epidemilogy Flashcards
Epidemiology
• science that evaluates occurrence, determinants, distribution, and control of health and disease in a defined human population
sporadic disease
examples?
example: histoplasmosis, gas gangrene
– occurs occasionally and at irregular intervals
endemic disease
examples?
example: gonorrhea, Dengue fever
– maintains a relatively steady low-level frequency at a moderately regular interval
holoendemic diseases
(example: malaria)
– occurs at a high, constant level throughout a population
epidemic
examples?
index case?
example: influenza, meningococcal infections
– sudden increase in frequency above expected number
– index case – first case in an epidemic
outbreak
– sudden, unexpected occurrence of disease
– usually focal or in a limited segment of population
pandemic
– increase in disease occurrence within large population over wide region
(usually worldwide)
Morbidity rate
- an incidence rate
- number of new cases in a specific time period per unit of population
new cases during a specific time/ # individuals in population
Prevalence rate
depends on?
- total number of individuals infected at any one time per unit of population
- depends both on incidence rate and duration of illness
Mortality rate
• number of deaths from a disease per number of cases of the disease # deaths due to given disease/ size of total population with disease
R0 or R(0), “R-naught”
number of secondary cases one case would produce in a completely susceptible population, measure of contagious
R(0) calculated as a function of?
3 primary parameters
• duration of contagiousness after a person becomes infected
• likelihood of infection per contact between a susceptible person and an infectious person or vector
• contact rate
Recognition of an Infectious Disease in a Population
use of?
• involves use of surveillance methods
• cases of a disease recognized by its characteristic disease syndrome
– set of signs and symptoms characteristic of a disease
– signs
• objective changes in body that can be directly observed
– symptoms
• subjective changes experienced by patient
Course of infectious disease
incubation, prodromal, period of illness, convalesence
incubation period
– period after pathogen entry but before
signs and symptoms appear
prodromal stage
– onset of signs and symptoms
– not clear enough for diagnosis
period of illness
– disease is most severe and has
characteristic signs and symptoms
convalescence
– signs and symptoms begin to disappear
Correlation with a Single Causative Agent
after recognition of infectious disease in a population, outbreak correlated with specific pathogen
clinical microbiologists help in isolation and identification of pathogen
2 types of epidemics
common source epidemics- rapid rise and fall
propogated epidemics- prolonged
propagated epidemics and threshold density
as the number of cases rises and more people recover there is a lower number os susceptible individuals= less cases
once this number drops below the threshold density the epidemic will slow down
threshold = minimum number of individuals necessary to continue propagating the disease
Herd immunity
resistance of a population to infection and to spread of an infectious organism because of the immunity of a large percentage of the population
factors decreasing the level of herd immunity
• level can be altered by introduction of new susceptible individuals into population
• level can be altered by changes in pathogen
– antigenic shift – major change in antigenic character of pathogen
– antigenic drift – smaller antigenic changes
The Infectious Disease Cycle questions to ask
- What Pathogen Caused the Disease?
- What was the Source and/or Reservoir of the Pathogen?
- How Was the Pathogen Transmitted?
- Why Was the Host Susceptible to the Pathogen?
- How Did the Pathogen Leave the Host?
- What Pathogen Caused the Disease?
- Kochs postulates (or modifications) are used if possible
- clinical microbiology lab – plays important role in isolation and identification of pathogen
- communicable disease – can be transmitted from one host to another
- What was the Source and/or Reservoir of the Pathogen?
source? reservoir? Period of infectivity?
- source – location from which pathogen is transmitted to host
- period of infectivity – time during which source is infectious or is disseminating the organism
• reservoir – site or natural environmental location in which pathogen is normally found
– sometimes functions assource of pathogen
Human sources/reservoirs are called?
potential time frames?
• carrier
– infected host
– can be casual (acute or transient) carrier – short time
– can be chronic carrier – long time
types of carriers
- active carrier – has overt clinical case of disease
- convalescent carrier – has recovered from disease but continues to harbor large numbers of pathogen
- healthy carrier – harbors pathogen but is not ill
- incubatory carrier – harbors pathogen but is not yet ill
Animal reservoirs
vectors?
- numerous diseases are zoonoses
- transmission to human can be direct or indirect
- vectors– organisms that spread disease from one host to another
modes of transmission
- Airborne Transmission
- Contact Transmission
- Vehicle Transmission
- Vector-Borne Transmission
Airborne Transmission
pathogen suspended in air and travels ³ 1 meter
• droplet nuclei
– small particles (1-4 micrometers in diameter)
– can remain airborne for long time (hours to days)
– can travel long distances
– usually propelled from respiratory tract of source organisms by sneezing, coughing, or vocalization
• dust particles also important route of airborne transmission
Contact Transmission
• coming together or touching of source/reservoir and host
• direct contact (person-to-person)
– physical interaction between source/reservoir and host
– e.g., kissing, touching, and sexual contact
• droplet spread
– large particles (>5 micrometers) that travel < 1 meter
Vehicle Transmission
• Vehicles = inanimate materials or objects involved in pathogen transmission
– water and food (spread pathogen to multiple hosts)
– fomites = inanimate objects: surgical instruments, bedding and eating utensils
Vector-Borne Transmission, types?
• external (mechanical) transmission
– passive carriage of pathogen on body of vector
– no growth of pathogen during transmission
• internal transmission
– carried within vector
– harborage transmission – pathogen does not undergo changes within vector (bubonic plaque: Yersinia pestis in flea)
– biologic transmission – pathogen undergoes changes within vector
(malaria: Plasmodium spp. in Anopheles spp. mosquito)
Why Was the Host Susceptible to the Pathogen?
– defense mechanisms of host
– pathogenicity of pathogen
escape routes from the host
• active escape
– movement of pathogen to portal of exit [helminths (worms) that migrate through the body of the host]
• passive escape
– excretion in feces, urine, droplets, saliva, or desquamated cells [microbes]
Control of Epidemics
Reduce or eliminate source or reservoir
Break connection between source and susceptible individuals
Reduce number of susceptible individuals
How to Reduce or eliminate source or reservoir
– quarantine and isolation of cases and carriers
– destruction of animal reservoir
– treatment of sewage
– therapy that reduces or eliminates infectivity of cases
How to Break connection between source and susceptible individuals
– chlorination of water supplies
– pasteurization of milk
– supervision & inspection of food & food handlers
– destruction of insect vectors with pesticides
How to Reduce number of susceptible individuals
– passive immunity following exposure
– active immunity for protection
both augment herd immunity
