Virus transmission

LECTURE 7:

Viro100: Virology3 Credit hoursNUST Centre of Virology & Immunology

Waqas Nasir Chaudhry

Virus infectionEpidermal

cells

Mesophyll cells

Bundle sheath cells

PhloemParenchyma

Companion cells

Phloem

Other host plants

Replication

Plant-to-plantmovement

Cell-to-cellmovement

Systemic

movement

Stages of viral infection cycle

• 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

• 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

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)

• 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

Transmission via vectors; generalprinciples

• 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

Vector feed on infected host Pick the virus Transfer to

uninfected host

If virus attach to Vector mouth

part

Transmission happens in

seconds and minutes

If virus cross the gut wall of vector

Virus enter into circulatory

system

Virus reach salivary

gland and secrete through

saliva

1

2 Circulative transmission

Non-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

• 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

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

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

Transmission of plant viruses

• Aphid• Nematode• Parasitic Fungi