Week 14 - Epidemiology of Infectious Diseases Flashcards
What is Epidemiology?
Science that evaluates occurrence, determinants, distribution and control of health and disease in a population
What is an Endemic Disease?
Maintains relatively low-level frequency at a moderately regular interval
What is an Epidemic?
Sudden increase in frequency
Index case - first case in an epidemic
What is an Outbreak?
Sudden, unexpected occurrence of disease
Usually focal or in a limited segment of population
What is a Pandemic?
Increase in disease occurrence within large population over wide region
What are Zoonoses?
Diseases of animals that can be transmitted to humans
What are the 3 measures of disease frequency?
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
What are the 2 types of Epidemics?
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.
Infectious Period
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
Types of Carriers
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
What are the 4 main ways pathogens are transmitted?
Airborne
Contact
Vehicle
Vector-borne
Vehicle Transmission
Inanimate materials or objects involved in pathogen transmission
E.g. water and food
Fomites
- common vehicles such as surgical instruments, eating utensils
Vector-borne Transmission
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
What are the 2 ways a pathogen can leave the host?
Active - movement of pathogen to portal of exit
Passive - excretion in faeces, urine, saliva etc.
What are the most and least virulent modes of transmission?
Vector-borne - most virulent
Direct - least virulent
Greater ability to survive outside host = more virulent
What are the 3 types of control measures for epidemics?
- Reduce or eliminate source or reservoir of infection
- quarantine and isolation of cases and carriers
- treatment of sewage
- destruction of animal reservoir - Break connection between source and susceptible individual
- chlorination of water
- pasteurisation of milk
- destruction of insect vectors with pesticides - Reduce number of susceptible individuals
- raises herd immunity
Herd Immunity
Resistance of a population to infection and to spread of infectious organism because of immunity of a large percentage of population
How can the level of Herd Immunity be altered?
Introduction of new susceptible individuals into population
Changes in pathogen
- antigenic shift - major change in antigenic character
- antigenic drift - smaller antigenic changes
What are the major reasons for emergence of new infections?
Changes in human behaviour Food handling practice Crowding Habitat disruption Excessive or inappropriate use of antimicrobial therapy
Difference between a Source and Reservoir
Source - location where the disease is transmitted
Reservoir - where the pathogen is normally found (can act as a source)
