillomaviruses are able to release proteins for manipulating the cell cycl
34
Q

Host Outcomes During Virus Invasion

A

could be productive or non-productive

35
Q
  1. Productive virus infection
A
  1. Productive virus infection: could entails either cell death, productive infection(persistent) or latent infection (persistent).
  • Virionsmay be released when an infected cell lysesreleasing new viruses which evade new host cells.
  • On the other hand, an infected cell may lyse, releasing new viruses over a long period (Hepatitis B virus infection) or over a short period (HIV virus infection
36
Q
  1. Non-producting infection
A
  1. Non-productive infection may results either in a latent infection, programmed cell deathor abortive infection.
  • Latent infection: though virus replication may be paused in host cells, its genes still persist and could be retained in daughter cells during cell division.
  • Abortive infection could result from a virus mutated genome which results in a defective gene.
  • Viruses with defective genome are unable to replicate until a helpervirusis available to provide missing functions.
37
Q

Host Barrier Mechanisms

A

Viruses cause disease in host after successfully bypassing 2 main host mechanisms:

  1. Body barriers defenses
  2. Immune system
    a. Outcomes are either destructive immune and inflammatory response to the presence of the virus.
    b. Kill cells of important tissue, e.g. brain
38
Q

Vertebrate surface barrier defenses against viruses

A
  • The mucosa surfaces of the respiratory, intestinal and urogenital tracts present significant barriers to viral infection.
  • These surfaces are bathed in fluids that contain antiviral substances and the fluid are viscous, limiting virus access to the cells.
  • Example: A virus in the lower respiratory tract must avoid removal by the mucociliaryescalator.
  • Also, viruses in the gastrointestinal tract must avoid inactivation by extremes of pH.
39
Q

Vertebrate immune barriers against viruses

innate

A
  • Viruses also must successfully evade the host innate and adaptive immune barriers to establish an infection.
  • Innate immune response
  1. Interferon: signals other immune cells such as cytokines
  2. Natural killer cells: patrolling immune cells that detect and kill virus infected cells
  3. Antigen presenting cells: (macrophages and dendritic cells)
40
Q

Vertebrate immune barriers against viruses

•Adaptive immune system

A

•Adaptive immune system

  1. Cytotoxic T-cells
  2. B-cells
  3. Plasma cells or antibodie
41
Q

Factors influencing host outcomes due to virus infection

A
  1. Nature of exposure
  2. health status
  3. Age
  4. Immune status
  5. Viral dose
  6. Genetics of host/viru
42
Q

Steps in viral disease progression

A
  1. Acquisition / Entry
  2. Initiation of infection
  3. Incubation period
  4. Viral replication

5.Immune system
response

a) Positive: Elimination of virus and virus infected cells
b) Negative: Causes an harmful immune response (immunopathology)
6. Viral production / release (If result is 5b above)
7. Resolution / Continuatio

Virology (20 decks)

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