Transmission of Infection

Question 1

What is the chain of infection?

Answer 1

infectious agent > reservoir > portal of exit > means of transmission > portal of entry > susceptible host

Question 2

What is virulence?

Answer 2

The ability of the microbe to cause disease

Question 3

What is dose?

Answer 3

The number of microbes entering the body

Question 4

What is the international unit for dose?

Answer 4

Infectious dose 50 (ID50)

Question 5

Why are viruses grown on tissues?

Answer 5

they cannot grow in agar

Question 6

What does a low id mean and what is ID?

Answer 6

low id = more infectious

id values compares infectivity of microbes across different species

ID50 is the amount of pathogen (dose) required to infect half of a sampled population

Question 7

What two categories do virulence factors usually include?

Answer 7

exotoxins
endotoxins

Question 8

What are examples of exotoxins?

Answer 8

P. gingivalis (protease)
S. aureus (enterotoxin & leukocidin)

Question 9

What is an example of a endotoxin?

Answer 9

Lipopolysaccharide
(released by P. gingivalis & E. coli)

Question 10

Where does e.coli usually infect?

Answer 10

urinary tract

Question 11

Which reservoir do most pathogenic microbes that infect humans come from?

Answer 11

other humans

Question 12

What are examples of different reservoirs for microbes?

Answer 12

1. humans
2. animals (anthrax)
3. environmental (clostridium tetani spores in soil)
4. fomites

Question 13

What type of bacteria is clostridum tetani?

Answer 13

gram positive bacillus

Question 14

When are microbes not easily recognised in patients?

Answer 14

when patients are health carriers (convalescent and asymptomatic)

Question 15

What is the incubation period?

Answer 15

the time between contamination and the development of symptoms

Question 16

What do long incubation periods mean?

Answer 16

• longer time periods when the infecting microbe may be spread to others
• greater spread of the disease because of more human contact

Question 17

What is a asymptomatic carrier?

Answer 17

An infected person with no clinical evidence of disease, though signs and symptoms of the disease may have been evident earlier

Question 18

What is a carrier?

Answer 18

(as well as their contacts) are usually not aware of their infectious state

Question 19

What is colonisation?

Answer 19

the presence of micro-organism (s) in or on a host, with growth and multiplication, but without any overt clinical expression (infection) at the time the micro-organism is isolated.

Question 20

What is an endogenous reservoir and why does it occur?

Answer 20

Caused by members of the normal flora if:
Flora becomes ecologically harmful due to population/gene expression shifts e.g. periodontal disease and dental caries

they become displaced to another body site or are allowed to invade deeper tissues (e.g. post-surgical infections)

Question 21

Is influenza endogenous or exogenous?

Answer 21

exogenous

Question 22

What did general dentists have raised antibodies to?

Answer 22

Flu A
Flu B
RSV

Question 23

What is the portal of exit?

Answer 23

Microbes must ESCAPE from the source to colonise a new host
* Mechanisms of escape vary depending upon the source.

Question 24

What can the mode of escape be?

Answer 24

• NATURAL (e.g. coughing or sneezing)
• ARTIFICIAL (e.g. blood donation or dental handpiece aerosols)

Question 25

What does COSHH classify human pathogens based on?

Answer 25

ability to cause infection (higher virulence) severity of disease vaccine and treatment availability risk of population spread

Question 26

What is the Ro?

Answer 26

reproductive number the number of cases one case generates on average over the course of its infectious period.

Question 27

Ro < 1 VS Ro > 1

Answer 27

When Ro< 1 infection will die out in the long run When Ro>1 infection will be able to spread in a population

Question 28

What factors affect the Ro?

Answer 28

Ÿ Duration of infectivity Ÿ Infectiousness Ÿ Number of susceptible people

Question 29

How do respiratory viruses exit?

Answer 29

Droplets & aerosols Respiratory secretions

Question 30

Airborne vs Droplet | precautions

Answer 30

Airborne precautions = less than 5 microns, tiny particles, travel further and remain in air longer (respiratory precautions) Droplet precautions = more than 5 microns, large particles (non respiratory precautions)

Question 31

What respiratory infections are accepted to be mostly transmitted by the airborne (aerosol route)?

Answer 31

a. tuberculosis, b. measles c. chickenpox

Question 32

If the particle is larger, will it fall slower or faster than smaller?

Answer 32

larger = slower

Question 33

What is an aerosol?

Answer 33

An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas

Question 34

What does survival depend on?

Answer 34

persistence, stability, retention of infectivity

Question 35

What is fomite mediated transmission dependant on?

Answer 35

Environmental factors Time between reservoirs Efficiency of transfer Dose of virus

Question 36

Why is fomite transmission difficult to prove?

Answer 36

respiratory transmission from asymptomatic people cannot be ruled out

Question 37

What is a barrier to fomite transmission?

Answer 37

hand hygiene

Question 38

How can aerosols be mitigated in dentistry?

Answer 38

-Use of rubber dam -High/low volume aspiration -Surgery ventilation

Question 39

How many air changes per hour?

Answer 39

10

Question 40

Where are the portals of entry?

Answer 40

*Respiratory tract (influenza) *Gastro-intestinal tract (norovirus) *Open/surgical wounds (staph. aureus) *Medical devices/Sharps injuries (Hep b) *Sexual contact (HIV)

Question 41

How can influenza virus enter?

Answer 41

*Respiration (aerosols) *Inhalation (all particles) *Direct contact (droplets) *In-direct contact (settled particles – face touching) * Role of conjunctiva uncertain

Question 42

What is a susceptible host?

Answer 42

Is a person who is at risk of infection as they are unable to fight the infection due to; *An underdeveloped immune system (neonate) *A declining immune system (elderly) * Disease or drugs that impair host defences *Breaks in the skin *Medical devices

Question 43

What are the risk groups?

Answer 43

Elderly, young children, pregancy & chronic medical conditions (diabetes, asthma, heart disease)