Lecture 10: Infectious Disease Epidemiology Flashcards
History of Infectious Disease
1350BCE - smallpox
1400s - bubonic plague
1500s - syphillis
1900s - smallpox, cholera, typhoid, tuberculosis
Susser’s Era in Epidemiology
Through different paradigms and intervention strategies, we have new understandings of infectious diseases.
Eras include:
- Sanitary
- Infectious Disease
- Chronic Disease
- Eco-epidemiology
Sanitary era (paradigm, analytic approach, prevention)
paradigm - miasma theory
analytic approach - clustering of mortality
prevention - sanitation
infectious disease era (paradigm, analytic approach, prevention)
paradigm - germ theory
analytic approach - laboratory
prevention - vaccinations
Chronic Disease era (paradigm, analytic approach, prevention)
paradigm - black box
analytic approach - risk ratios
prevention - host, agent, environment
Eco-epidemiology era (paradigm, analytic approach, prevention)
Paradigm - systems theory
analytic approach - determinants at many levels
prevention - contextual to molecular
system theory
there are a multitude of risk factors in various systems. Ex. transportation, government, healthcare, etc.
Infectious disease
An illness due to a specific infectious agent or its toxic products.
Arises through transmission of agent/toxic product from infectious person/animal/reservoir to a susceptible host.
How can transmission happen?
- directly, from other infected animals/humans
- indirectly, from intermediate host, vector, or the environment
Susceptible
uninfected, but able to become infected if exposed
Infectious
infected and able to transmit infection to other susceptible individuals
Immune
possessing cell-mediated or humoral or antibody protection against an infection
diseased/clinical infection
presence of clinical signs of pathology (A disease that has recognizable clinical signs and symptoms)
Latent infection/subclinical infection
Remains in the body but you are not infectious
Carrier
Protracted infected state with shedding of the infectious agent.
Epidemic
excess in occurrences of cases beyond of what is normally expected in a community or region
Outbreak
an epidemic limited to localized increase in the incidence of a disease
Endemic
The constant presence of a disease or infectious agent within a given geographic area or population group
Pandemic
An epidemic crossing international boundaries and affects many people
Pathogens
- bacteria
- virus
- chemical toxins
- parasitic (multi-cell)
- Fungal
- Protozoa
- Prions
Bacteria
- fatty membrane contains DNA and cellular machinery
- obtain energy from sugars, fats, and proteins
- live and multiply outside of host
Different Shapes of Bacteria
bacilli - rod shape
cocci - round
spirochete - spiral
virus
- not affected by antibiotics
- smallest infectious disease agent
- thin protein coating around genetic material
- requires energy from host cells
- cannot replicate outside host cell
Chemical toxins
- organic and inorganic
- allergens, carcinogens, teratogens
Parasitic (multi-cell)
- defined nucleus
- single or multi-celled
- ex. malaria, head lice
Fungal
includes yeast infections, candida
Protozoa
- single cell organisms
- can be free-living or parasitic in nature
- transmission - live in a human’s intestine; fecal-oral route
Prions
- relatively new discovery
- non-living
- infectious agent with protein-like qualities
- thought to be responsible for mad cow disease
- difficult to disinfect
- Ex. TSEs
Transmission Routes
Direct Transmission
Indirect Transmission
Direct Transmission Routes
- mucous membrane to mucus membrane (STD)
- Across placenta
- Transplants
- skin to skin
- Sneezes, coughs
Indirect Transmission Routes
- water-borne
- proper air-borne
- food-borne
- vectors
- objects
Six-Link Chain of Infection
These interactions cause communicable disease to occur:
1. Pathogens
2. Reservoir
3. Portal of Exit
4. Transmission
5. Portal of Entry
6. Susceptible Host
Pathogens (Definition and public health approach)
- virus, bacteria, or parasite that causes disease in humans
- kill agent (antibiotics)
Reservoir (definition and public health approach)
- location which pathogen lives and reproduces
- eliminate reservoir (remove garbage)
Portal of Exit (Definition and public health approach)
- route pathogen takes to leave infected host
- knowledge to understand transmission and control
Transmission (Definition and Public Health Approach)
