transmission of infection

Question 1

what is in the chain of infection?

6

Answer 1

• susceptible host
• infectious agent
• reservoirs
• portal of exit
• means of transmission
• portal of entry

Question 2

give examples of reservoirs

6

Answer 2

• humans
• equipment/formites
• the environment
• food
• water
• animals

Question 3

give examples of portal of exit

4

Answer 3

• blood and other bodily fluids
• skin scales
• coughing and sneezing
• droplets and aerosols

Question 4

give examples of mode of transmission

6

Answer 4

• direct physical contact
• contaminated objects
• air via coughing
• contact with blood or body fluids
• contaminated food/water ingestion
• insects or animals

Question 5

give examples of portal of entry

6

Answer 5

• open/surgical wounds
• broken skin
• eyes or mouth
• respiratory tract
• intestinal tract
• tubes inserted into the body

Question 6

give examples of susceptible hosts

5

Answer 6

• under-developed immune system (infants)
• weakened immune system (elderly, cancer pts)
• breaks in skin
• tubes inserted into the body
• drugs/disease that lower defence against infections

Question 7

give examples of infectious agents

3

Answer 7

• bacteria
• viruses
• fungi

Question 8

what is virulence?

Answer 8

the ability of a microbe to cause disease

Question 9

what is infectious dose?

Answer 9

the number of microbes entering the body
* usually expressed as ID50 which is the international reference standard to compare infectivity of microbes across different species and under different conditions

Question 10

what are the two virulence factors?

Answer 10

exotoxins
endotoxins

Question 11

what are exotoxins?

Answer 11

• produced by gram positive and negative bacteria
• secreted into the external environment
• p. gingivalis (protease)
• s. aureus (enterotoxin and leukocidin)

Question 12

what are endotoxins?

Answer 12

• produced by gram negative bacteria
• released from cell walls
• p. gingivalis and e. coli (lipopolysaccharides)

Question 13

what is the incubation period?

Answer 13

the time between contamination and the development of symptoms

Question 14

what do longer incubation times permit?

2

Answer 14

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

Question 15

what is an asymptomatic carrier?

Answer 15

an infected person with no clinical evidence of disease

Question 16

what is an endogenous reservoir?

2

Answer 16

caused by members of the normal flora if:
* the flora becomes ecologially harmful due to population/gene expression shifts e.g. periodontal disease or caries
* they become displaced to another body site or are allowed to invade deeper tissues e.g. post-surgical infections

Question 17

what is an exogenous reservoir?

Answer 17

diseases caused by microbes from external sources e.g. influenza

Question 18

what are the four hazard groups that COSHH classifies human pathogens?

Answer 18

• ability to cause disease
• severity of the disease that may result
• vaccine and treatment availability
• risk of population spread

Question 19

what is R0 value? what does it mean if it is more or less than 1?

3

Answer 19

• the number of cases one case generates on average over the course of its infectious period
• when R0>1, the infection will die out in the long run
• when R0<1, the infection is able to spread in a population

Question 20

what factors affect R0?

3

Answer 20

• duration of infectivity
• infectiousness
• number of susceptible people exposed to pathogen

Question 21

what is the cut off size for an aerosol?

Answer 21

5μm

Question 23

what is the difference between an aerosol and a droplet?

Answer 23

• aerosols are small enough to remain airborne for long periods of time
• droplets are larger so fall to the ground relatively quickly

Question 24

what factors affect aerosol transmission?

3

Answer 24

• physiochemical properties of virus-laden aerosols (size, viral load and infectivity, other chemical components, pH value, electrical charge)
• viral determinants of virus survival and transmission (envelope and capside structure)
• environmental determinants of virus survival (temperature, humidity, pH, ventilation and airflow, salinity, surface materials, UV radiation)

Question 25

how can transmission of aerosols be mitigated in dentistry?

3

Answer 25

• use of rubber dam
• high/low volume aspiration
• surgery ventilation (10 air changes per hour)