Mechanisms of Viral Virulence and Host Resistance

The outcome of a virus infection (asymptomatic infection, disease or death) depends on characteristics of both the _______ and the _________

virus; host

Infectivity/transmissibility/contagiousness

measure of ease of transmission

Host resistance

the ability of the host to control or eliminate viral infection or viral- induced disease

Pathogenicity

ability to cause disease; general ability of a virus to infect members of a species

True or false: a virus is not pathogenic if a host was infected and it did not cause disease

false!

Viral virulence

a measure of pathogenicity of

viruses; degree of severity of a infection linked to clinical signs

Three examples of measures of viral virulence

-asymptomatic

-mild, moderate, severe

-death

Five factors that impact virulence:

1.) Infectious dose

2.) Route of infection

3.) Tissue tropism

4.) Virus dissemination

5.) Genetic determinants of virulence encoded in individual virus strains

Infectious dose

minimum number of microbes required for infection to proceed in under controlled conditions

LD50

lethal dose which is required to kill 50% of infected animals

ID50

infectious dose which is required to infect 50% of inoculated animals

Route of infection

way in which the virus enters the host; right route of infection is important for infectivity (such as a respiratory virus entering through the nose rather than being ingested and destroyed by GI acid)

Tissue tropism

the range of tissue types that a virus can infect; cells must be susceptible and permissive to support viral replication

Cells are susceptible to viruses if...

they have the corresponding receptor to a receptor binding protein on the virus

Cells are permissive to viruses if...

they provide intracellular components necessary for virus replication

Pantropism

the ability of some viruses to infect multiple cell types in multiple organ systems

Virus dissemination

how a virus spreads through a host body or from one host to another

The more locally ________ a viral infection is, the more likely it will infect other animals

widespread

ex: if a virus stays local to the respiratory tract, it only has one way to infect other animals

if a virus spreads throughout the body and infects multiple organ systems, there will be more routes for sheddings and therefore more likely to infect other animals

Genetic determinants of virulence encoded in individual virus strains

some viruses have properties encoded in their genome that make them more virulent

Two viruses that have virulence factors encoded in their genome:

1.) Canine Parvovirus

2.) Influenza Virus

Canine Parvovirus genetic virulence factor

amino acid changes in the capsid protein that binds to the receptor on cells was responsible for a species switch from feline panleukopenia virus to canine parvovirus

Influenza A Virus genetic virulence factor

has a segmented RNA genome; for every viral protein there is a viral gene segment, so genes are not like "pearls on a chain", but individual "pearls"; when a point mutation on a gene segment occurs, this can lead to an epidemic

antigenic drift

Minor change in influenza virus antigens due to gene mutation

antigenic shift

major change in influenza virus antigen due to gene reassortment (when two strains of influenza infect the same animal and switch genes)

Host resistance

the ability of an organism to withstand a virus infection

Five factors affecting host resistance:

1.) expression of cellular receptors for viruses

2.) metabolic activity

and maturation status of the cell

3.) factors influencing the immune status

4.) age

5.) nutrition and husbandry conditions

host resistance factors: expression of cellular receptors for viruses

in order for a cell to be infected by a virus, it must have the corresponding receptor; if the cell does not have the receptor, the virus cannot infect it

Ex: Rabies: conserved receptor on neurons in multiple species, but restricted tropism in body

Influenza: Restriction usually to airway epithelial cells (exceptions are highly virulent strains which spread systemically)

host resistance factors: metabolic activity and maturation status of the cell

cells that are actively replicating are more likely to be infected by viruses

Three examples of actively replicating cells:

1.) lymphocytes

2.) epithelial cells in the GI tract

3.) fetal and neonatal tissues

host resistance factors: factors influencing the immune response

things such as pregnancy, stress, and corticosteroids (which suppress the immune system) will make an animal more susceptible to viral infection

host resistance factors: age

older animals (declining immune response) and younger animals are more likely to be infected by viruses

Two reasons young animals are more susceptible to viral infection:

1.) immature immune system

2.) rapidly replicating cells in the heart, brain, and intestines

What might protect young animals from certain viruses?

milk proteins in suckling animals will protect the GI tract of young animals from enveloped GI viruses

host resistance factors: nutrition

inadequate dietary intake makes an animal more susceptible to viral infection

Why would inadequate nutrition make an animal more susceptible to viral infection?

inadequate nutrition leads to weight loss, growth failure, and lowered immunity, which increases the incidence, severity, and duration of viral infection