What are some mechanisms of viral transmission?
Fecal-oral: food, water, dirty hands
Contact: lesions, saliva, fomites
Zoonoses: animals, insects
Blood: direct contact, blood products, organ transplants
Sexual: mucous membranes, blood
Maternal-neonatal: birth, breastmilk
Genetic: prions, retroviruses
What factors affect the susceptibility and severity of a viral disease?
Nature of exposure: route; i.e., aerosol vs. scratch, etc.
Viral dose: more virus increases risk of disease
Status of the person: age, general health, immune status
Virus-host interactions: unique genetic features of each
What are some general sites of entry for virus?
How do viruses use the respiratory tract as a site of entry?
- Interact with cilia, OR
- Cross basement membrane → lymph and blood system
How do viruses use the GI tract as a site of entry?
M cells sample the gut contents and present it to underlying immune cells. Viruses can infect M cells and easily reach the blood stream.
ex) Reovirus attaches to M cells and intracellular vesicles after fecal-oral transmission
What are the stages of virus dissemination throughout the body?
1° viremia = low levels of virus in the blood → replication in internal organs; often asymptomatic (incubation stage) →
2° viremia: high levels of virus in blood → dissemination of virus to organs, where it is shed
ex) Chicken pox and shingles (VZV): mucosal surfaces → lymph nodes → liver, spleen, respiratory system → skin → latency in neuron
What are some patterns of viral infection?
Viral infection may be unnoticed, cause illness, induce autoimmunity, be persistent, or be lethal. A successful virus, however, will avoid destruction by the immune system AND avoid destroying the host before replication is finished. Patterns include:
- Chronic: persistent, latent (VZV), and/or slow
(Blue = virus shedding, green = disease symptoms)
What is injury induced by viruses?
Symptoms of viral disease (e.g., fever, tissue damage, rash, aches, pains, nausea, etc.) are MAINLY caused by the host response to infection.
Cell injury, on the other hand, is caused DIRECTLY by viruses (e.g., cell lysis, Norwalk virus, cell inactivation) and INDIRECTLY by the host (e.g., immunopathology).
What is cell inactivation?
A virus-induced cell injury, directly caused by the virus (as opposed to the host).
Essential cell functions halted by virus → infected cells susceptible to apoptosis → loss of cell functions → organ damage/failure
What is immunopathology?
A virus-induced cell injury, indirectly caused by host immune responses.
Immune pathology is usually caused by T cells and antibody complexes. Vaccination can make some viral infections worse.
ex) dengue fever, RSV