- method of traveling from reservoir/infected host to susceptible host
- block movement of pathogen (quarantine)
Portal of Entry (Definition and Public Health Approach)
- route pathogen enters host
- block point of entry (mask)
Susceptible host (Definition and Public Health Approach)
- entry into host that is not immune
- improve resistance (vaccination)
Characteristics of Agent
- toxicity, virulence, infectivity
- susceptibility to antibiotics
- ability to survive outside body
Characteristics of Environment
- climate
- physical structures
- population density
- social structure
Characteristics of Host
- age
- prior exposure
- susceptibility
- co-infection
- immune response
Interventions for agent
- eradicate
- genetically modify
Interventions for environment
- housing quality
- sanitation, water
- prevention services
Interventions for Host
- treat, isolate
- immunize
- nutrition
Age-environment intervention
- remove breeding grounds
- improve sanitation
Host-environment intervention
- educate
- change activity patterns
- quarantine
Host-agent intervention
- protect
- educate
- alter exposure
Surveillance of Infectious Diseases
- influence decreased 2020-2021
- peaks end of december and early january
- increasing after that
- STIs have increased such as Gonorrhea and Chlamydia
Coronaviruses
group of viruses that are common in humans and are responsible for up to 30% of common cold
Outbreaks that have occurred due to coronaviruses
SARS (2003)
MERS (2012)
COVID-19 Seroprevalence in Ontario
- PHO created the COVID-19 serosurveillance program
- see what proportion of the Ontario population was infected with COVID and developed the antibodies for it
Serology Testing
a lab test that measures the antibodies specific to Covid-19
(looks as the presence or levels of antibodies)
Investigation and Control of Epidemics/Pandemics
- Investigation
- Identifying cases
- Management and Control
- Surveillance and Response
Principles of Surveillance
- incidence and prevalence
- case-fatality
- mortality rate and premature mortality
- bed-disability days
- medical costs
- preventability
- epidemic potential
- information gaps on new diseases
Sources of Data
- mortality an morbidity
- hospital records
- laboratory diagnoses
- outbreak reports
- vaccine utilization
- sickness absence report
- biological changes in agent, vectors, or reservoirs
- blood banks
Factors that Influence Effectiveness of Surveillance Systems
- Number of conditions (fewer)
- Amount of information per case (little)
- Burden on reporter (small)
- Decision-Makers’ interest in surveillance data (high)
- goals for surveillance (clear and support)
- reporting strategy for serious but common conditions (enough to meet goals)
- usefulness of data locally (high)
- use is limited to analysis of data and archiving (data are well used)
- usefulness to decision-makers for prevention action (high)
Mathematical Models of Infectious Disease Transmission Dynamics
Systematic way of identifying and applying assumptions to data on disease transmission to quantify how an epidemic may evolve.
What do mathematical models help with?
- Estimating transmission parameters
- effectiveness of interventions
- predict cost effectiveness of interventions
Reproductive Rate, R
Average number of new infections caused by 1 infected individual.
Basic Reproductive Rate, R0
in an entirely susceptible population
> 1 = epidemic
1 = endemic
< 1 = infection dies out
Effective Reproductive Rate, R
In a population where <100% is susceptible.
= proportion susceptible x R0
Hallmarks of an epidemic
- number of infections increase exponentially during the early phase
- epidemic curve is unimodal and peaks when the susceptible pool has been sufficiently depleted (Rt < 1)
Calculating R0
R0 = BcD
B = probability of transmission per contact
c = contact rate
D = duration of infectivity
SIR model
People can be in one of three states
S - susceptible to the infection
I - infected/infectious
R - recovered/immune
Assumptions for the SIR model
- fixed population
- latent period is 0
- infectious period = disease period
- after recovery, individuals are immune
Herd Immunity
The proportion of the population that need to be immune in order to stabilize an infectious disease.
HIT = (R0 - 1) / R0
or
HIT = 1 - (1/R0)
Things to consider when interpreting the herd immunity number
virulence of the disease
re-emergence
susceptibility
interventions in place
