WEEK 5 - Infectious Disease Epidemiology Flashcards
Infectious disease Epidemiology - Definition
The study of the occurrence, spread and control of infectious diseases within defined population groups
- this data is important to help control the spread of infection
List of KEY DEFINITIONS
inc. disease classification by incidence
Prevalence - the proportion / % of diseased indivuals in a population at a specific time (ALL cases)
Incidence - the number of NEW cases / diseased individuals within a population
Mortality - incidence of death in a pop.
Morbidity - incidence of illness in a pop.
R0 = no. of cases that arise from 1 infected indivual
- i.e. R0 of 3 means 1 infected person will infect 3 susceptible people
Classification of disease by incidence:
Sporadic level - occasional cases occuring irregularly
- i.e. random pop-ups
- no sustained spread + go away very quickly
Endemic - disease is always present (have background level of disease)
- persistent occurence
- have a cluster of cases spread around
Epidemic - person to person transmission within a defined area
Pandemic - sustained widespread person to person transmission
- spreading across 2 or more regions
What are notifiable diseases
Diseases that are not typically seen in the UK
URGENT CASES:
- If person is classed as a vulnerable / risk group (e.g. age, immunocompromised)
- requires urgent GOV contact
- within 24hrs if suspected (don’t need to wait for confirmed diagnosis)
NON-urgent CASES:
- Use an online system to notify GOV
- Report within 3 days
- Have 34 notifiable diseases in England and Wales
List the 3 key transmission routes
- Direct contact
- Indirect contacts
- inc. fomites, vehicle, vectors - Aerosol and Droplets (common route)
NOTE:
- most infectious disease can be transmitted via any of the above
Explain how infectious disease may be transmitted via direct contact
Short Range Transmission Route
- Contact between 2 mucosal surfaces
- Making contact with wound of infected
Explain how infectious disease may be transmitted via indirect contact (fomute)
Short and Long Range Transmission Route
Fomite (inanimate object):
- can harbour microorganisms for some time (if organim produces spores)
- e.g. mouse, laptops, keyboards
Objects frequently touched are points of contact between indiviuals
- If infected sneezes / coughs onto fomite + someone else touches / uses object = spread
- aerosols / droplets infect objects
Vehicle Transmission:
- Transmission via food / water
- e.g. contaminated with toxins, infectious agents, poisons
Vectors:
- Transmission via blood feeding animals
- e.g. mosquitos
Droplets and aerosols INFO
Transmission Route
Aerosols:
- Can travel within AND beyond 1m
- Can float / be sustained in air for HOURS
- When sneeze / cough plume of aerosol are released
- if enter air cirulation / ventilation aerosol will be moved around
- They are inhaled:
- <5 microns = enter LRT
- >100 microns = can enter URT
- Larger the aerosol is (>100) its likely to drop to the ground
Droplets:
- Can travel within 1m ONLY
- Do NOT suspend in air (fall to ground within 5 seconds)
- If >100 microns = can NOT be inhaled
NOTE:
Droplets = short-range transmission
Aerosols = short and long range transmission
- travel further + supsend in air longer
Explain how infectious disease may be transmitted via droplets and aerosols
3 Phases
Phase 1:
- Generation of viral / microbial load
- transmission depends on how much viral load the particle carries
- Exhalation of aerosol / droplet
Phase 2:
- Transport of particle
- Transport is affected by: size, environmental factors (temp., humidty, ventilation, airflow, UV)
- Good ventilation ↓ viral load / infectious dose entering susceptible person
Phase 3:
- Inhalation of particle
- smaller it is = easier to be inhaled + get into lungs
- Deposition mechanisms
- Infection occurs
How does infectious diseases spread within populations
i.e. what affects R0
- Contaigousness
- Susceptibility to infection
- Prior exposure to microorganism
- Social and age-related patterns
- Microbiology of the infectious agent
- i.e. microbial factors
List 3 Reservoirs of Infection
Reservoir - site where infectious agents reside + multiply in
- Animals (zoonosis)
- Can transmit infectious agents to humans
- Can NOT eradicate zoonotic infections as it requires control of the animal pop.- via vaccination or cull
- Humans frequnetly have contact with animals
= even if eradicate disease in humans, will still appear again as its still present in animals - Inc. wild life birds, domesticated animals
- Humans frequnetly have contact with animals
- via vaccination or cull
- Human carriers
- Have infection but show NO SIGNS of disease
Can be an:- Acute carrier = infectious during incubation period
- Chronic carrier = harbour the pathogen forever
- Environmental
- Bacteria / microorganisms that reside in soil, water etc.
- These organisms can survive for years
- Organisms usually sporolate
Why did SARS-CoV-2 cause a pandemic when SARS-CoV-1 only caused an epidemic?
CoV-1 cases emerged due to wet markets (livestock was kept in close proximity to humans)
- Very few had mild illness when contracted virus, usually required ITU admission
- CoV-1 had a ↓ mortality rate
- Didnt become a pandemic despite global spread as cases were controlled + it wasn’t sustaine transmission
- ALL infected displayed symptoms
- Infected was ONLY infectious once symptoms appeared
CoV-2 was a milder disease in comparison to CoV-1
- had flu-like symptoms, caused pneumonia, shortness of breath etc.
- affected URT and LRT
- had BETTER TRANSMISSION = why it became a pandemic
- rapid GLOBAL SPREAD
- had a ↑ R0
- had ↓ mortality than CoV-1 but had ↑ no. of cases
- some infected were ASYMTPOMATIC = ↑ spread (you were infectious before symtptoms appeared)
