Communicable diseases: public health concepts

Question 1

What are the stages of the chain of infection?

Answer 1

1) Infectious agent
2) Reservoir
3) Portal of exit
4) Mode of transmission
5) Portal of entry
6) Susceptible host

Question 2

What is a reservoir?

Answer 2

examples: human, animal, arthropod, soil, plant, an object in which the agent can live and multiply

It isn’t necessarily the source or vehicle of infection

Question 3

What are zoonoses?

Answer 3

infectious diseases transmissible from animal reservoirs to humans

Question 4

What is the portal of exit?

Answer 4

the path by which the agent leaves the reservoir/host - usually corresponds with the site at which infection/the agent is localised

Question 5

What is the path of entry?

Answer 5

path by which the agents enters the susceptible host

Question 6

What is direct transmission?

Answer 6

Direct route = via contact with skin, mucous membranes, blood or other bodily fluids
Respiratory route = droplet spread: sneezing, coughing, singing, talking - usually limited to spread of 1m or less
Faecal-oral route = faecal material transferred directly to mouth e.g. on hands

Question 7

What is indirect transmission?

Answer 7

Vehicle-bourne route = via contaminated inanimate objects or materials (fomites) e.g. toys, cooking utensils, food or water (faecal-oral route), surgical utensils = the agent may or may not multiply or develop on the vehicle prior to transmission
Vector-borne route = either “mechanical” (e.g. simple carriage by a crawling/flying insect from one point to another) or “biological” involving some form of multiplication or development of the agent before transmission is possible

Question 8

What is airborne spread?

Answer 8

Dissemination of fine microbial aerosols (droplet nuclei or dust) which may remain suspended in the air for some time

Question 9

What is the epidemiological framework called?

Answer 9

Epidemiological triangle - interaction between host, agent and environment

Question 10

What needs to be considered about the host in terms of the epidemiological triangle?

Answer 10

its susceptibility, its likelihood of exposure, and response to agent - these factors will determine severity of infection and outcome - incl. sex; age; genetics; nutrition ; immunological status; co-morbidities; occupation; psychologically factors; ethnicity

Question 11

What needs to be considered about the agent in terms of the epidemiological triangle?

Answer 11

organism, infectivity, pathogenicity, virulence, reservoir, and ability to survive in the environment

Question 12

What needs to be considered about the environment in terms of the epidemiological triangle?

Answer 12

extrinsic factors that affect the agent and the opportunity for exposure e.g. geology, climate, physical surroundings, over-crowding, sanitation, occupational factors

Question 13

Define infectivity:

Answer 13

likelihood that a person will become infected following exposure - typically varies during the period of communicability (and may be maximal before the onset of the symptoms)

Question 14

Define pathogenicity:

Answer 14

likelihood that an infected person will suffer clinical disease - proportion of infected persons who develop clinically apparent disease

Question 15

Define virulence:

Answer 15

likelihood that an infected person will become severely ill or die - proportional of infected persons who become severely ill or die

Question 16

Define incubation period:

Answer 16

time between acquiring and onset of symptoms- may be affected by infecting dose - important in outbreak control in order to determine likely time of exposure and potential for further cases to become apparent

Question 17

Define latent period:

Answer 17

time between acquiring infection and becoming capable of transmitting the infections to others

Question 18

Define period of communicability:

Answer 18

infectious period - time during which an infected person is capable of transmitting the infection