Week 14 - Epidemiology of Infectious Diseases

Question 1

What is Epidemiology?

Answer 1

Science that evaluates occurrence, determinants, distribution and control of health and disease in a population

Question 2

What is an Endemic Disease?

Answer 2

Maintains relatively low-level frequency at a moderately regular interval

Question 3

What is an Epidemic?

Answer 3

Sudden increase in frequency

Index case - first case in an epidemic

Question 4

What is an Outbreak?

Answer 4

Sudden, unexpected occurrence of disease

Usually focal or in a limited segment of population

Question 5

What is a Pandemic?

Answer 5

Increase in disease occurrence within large population over wide region

Question 6

What are Zoonoses?

Answer 6

Diseases of animals that can be transmitted to humans

Question 7

What are the 3 measures of disease frequency?

Answer 7

Morbidity Rate
- an incidence rate
- number of new cases in specific time period per unit of population
- calculated by no. of new cases during specific time/no. individuals in population
Prevalence Rate
- total number of individuals infected at any one time
- depends on both incidence rate and duration of illness
Mortality Rate
- number of deaths from a disease per number of cases of the disease
- calculated by no. of deaths due to disease/size of total population with disease

Question 8

What are the 2 types of Epidemics?

Answer 8

Common source epidemic
- all comes from one location e.g. food poisoning
- sudden onset of symptoms
- everyone develops symptoms at same time
Propagated epidemic
- takes long time for epidemic to occur
- one person passes it onto another
- this person then passes to 2 family members etc.

Question 9

Infectious Period

Answer 9

Incubation period - time between infection and appearance of signs and symptoms
Prodromal phase - mild, non-specific symptoms that signal onset of disease
Illness - disease is most severe and displays characteristic symptoms
Convalescence - decline of symptoms

Question 10

Types of Carriers

Answer 10

Active - has overt clinical case of disease
Convalescent - recovered from disease but still has large number of pathogen
Healthy - has pathogen but is not ill
Incubatory - has pathogen but is not yet ill

Question 11

What are the 4 main ways pathogens are transmitted?

Answer 11

Airborne
Contact
Vehicle
Vector-borne

Question 12

Vehicle Transmission

Answer 12

Inanimate materials or objects involved in pathogen transmission
E.g. water and food
Fomites
- common vehicles such as surgical instruments, eating utensils

Question 13

Vector-borne Transmission

Answer 13

External (mechanical) transmission
- passive carriage of pathogen on body of vector
- no growth of pathogen during transmission
Internal transmission
- carried within vector
2 types of internal
1. Harborage - pathogen doesn’t undergo changes within vector
2. Biologic - pathogen undergoes changes within vector

Question 14

What are the 2 ways a pathogen can leave the host?

Answer 14

Active - movement of pathogen to portal of exit

Passive - excretion in faeces, urine, saliva etc.

Question 15

What are the most and least virulent modes of transmission?

Answer 15

Vector-borne - most virulent
Direct - least virulent
Greater ability to survive outside host = more virulent

Question 16

What are the 3 types of control measures for epidemics?

Answer 16

1. Reduce or eliminate source or reservoir of infection
   - quarantine and isolation of cases and carriers
   - treatment of sewage
   - destruction of animal reservoir
2. Break connection between source and susceptible individual
   - chlorination of water
   - pasteurisation of milk
   - destruction of insect vectors with pesticides
3. Reduce number of susceptible individuals
   - raises herd immunity

Question 17

Herd Immunity

Answer 17

Resistance of a population to infection and to spread of infectious organism because of immunity of a large percentage of population

Question 18

How can the level of Herd Immunity be altered?

Answer 18

Introduction of new susceptible individuals into population
Changes in pathogen
- antigenic shift - major change in antigenic character
- antigenic drift - smaller antigenic changes

Question 19

What are the major reasons for emergence of new infections?

Answer 19

Changes in human behaviour
Food handling practice
Crowding
Habitat disruption
Excessive or inappropriate use of antimicrobial therapy

Question 20

Difference between a Source and Reservoir

Answer 20

Source - location where the disease is transmitted

Reservoir - where the pathogen is normally found (can act as a source)