1. Viro100: Virology 3 Credit hours NUST Centre of Virology & Immunology
LECTURE 7:
Virus transmission
Waqas Nasir Chaudhry
Viro100:
Virology
3 Credit hours
NUST Centre of Virology & Immunology

2. Stages of viral infection cycle
Virus infection
Epidermal
cells
Replication
Cell-to-cell
movement
Other host plants
Mesophyll
cells
Systemic
movement
Phloem
Bundle
sheath cells
Plant-to-plant
movement
Phloem
Parenchyma
Companion cells

3. A minimum proportion of the virions produced in infected hosts must be transmitted to new hosts in which more virions can be manufactured.
Viruses are dependent upon chance to encounters with susceptible cells, to which they may bind if receptors on the surface of those cells come into contact with virus attachment sites
Viruses can’t move, hence they opt number of possibilities for transmission

4. Some virus infections modify the behaviour of their hosts in order to increase the probability of transmission
Some plant feeding insect larvae infected with baculoviruses become more mobile in the late stages of infection, thus aiding virus dispersal
Mammals infected with rabies virus often become aggressive; this change in behaviour increases the likelihood of the host biting another individual and transmitting virus in the saliva

5. Transmission types Horizontal transmission
In Horizontal transmission virus travels from one host to another between two individuals, generally in the same generation
Direct contact can occur due to touching, biting or licking or sexual intercourse (HIV, HCV, Influenza)
Indirect contact can occur through vectors (Dengue Virus, CLCV)
Vertical transmission
Viruses that can be transmitted directly from a parent to members of the next generation (HIV, Rubella Virus)

6. Viruses may be moved over long distances in a variety of ways
Rivers and winds
Bird migration (avian influenza viruses)
Human travel (SARS virus)
Animal export (monkey pox virus)
Once virions have entered a multicellular organism they may have further to travel before suitable host cells are encountered
Escape from host defense mechanisms
Minimum amount of virus required for infection of a host is known as the minimum infective dose
severe acute respiratory syndrome (SARS)

7. Transmission via vectors; general principles
Many viruses of plants and animals are transmitted between hosts by organisms that feed on them; these organisms act as vectors
Most vectors of viruses are arthropods, the arthropod transmitted viruses of vertebrates are sometimes referred to as arboviruses
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
Why is HIV not vector‐borne?
Mosquito RNAi is the major innate immune pathway controlling arbovirus infection and transmission.

8. Vector feed on infected host
Transfer to uninfected host
Pick the virus
Transmission happens in seconds and minutes
If virus attach to Vector mouth part 1
Non-Circulative transmission
Virus reach salivary gland and secrete through saliva
Virus enter into circulatory system 2
If virus cross the gut wall of vector
Circulative transmission
Some circulative viruses replicate in one or more tissues and organs of their vectors; thus there are viruses that can replicate in both invertebrates and plants

9. Some viruses are sexually transmitted (male to female and vice versa)
Cross-section of a mosquito . Some of the organs and tissues that may become infected by a virus acquired in a blood meal are indicated
If the reproductive organs of the vector are infected there may be possibilities for vector-to-vector transmission
Some viruses are sexually transmitted (male to female and vice versa)
Some are transmitted to the next generation within the egg; the latter is known as transovarial transmission

10. Transmission of plant viruses
Plant viruses are classified
as non-persistent
semi-persistent
and persistent
In non-persistent transmission, viruses become attached to the distal tip of the stylet of the insect and on the next plant it feeds on, it inoculates it with the virus
Semi-persistent viral transmission involves the virus entering the foregut of the insect
Those viruses that manage to pass through the gut into the haemolymph and then to the salivary glands are known as persistent

12. Transmission of plant viruses
Aphid
Nematode
Parasitic Fungi