Communicable diseases Flashcards
Epidemiologic triad
- Host (demographic, socioeconomic, biological)
- Agent (chemical,biological,physical,nutrient,mechanical,social)
- Environment (physical, biological, social)
Definition of communicable diseases
An illness caused due to specific infectious (biological) agent or its toxic products capable of being directly or indirectly transmitted from man to man, animal to man animal to animal or from the environment to man
Roles of epidemiology
- Monitor public health (mortality, morbidity rates)
- Respond to outbreaks, pandemics, epidemics (determine cause of outbreak, put in control measures)
- Investigate emerging and reemerging diseases (determine risk factors, recommend control measures)
Why is it important to study epidemiology of communicable diseases?
Changes of pattern of infectious disease
Discovery of new infections
Possibility that some chronic diseases have an infective origin
Definition of infection
Entry and development/multiplication of an infectious agent in the body of a man/animals
An infection does not always cause illness (asymptomatic)
Levels of infection (gradients)
- Colonisation- prescence of infectious agent doesn’t mean infection straight away, e.g S.aureus in skin and normal nasopharynx
- Subclinical- infection but no signs/symptoms e.g polio
- Latent infection- prescence of infectious agent but no symptoms e.g herpes simplex virus
- Manifest/clinical infection- infectious and showing symptoms
Meaning of contamination
Presence of an infectious agent on a body surface/ clothing/bedding/toys/surgical instruments/water/food
Meaning of infestation
Lodgement, development and reproduction of arthropods on the surface of a body/clothing
e.g lice/mites
Term can be used to describe invasion of gut by parasitic worms e.g ascariasis
Meaning of contagious disease
Disease transmitted trhough contact
e.g leprosy, scabies, STIs
Meaning of reservoir host
Person/animal/arthropod/soil/plant/substance in which an infectious agent normally lives and multiplies
natural habitat of infectious agent
depends on it for survival
reproduces so it can be transmitted to a susceptible host
Meaning of incidence
Number of new cases in a given time period expressed as percent of infected per year (cumulative incidence)
or incidence density= number of cases per person time of observation
Meaning of prevalence
Number of cases at a given time expressed a as percent at a given time
Incidence x duration
What disease has a short duration?
Measles
What disease has a long duration?
Chronic hepatitis B
Meaning of epidemic
Unusual occurrence of disease in a community
specific health related behaviour/ health related events
in excess of expected occurrence
Meaning of endemic
Constant presence of disease/infectious agent within a geographic area/population group
It is the usual/ expected frequency of disease within a population
Meaning of pandemic
An epidemic affecting large proportion of population occuring over a wide geographic area e.g section of nation/continent/world e.g influenza pandemics
Meaning of exotic diseases
imported into a country where they do not normally occur in e.g rabies in UK
Meaning of sporadic
cases are ‘scattered about’
Irregular, from time to time
Cases are few and separated widely in time
show no/little connection with each other/ recognizable source of infection e.g polio/meningococcal meningitis
What can a sporadic disease lead to?
Starting point of an epidemic
If conditions are favourable for its spread
Meaning of epizootic
Epizoic= outbreak (epidemic) of a disease within an animal population e.g rift valley fever
Meaning of Enzootic
Endemic occurring in animals e.g bovine TB
What is a nosocomial infection?
Hospital acquired infection
Infection originating in a paitent while in hospital
Has to be a new disorder unrelated to primary condition of patient
e.g surgical wounds, hepatitis B, UTI
What are the 2 types of epidemic?
- Common source
2. Propagated
What is a common source epidemic?
Rapid rise and fall of incidence
Arises from a single source e.g food poisoning/legionnaire’s disease
What is a propagated epidemic?
Slow rise and fall of incidence
Disease may be transmitted from each contaminated individual
e.g strep throat
What is an opportunistic infection?
Infection by organisms that take the opportunity provided by a defect in the host (e.g weakened immune system) to infect the host and cause disease
This disease wouldn’t occur in individuals with a healthy immune system
Example of opportunistic infections
Common in AIDS
M.tuberculosis
Herpes simplez virus
Meaning of eradication
Termination of all transmission of infection by extermination of infectious agent
Through surveillance and containment
Meaning of elimination
Eradication of disease in a large geographic area
What diseases are currently easily eliminated?
