Viral Pathogenesis

Question 1

How do viruses make us sick

Answer 1

• how do viruses get into our bodies
• why are viral infections so miserable
• how exactly do viruses damage the host
• what do we know about how viruses cause disease

Question 2

where do we encounter viruses

Answer 2

• people
• animals
• insects
• food and water

Question 3

mechanisms of viral transmission

Answer 3

• respiratory: aerosols
• 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

Question 4

Susceptibility and severity of viral disease depend on

Answer 4

• the nature of exposure- route- aerosol vs scratch
• the viral dose- more virions increases risk of disease
• the status of person- age, general health, immune status
• the virus-host interactions- unique genetic features of each

Question 5

Routes of entry, disemination and shedding in host example: smallpox

Answer 5

-acquired through respiratory tract, disseminates in blood and sheds from postules on the skin

Question 6

Virus sites of enry

Answer 6

• in and out of conjunctiva
• in and out of respiratory tract
• in of alimentary tract
• in and out of urinogenital tract and anus
• scratch injury
• arthropod (bug)
• capillary

Question 7

Sites of virus entry in the respiratory host

Answer 7

• turbinate baffles
• tonsillar lymphoid tissues
• cervical lymph node
• esophagus
• trachea
• bronchi
• bronchioles
• bronchial lymph node
• alveolus

Question 8

Virus entry into M cells in GI tract

Answer 8

• M cell sample the gut contents and present it to underlying immune cells
• viruses can infect M cells and easily reach the blood stream
• Reovirus attach to M cells

Question 9

Virus Dissemination

Answer 9

• virus may spread from the surface of the body to lymph nodes and the blood stream
• primary viremia leads to
• replication in internal organs
• may occur without symptoms (incubation stage)
• secondary viremia disseminates the virus to organs where it is shed
• transmission may be by direct contact or through the environment (air, water, objects)
• exposure to infected blood is now a common route of transmission

Question 10

Excretion of HIV-1

Answer 10

• HIV was measured by PCR
• most virus was in blood plasma and lymphocytes and in CSF
• sperm had less virus than semen
• in ear wax- weird

Question 11

Virus Host interactions

Answer 11

• virus infection may be unnoticed, cause illness, induce autoimmunity, be persistent, or be lethal
• a successful virus will avoid destruction by the immune system and avoid destroying the host before replication is finished

Question 12

General patterns of infection

Answer 12

• acute- common cold
• persistent
• latent-varicella-zoster
• slow
• transforming

Question 13

Injury induced by viruses

Answer 13

• symptoms of viral disease (fever, tissue damage, rash, aches, pains, nausea) are mainly caused by the host response to infection
• virus replication -> cell injury <-host response
• cell injury is caused directly by viruses and indirectly by the host
• direct effects: cell lysis

Question 14

Direct effect of virus: cell inactivation

Answer 14

• virus infection may halt essential cell functions
• infected cells are susceptible to apoptosis
• loss of cell functions can lead to organ damage or failure

Question 15

Indirect effects of immunopathology

Answer 15

• the host immune response to a virus may be the sole cause of disease
• immune pathology usually caused by T cells and antibody complexes
• caution: vaccination can make some viral infections worse
• corneal scarring caused by immune response to chronic HSV reactivation

Question 16

Factors that contribute to a viral infection

Answer 16

• Host: immune status, route of exposure, age, habits, barriers to dissemination, contagiousness
• Virus: antigenic diversity, infectious dose, cell killing or inactivation, pattern of infection, ability to diseeminate, shedding