Spread and replication

Question 1

surface infection

Answer 1

multiply in epithelial cells at site of entry
does not go deeper in tissue
can spread to other areas (ie. fluid flow -coughing, sneeze, mucous or GI and resp tract)
large area of body may be covered
Innate IR is involved
not enough time for adaptive response

Question 2

systemic infection

Answer 2

shed via blood/lymph
migration from surface to deeper tissue
enduring increased immune response
eg. meningococci from nasal mucosa to meninges (membrane layers of the brain)

Question 3

Factors affecting location of infection

Answer 3

-temperature eg Rhinovirus URT (33C) not LRT (37C)
mycobacterium leprae -nasal mucosa, skin and superficial nerves
-budding eg influenza, parainfluenza viruses -released from the lung epithelial cells not basal layers
-systemic control -failure to spread and multiply at site of primary infection
measles RT but replicates in nerve cells
Typhoid -GI but replicates in liver/spleen
HAV -alimentary canal -liver
Mumps -resp canal -salivary glands

Question 4

mechanisms of systemic spread

Answer 4

spread to blood and lymph

stopped by physical barriers, antimicrobial compounds, local macrophages and phagocytosis

Question 5

spread from blood

Answer 5

easy access, transient bacteremia
filtered out and destroyed by macrophages (liver and spleen)
localization at less well defended sites -congenital abnormal heart valves -viridans streptococcus “attacked by their heart”
if free in blood can encounter Ab and phagocytes unless associated with circulating cells in blood stream eg. rubella and listeria

circulating pathogen invades characteristic target organs and tissue
reasons not fully known, possibly:
tissue receptors
only some organs suitable for replication
areas with inflammation could trap microbes

Question 6

Spread via nerves

Answer 6

viruses -> CNS to PNS (rabies, HSV, VZV)
travel in axons to CNS and back to peripheral nerves during recurrent outbreaks
host defenses unable to control viral spread in nerves

Question 7

spread via CSF

Answer 7

spread in CSF rapid once BBB is crossed

Question 8

spread in pleural and peritoneal cavities

Answer 8

microbes can spread from one visceral organs to another through the peritoneal or pleural cavity

injury/ disease in abdominal organ may lead to infections eg peritonitis (abdominal organ failure)

Question 9

Genetic determinants of spread and replication for host

Answer 9

narrow vs broad host range
only humans or closely-related primates eg measles virus
wide range of hosts eg rabies virus
genetic determinants of susceptibility eg humans, sickle cells anemia and malaria (HAEM resistant)

Question 10

genetic determinants for spread and replication of pathogen

Answer 10

virulence:
adhesion 
cell penetration 
toxin production 
interaction with immune system 
attenuation, serial passaging on artificial media -(losing virulency)