5.59 Flashcards
Epidemiology
• science that evaluates occurrence, determinants,
distribution, and control of health and disease in a defined
human population
• sporadic disease (example: histoplasmosis, gas gangrene)
– occurs occasionally and at irregular intervals
• endemic disease (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 (example: influenza, meningococcal infections)
2
– 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
equation # new cases during a specific time over # individuals in population
Prevalence rate
2
- 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
equation
# deaths due to given disease over
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
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
(2)
• involves use of surveillance methods
• cases of a disease recognized by its characteristic
disease syndrome
cases of a disease recognized by its characteristic
disease syndrome
(3)
– set of signs and symptoms characteristic of a disease
– signs
– symptoms
– signs
• objective changes in body that can be directly observed
– symptoms
• subjective changes experienced by patient
• incubation period
– period after pathogen entry but before
signs and symptoms appear
• prodromal stage
2
– 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
skipped
Correlation with a Single Causative Agent
(2)
• after recognition of infectious disease in a population, outbreak
correlated with specific pathogen [e.g. HIV/AIDS (early 1980s), SARS
CoV (2003), SARS-CoV-2 (2019-2020)]
• clinical microbiologists help in isolation and identification of pathogen
Recognition of an Epidemic
• 2 types of epidemics
common source epidemic
propagated epidemic
threshold density =
the 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
Herd immunity
• level can be altered by
introduction of new susceptible
individuals into population
Herd immunity
• level can be altered by changes in
pathogen
– antigenic shift –
major change in antigenic character of pathogen
– antigenic drift –
smaller antigenic changes
- What Pathogen Caused the Disease?
3
• Kochs postulates (or modifications) are used if possible • clinical microbiology lab • communicable disease
clinical microbiology lab
– plays important role in
isolation and identification of
pathogen
communicable disease
– can be transmitted from one
host to another
source
– location from which pathogen
is transmitted to host
• period of infectivity
– time during which source is
infectious or is disseminating
the organism
• reservoir (2)
– site or natural environmental location in which pathogen is normally found – sometimes functions as source of pathogen
Human sources/reservoirs
• carrier
(3)
– infected host
– can be casual (acute or transient)
carrier – short time
– can be chronic carrier – long time
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
3
- numerous diseases are zoonoses
- transmission to human can be direct or indirect
- vectors– organisms that spread disease from one host to another
- Airborne Transmission
3
- pathogen suspended in air and travels ³ 1 meter
- droplet nuclei
- dust particles also important route of airborne transmission
• droplet nuclei
4
– 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
Contact Transmission
3
- coming together or touching of source/reservoir and host
- direct contact (person-to-person)
- droplet spread
• direct contact (person-to-person)
2
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
• external (mechanical) transmission
(2)
– passive carriage of pathogen on body of vector
– no growth of pathogen during transmission
. Vector-Borne Transmission
• internal transmission
(3)
– 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?
• two main factors
– defense mechanisms of host
– pathogenicity of pathogen
- How Did the Pathogen Leave the Host? (2)
- active escape
* passive escape
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 (3)
- Reduce or eliminate source or reservoir
- Break connection between source and susceptible individuals\
- Reduce number of susceptible individuals
- Reduce or eliminate source or reservoir
4
– quarantine and isolation of cases and carriers
– destruction of animal reservoir
– treatment of sewage
– therapy that reduces or eliminates infectivity of cases
- Break connection between source and susceptible individuals
(4)
– chlorination of water supplies
– pasteurization of milk
– supervision & inspection of food & food handlers
– destruction of insect vectors with pesticides
- Reduce number of susceptible individuals
2
augment herd immunity:
– passive immunity following exposure
– active immunity for protection
