Viruses and what they do -

An overview

Wednesday, August 25, 2010

Viruses (Encyclopedia Britannica)

..infectious agents of small size and simple composition that can multiply only in living cells of animals, plants and bacteria. Viruses are obligate parasites that are metabolically inert when they are outside their hosts. They all rely, to varying extents, on the metabolic processes of their hosts to reproduce themselves. The viral diseases we see are due to the effects of this interaction between the virus and its host cell (and/or the host’s response to this interaction).

Viral Genomes

Nucleic Acid

DNA

RNA

Double Stranded

Positive

Negative

RNA DNA

Single Stranded

Double Stranded

Single Stranded

SS RNA genomes

Positive(sense)

Negative(anti-sense)

AUG GCA CGA

UAC CGU GCU

met ala arg

• +ve (sense) and -ve (anti-sense) RNA genomes

Virion

capsomeres

Capsid

envelopedVirus or Virion

envelope

capsid

“naked” virus particle or Virion

herpesadeno

Proteins produced by viruses

• Structural proteins• Non-structural proteins

Some viral shapes

adenovirus

parvovirus

papillomavirus

100 nm1 nm = 1 millionth of a mm100 nm = 1 ten thousandth of a mm

“naked” viruses

Some viral shapes

herpesvirus

parainfluenzavirus

influenzavirus

poxvirus

1 nm = 1 millionth of a mm100 nm = 1 ten thousandth of a mm

100 nm

Enveloped viruses

Taxonomy

• What is it?• On what is it based?• Is it important?• Do I need to remember all the details?

International Committee onTaxonomy of Viruses

Viruses with ss DNA genomes

Circoviridae

Parvoviridae

porcine circovirus

canine parvovirus-2

feline panleukopenia virus

porcine parvovirus (SMEDI)

Viruses with ds DNA genomes

Poxviridae

African swine fever virus

Herpesviridae

Adenoviridae

Papovaviridaepapillomaviruses

adenoviruses

bovine herpesvirus-1,2

porcine cytomegalovirus

equine herpesviruses -1,4

african swine fever virus

poxviruses

malignant catarrhal fever virus

Viruses with ds RNA genomes

Reoviridae

Birnaviridae

rotaviruses

bluetongue virus

african horse sickness

infectious bursal disease (chickens)

infectious pancreatic necrosis (salmonid fish)

Viruses with +ve RNA genomes

PicornaviridaeCaliciviridae

Coronaviridae

ArteriviridaeFlaviviridae

Togaviridae

foot and mouth disease virus

porcine enteroviruses

feline calicivirus

equine arterivirus, PRRS

pestiviruses (BVD)

coronaviruses

equine encephalitis viruses

flaviviruses (WNV)

Viruses with -ve RNA genomes

Paramyxoviridae

Rhabdoviridae

Orthomyxoviridae

Filoviridae

Bunyaviridae

parainfluenza viruscanine distemper virus

respiratory syncytial virus

rabies virus

vesicular stomatitis virus

influenzaviruses

Ebola virus

Haantan virus

Hendra, Nipah viruses

Viruses with reverse transcriptase

Retroviridae

Hepadnaviridae

feline leukemia virus

feline, bovine immunodeficiency

viruses

bovine, avian leukosis viruses

caprine arthritis-encephalitis virus

How new serotypes arise – gradual changes in external proteins due to pressure by neutralizing antibodies

Antigenic classification:Serotypes and Groups

antibodies to all viral proteins

selective pressureonexternal viral proteins

antibodies to external proteins neutralize virus

no selective pressure on internal proteins

antibodies to all viral proteins

selective pressureonexternal viral proteins

antibodies to external proteins neutralize virus

selective pressure forces selection of virions with slightly different external proteins

antibodies to all viral proteins

selective pressureonexternal viral proteins

antibodies to external proteins neutralize virus

selective pressure forces slight change in external proteins

virus, including changed virus passed on to new host

process repeated, over time…..

serum from original cat

neutralizesneutralizes neutralizes neutralizes

does not neutralize

NOTE: Only external proteins change. Internal proteins do not change

process repeated, over time

serum from original cat

neutralizesneutralizes neutralizes neutralizes

does not neutralize

new serotype

NOTE: Only external proteins change. Internal proteins do not change

same serotype

Serotype - all isolates of a virus that can be neutralized by a

common antiserum are said to belong to the same serotype.

…..because of changes in external protein (internal proteins do not change)

external proteins are called TYPE SPECIFIC antigensinternal proteins are called GROUP SPECIFIC antigens

process repeated, over time

new serotypesame serotype

different serotypes

same group

Groups, types (sero-types), isolates and ‘strains’

Group

Type -A

Type - B

Type - C

Group specific antigen

Type -A specificantigen

Type - C specificantigen

isolate

Group and type specific antigens

group specific antigen

type specific antigen

“naked”virus(eg FMDV)

enveloped virus(eg influenza, FeLV)

Serotypes and neutralizing antibody (eg. FMDV)

serotypes of FMD virus

A OC SAT1 SAT2 SAT3 Asia

antibodies against receptor binding protein of serotype A will neutralize viruses of serotype A but not of serotype C

receptor

receptor binding protein on viral surface

example - influenza

group specific antigen

type specific antigen

serotype H1 serotype H5 serotype H7

test based on group specific antigen will detect all three

vaccination against one serotype will not protect against others

Infection of a cell

Stage Biological implications

Host defenses

Drug intervention

Distribution of the CCR532 mutation in human populations

from PLoS Biology, Nov 2005

Errors in replication lead to “quasispecies”

persistentinfection

mixture of variant viruses(quasispecies)

inclusion bodies

Release of virus

Release by lysis of cell(cytopathic)

or by budding (withoutdeath of cell, non-cytopathic)