Viruses

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Viruses

First discovered in tobacco

Found to be transmissible

Not removed by filtration (not bacteria)

Must be smaller

Virus

Small obligate intracellular parasites of translation

NB a virus outside of a cell is INERT

Needs ribosomes from infected cell

Bacteria 1-10u

Observed by light microscope

Virus 20-100 nm

Observed by electron microscope

Genome is DNA or RNA

RNA viruses can change very quickly

Foot and mouth

Influenza

Ebola

Rabies

HIV

Classification

Genome

Morphology

Ecology

Nucleic acid

DNA or RNA

Linear or segmented

Single or double stranded

Positive or negative sense

Positive sense codes for protein

mRNA is positive sense

Codes directly for protein

If you put just the viral genome into cytoplasm it can often make virus

Negative sense = complimentary strand

?

Symmetry of virus

Helical

Icosahedral (20 sides)

Some viruses have an envelope

Bud through cell membrane

Acquires lipid membrane from infected cell

Very fragile- inactivated by soap

HIV

Influenza

Poxviruses

Big DNA virus

Complex structure

Host range varies

Very narrow

Very wide

Wild animal reservoirs

Zoonotic

Control of viruses

Not usually antimicrobials

HIV is controlled by antimicrobials

Vaccination is main approach

Live or killed

Eradication

Smallpox

Rinderpest

Usually involves killing infected

Biosecurity

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Viral strategies for survival, infection + persistence

Definition of pathogenesis

Viruses

Obligate intracellular parasites

Exclusively replicate in cells

Viral replication may lead to altered cell function

Immune response

Targets viral antigens both within + outside the cell

As well as clearing infection these responses may damage both infected + uninfected cells

Pathogenesis

Mechanism by which the combined effects of virus replication + host immunity may lead to clinical disease

Important features

High adapted with ecological niche virus is in

Highly specific for host-virus combination due to co-evolution

Rarely completely understood

New therapeutics

Identification of control points

Methods and interpretation of diagnosis

Viruses pass through host

Ingestion/Inhalation/Injection

Crossing mucosal barriers

Transmitted through the blood

Identifying target cells

Getting into target cells

Replicating

Leaving target cells

Exiting the body

Canine distemper

Envelope virus

Inhalation

Cross respiratory mucosa

Lymph system

Macrophages

Lymphoid tissue

Blood (Access to whole body)

CNS (epithelial surfaces)

Reinfects respiratory mucosa

Release

Infection

Skin

Trauma

Vertebrate

Arthropod

Mechanical vector

Biological vector

Mucous membranes

Oral / intestinal

Respiratory

Reproductive

Limited local replication at site of infection

Localised epithelial spread

Further more widespread replication limited by

Temperature

Lack of basal shedding

Lack other target cells permissive for replication

Systemic infections

Epithelium to lymph nodes

Right atrium = blood system

Extremely rapid virus dissemination throughout the bnody

Primary viraemia

Low level

Early infection of limited target cells

Release of lots of viral progeny

Secondary viraemia

Further large scale target cell infection

Neurological spread

Target cell infection- the receptor

Cellular receptor largely determines

Who it infects

Which cells it infects

Uptake into cell

Target cell replication

Profound or subtle effect on cell function

Uncoating genome

+/- genome integration

Genome replication

Protein production

Virus assembly

Virus release for transmission

Scabs

Blood

Reproductive

Saliva

Faeces

Respiratory

Why do we get disease?

Extent of disease determined on

Host factors

Age

Immunity / vaccination

Genetics

Diet

Viral factors

Dose

Route

Strain

Mutation

Not an inevitable consequence of viral infection

Pressures that drive evolution of virulence

Optimal virulence

Enough to be transmitted

Not enough to kill host too quickly

Persistent infection

??

??

Organisms with a DNA genome proofread new sequences

Organisms with a RNA genome don't proof read

Error rate 1000-1000 times more

Faster mutation

Viruses released from one organism are different

Allows evolution

Escape immunity

Persistant infection

??

Genomic integration

RNA genome

Make a DNA copy of RNA genome

DNA copy added to cells genome

Provirus- any time cell replicates????

Retroviruses

Can cause tumours

?

Herpesviruses

Down regulation of viral protein synthesis

Replication in immuno-privileged sites

???

Lifelong infection

Down regulation of viral protein synthesis

Cells produce viral proteins

Switch off host proteins

Immunotolerance

BVD

Pregnant cow infected

Can cause abortion

Between 2-4 months of pregnancy

Calf becomes immune intolerant so is unable to produce a immune response to it

Sheds lots of virus its whole life

RNA virus

Mutates to very virulent form and kills calf at around 1yr old

Causal paths + webs

??

Vaccinated?

Colostrum?