Polio,measles, diphtheria
What is the attack rate?
Proportion of non-immune exposed individuals who become clinically ill
What is a primary and second case?
Primary case= person who comes into and jnfects population
secondary case= those that subsequently contract the infection
further spread= waves/generations
What is the reproductive rate/number (R)?
Average number of new infections caused by 1 infected individual
Meaning of R0?
Basic reproductive rate in an entirely susceptible population
What is the effective reproductive rate in a population where <100% are susceptible?
R= proportion susceptible x R0
What does it mean if R0>1?
Infection spreads (epidemic)
What does it mean if R0=1?
Infection remains constant (endemic)
What does it mean if R0<1?
Infection dies out
Determinants of R0 formula
R0= B (beta) x c x D
B= probability of transmission per contact between an infected and susceptible individual c= contact rate D= duration of infectivity
What is the mathematical SIR model
Susceptible-infectious-recovered model
epidemiological model that computes the theoretical number of people infected with a contagious illness in a closed population over time
Assumptions of SIR model
Latent period= 0
population is fixed- no births/deaths
Infectious period= duration period
After recovery, individuals are immune
Parameters needed to use model
effective contact rate, natural mortality rate, recovery rate, and disease-induced death rate
What are the 3 different types of reservoirs?
Human- look at diagram
Animal
Non-living
What is ‘case’?
A person in the population/study group identified as having the particular disease/health condition/disorder/condition under investigation
What is a ‘carrier’?
Infected person/animal that is infected with a specific infectious agent
absence of visible clinical disease
Acts as a potential source of infection to others
How does someone become a carrier?
Inadequate treatment/immune response
disease is not completely eliminated
What are the 3 elements that form a carrier state?
- Presence of disease agent in body
- Absence of recognisable symptoms and signs of disease
- Shedding of disease agent in discharge/excretion
What can act as a reservoir of infection?
- Soil- can harbor agents that cause anthrax, tetanus
2. Inanimate matter
DIfferent modes of transmission
- Direct transmission- direct contact, droplet infection, contact with soil, inoculation into skin/mucosa, vertical
- Indirect- fomite, vector(look at diagram), air borne, vehicle borne
Meaning of a susceptible host
An infectious agent seeks a susceptible host to aim for successful parasitism
What 4 stages are required for successful parasitism?
- Portal of entry into host
- Site of election inside body
- Portal of exit out of host
- Survival in external environment
Meaning of virulence
Degree of pathogenicity
How is virulence numerically expressed?
ratio of number of cases: total number infected
determined by immunoassay (measures concentration of antibody in body)
Meaning of case fatality rate?
Proportion of infected individuals who die from infection
no. of deaths/no of cases
depends on severity of infection (virulence)
heavily influenced about no of mild cases not diagnosed
Meaning of serial interval (generation time)
Gap in time between the onset of the primary and secondary cases
Interval between recipient of infection and maximal infectivity of host
What is the infectious period?
length of time a person can transmit disease
What is the incubation period?
Time interval between invasion by infectious agent and appearance of first sign/symptom of disease
What is the latent period?
Period between being infected to becoming infectious
What is a transmission probability ratio (TPR)?
Measure of risk of transmission from an infected to a susceptible individual during contact
TPR of differing types of contacts, infectious agents, infection routes and strains can be calculated
4 types of TPRs
p00
p01
p10
p11
p00
p00= tp from unvaccinated infective to unvaccinated susceptible
p01
p01= tp from vaccinated infective to unvaccinated susceptible
p10
p10= tp from unvaccinated infected to vaccinated susceptible
p11
p11= tp from vaccinated infected to vaccinated susceptible
How are diseases investigated in the clinic?
- Collate symptoms
- Foods eaten
- Location foods eaten in
- Secondary attacks
How are diseases investigated in the lab?
- Preliminary culture
- Attack rate analysis
- Antibiotic analysis
- definitive ID
- Sample culture
- Reference culture
- Deposit
How are diseases investigated in the environment?
- Location visit
- Sample collection
- Food source visit
- Sanctions
