7. Infection transmission

Question 1

What is a ‘common source’ in terms of infection?

Answer 1

Source (environmental, food/water, animal, human) of infection that gives rise to a number of cases in 1 area.

Question 2

Give an example of indirect person-to-person transmission.

Answer 2

malaria transmitted person-to-person via a vector (female mosquitoes)

Question 3

What is the endemic level of a disease?

Answer 3

the usual background rate of a disease in a community

Question 4

What is a disease ‘outbreak’?

Answer 4

2 or more cases linked in time and place

Question 5

What is a disease ‘epidemic’?

Answer 5

• rate of infection greater than usual background rate

- caused by antigenic drift (accumulation of mutations within genes that code for antibody-binding sites)

Question 6

What is a disease ‘pandemic’?

Answer 6

• very high rate of infection spreading across many regions (e.g. countries/continents)
• • caused by antigenic shift (2 or more different strains of a virus combine to form a new subtype having a mixture of the surface antigens of the original strains)

Question 7

What viral characteristic determines the size of an outbreak?

Answer 7

Basic reproduction number (R0): average no. of cases 1 case generates over the course of its infectious period, in an otherwise uninfected, non-immune population.
If R0 >1 - increase in cases
If R0 = 1 - stable no of cases
If R0 <1 - decrease in cases

Question 8

Name 3 possible reasons for outbreaks/epidemics/pandemics.

Answer 8

1. new pathogen (or antigens/virulence factors/antibacterial resistance)
2. new hosts (immunocompromised, healthcare effects)
3. new practice (social eg tattoos, healthcare)

Question 9

What is the infectious dose of a pathogen? How does this vary?

Answer 9

No. of microorganisms required to cause infection. Varies by:

• microorganism
• presentation of microorganism
• immunity of potential host

Question 10

Give an example of a pathogen with a very high and a very low infectious dose.

Answer 10

• very high: salmonella (>10^5 organisms)

- very low: E. coli (<10 organisms)

Question 11

What interventions at the pathogen/vector level can reduce transmission?

Answer 11

1. reduce/eradicate pathogen
   - antibacterials, inc. disinfectants
   - decontamination
   - sterilisation
2. reduce/eradicate vector
   - eliminate vector breeding sites

Question 12

What interventions at the patient level can reduce transmission?

Answer 12

1. improved health
   - nutrition
   - medical treatments
2. immunity
   - passive, e.g. maternal antibody, intravenous immunoglobulin (e.g. for immunocompromised)
   - active, i.e. vaccination - protects individual and herd immunity

Question 13

What interventions at the practice level can reduce transmission?

Answer 13

avoidance of pathogen or its vector

1. geographic (‘don’t go there’)
2. protective clothing, equipment, e.g.
   - long sleeves against mosquito bites
   - personal protective equipment in hospitals(gowns, gloves, masks)
3. behavioural
   - safe sex
   - safe disposal of sharps

Question 14

What interventions at the environmental level can reduce transmission?

Answer 14

environmental engineering: safe water, safe air, good quality housing, well-designed healthcare facilities

Question 15

What are positive and negative consequences of disease transmission control?

Answer 15

Good:
i) decreased incidence or elimination of disease/organism (smallpox, polio, dracunculiasis)

Bad:

i) decreased exposure to pathogen… decreased immune stimulus… decreased antibody… increased susceptibility… outbreak
ii) later average age of exposure… increased severity