Viral pathogenesis

Question 1

Incubation period

Answer 1

Initial period before disease symptoms are obvious
- can be transmitted
- Signs are present:
Viral genomes replicating
Host is responding

Question 2

Incubation periods for various diseases

Answer 2

Arbovirus - short (>1week)
Measles - Medium (7-21 days)
Rabies- Long (weeks to months)
Prion diseases; measles/subacute sclerosing panencephalitis - Very long (Usually fatal or years)

Question 3

Disease patterns

Answer 3

Acute non persistent
Latent
Persistent - asymptomatic
Persistent pathogenic
Insidious infections with fatal outcomes

Question 4

Dissemination of infection

Answer 4

movement of virus once it penetrates the epithelium

Directional release of virus from polarised cells can influence the outcome of the infection

Question 5

Apical release

Answer 5

Virus release to internal lumen side of epithelial cells
Virus does not usually invade underlying tissues

Question 6

Basolateral Side

Answer 6

Virus releases to external tissues
Virus has access to underlying tissues and this may allow systemic spread

Question 7

Hematogenesis Spread

Answer 7

Entry of viruses via bloodstream
Can occur via dissemination from epithelial infection or inoculation by a vector bite into capillaries

Replicated viruses in extracellular fluid can be taken up by Lymphatic vessels and drained into the circulatory system

Question 8

Lymphatic systems in virus spread

Answer 8

Lymphatic systems drain extracellular fluid into the bloodstream giving a pathway to viruses

With lymphatic vessels viruses can also be exposed to lymphocytes they may be tropic for

Question 9

Viraemia

Answer 9

describes the presence of
infectious virus particles in the blood

amount of virus in blood

Question 10

Active viremia

Answer 10

Class of Viremia
produced by virus replication

Question 11

Passive viremia

Answer 11

virus particles are introduced
into the blood without viral replication at the site
of entry

Question 12

Secondary viremia

Answer 12

subsequent disseminated
infections; appearance of a high concentration of
infectious virus in the blood

Question 13

Pathogenesis of measles from virus infection to recovery

Answer 13

1. virus entry via respiratory tract
2. primary replication
3. spread and tropism
4. cell injury and clinical illness
5. recovery from infection - immunosuppression
6. virus spreading and transmission

Question 14

Organ invasion

Answer 14

Post dispersal from infection site into blood invasion of new cells and tissues is required for replication

Question 15

3 main types of blood vessel/tissue junction for virion entry

Answer 15

continuous endothelium and basement membrane found in CNS, connective tissue, muscle, skin and lungs

Fenestrated epithelium found in choroid plexus, intestinal villi, renal glomerulus, pancreas, and endocrine glands

Sinusoid, lined with macrophages of the reticuloendothelial system, as found in adrenal glands, liver, spleen and bone marrow

Question 16

Liver, spleen, bone marrow and adrenal glands

Answer 16

Organs all characterised by the many gaps (sinusoids), lined with macrophages

Macrophages are often infected by viruses in the blood, making these areas highly susceptible

Question 17

CNS, Connective tissue and skeletal and cardiac muscle

Answer 17

endothelial cells are backed by a dense basement membrane

In the CNS the basement membrane is the foundation of the BBB

Viruses may pass though via the CSF into the brain

May also cross BBB in cells or via transcytosis

Question 18

Neural spread

Answer 18

Spread via the infection of nerve endings

Termed “neurotropic”

Virus usually infects cells of the PNS first and then spreads to the spinal cord

Question 19

Aseptic meningitis

Answer 19

inflammation of the meninges caused by increased mononuclear cells
- viral infection most common cause
- non infectious causes do occur

Question 20

Encephalitis

Answer 20

Infection and inflammation of the brain

May be cause of an acute infection or the complication of a latent virus

Mostly caused by enterovirus, HSV, rabies, arboviruses

Question 21

encephalomyelitis

Answer 21

inflammation of the spinal cord and brain

Question 22

meningoencephalitis

Answer 22

Encephalitis with meningitis

Question 23

Symptoms of meningoencephalitis

Answer 23

Symptoms: headache, fever, confusion, drowsiness; more
advanced disease includes convulsions, seizures, hallucinations,
memory loss and coma; can be fatal

Question 24

Acute flaccid paralysis

Answer 24

Acute flaccid paralysis
- symptom of CNS viruses
- May be caused by anterior horn cell infection with wild or vaccine polio virus, or other viruses ʹ e.g. Enterovirus 71

Question 25

HIV

Answer 25

HIV enters the nervous system early, at the time of initial infection, and may immediately cause symptoms, or may cause symptoms across a person’s lifetime

Question 26

HIV in the nervous system

Answer 26

easily crosses the BBB
Causes: AIDS Dementia Complex (brain), Vacuolar Myelopathy (spinal cord), Peripheral Neuropathy (nerve), Meningitis (acute or chronic)

Question 27

Zika virus in babies

Answer 27

Zika virus infection during pregnancy can cause infants to be born with microcephaly and other congenital malformations, known as congenital Zika syndrome and preterm birth or miscarriage

Question 28

Viral shedding

Answer 28

Viruses shed from an infected host can infect new susceptible hosts

Viruses may be shed from primary site of multiplication or from target organ

Healthy individuals may shed viruses continuously – HSV in saliva; CMV in urine and
breast milk; viruses infecting GIT in faeces and vomit

Question 29

R0

Answer 29

R0 is the average number of secondary cases generated
by one primary case in a susceptible community

The higher the R0, more persons infected from one
primary case

Question 30

Types of viral transmission

Answer 30

latrogenic: activity of a healthcare worker leads to infected patient
Nosocomical: individual is infected in hospital or healthcare settings
Vertical: from parents to offspring
Horizontal: all other forms of transmission
Germ line: transmission as part of the host genome