Influenza + outbreaks 5 - 7 Flashcards
What is the definition of an outbreak?
An epidemic limited to localised increase in the incidence of a disease e.g. in a village, town or closed institution
2 or more persons with the same disease or symptoms or orgnanism isolated from a diagnostic sample who are linked through common exposure, personal characteristics, time or location
Greater than expected rate of infection compared with the usual background rate for the particular place and time
What are the components of an outbreak investigation?
Prevention Surveillance/monitoring Detection Outbreak investigation Specific control interventions
What is the infectious disease epidemiology framework comprised of?
Reservoir/source Portal of exit Mode of transmission Portal of entry Susceptible host
What is direct transmission?
Touch (Ebola)
Respiratory (Influenza)
Faecal-oral (polio)
What is indirect transmission?
Faecal-oral (contaminated water)
Blood borne (HIV)
Airborne (influenza)
Vector borne (malaria)
What are methods in identifying a potential outbreak?
Report from clinician or laboratory
Report from a patient or member of public
Routine surveillance systems
Media reports
Which organisations are responsible for communicable disease surveillance?
PHE
ECDC = European Centre for DIsease Prevention and Control
WHO
What are the 2 types of surveillance?
Passive = routine systems which rely on clinicians remembering to send in a case report or returns from laboratories Active = Surveillance team actively seeks out reports e.g. an outbreak inestigation
What is the purpose of surveillance?
Time
Purpose
Place
What are the different epidemic curves?
Point or common source (one single peak) Continuing source (multiple peaks) Propagated source (infections occur over several incubation periods) Mixed source (multiple peaks e.g. cholera)
What can be done in an environmental investigation?
Environmental samples may identify or confirm the organism/agent and its source eg. food, water, soil
Environmental Health Department of Local Authority is responsible for this in England and Wales
What are the control measures formed by Public Health Act 1984; Public Health Infectious Disease Regulations 1988?
Aimed at reducing the spread of infection Safe Effective Appropriate to the risk Timely
What are the challenges during the outbreak investigation process?
Rapid response Multi-disciplinary working Case ascertainment Lack of resources Difficulty contacting cases to collect information Lack of environmental samples Lack of options for control activity Working with the media Risk communication Legal implications
What is the historical significance of influenza pandemics?
1918-1919 influenza pandemic - clinical disease in one third of the world’s population; total estimated deaths: 50-100 million
Complicated, severe and fatal cases increased with time
Human flu strains now dominated by its descendants
Why did the 1918-19 influenza pandemic kill so many young adults?
WW1 - Giant petri dish; transmission and global displacement of people
Working age population
Robust response associated with increased severity
New technologies e.g. trains - increased migration - around 10 million deaths in India and bicycles and roads attributed to transmission in Africa
Where is an example of a place with no herd immunity?
No previous exposure to H1N1 strains in this group means lack of herd immunity
Around 40% Pacific Islands had no previous influenza exposure
Describe the 2009 influenza pandemic in the UK
First case in 27 April 2009 and 457 confirmed deaths
What is the pathogenesis of influenza?
Hx of close exposure to poultry
Ill for 3-8 days; cough, SOB, muscle pain, abdo pain, vomiting, diarrhoea
Deaths from respiratory failure in week 2-4
Describe the pathogenesis of severe influenza
High viral load - enteric (gut) infectino; viral spread via the bloodstream
Early, strong release of cytokines + chemokines from infected cells
Initial interferon increase causes more of a broad immune response e.g. neutrophils but then immune response switches to an inappropriate immune response
What happens to the viral immune response as the bacterial immune response increases?
Decreases
Increased immune response to virus and bacteria causes tissue damage
Immune response drives severity causing symptoms
Why is severe flu both an immune failure and excess?
Virus-related - high viral load; delay in use of antivirals
Absence of prior immunity - innate immunodeficiency
Weakened host immune response
Vaccinate and use antivirals early and often
Describe the evolution of new ‘flu’ strains
Reassortment of genetic material from each virus causes evolution of flu strains
What is the phased UK pandemic plan?
Prepare
Contain
Treat
What is the National Pandemic Flu Service (NPFS) England?
Telephone/internet-based - “voucher” access to antivirals - 25% needed to see the GP
Why is there a need for models specifically designed for the transmission dynamics of infectious disease?
Simplification of a system and a tool to analyse data
Quantification of uncertainty is a key strength of modelling
Infectious disease epidemiology must take account of transmission dynamics - this is why modelling is essential
Describe a compartmental model for an acute immunising infectious disease e.g. influenza, incorporating interventions
Susceptible -> Prophylaxis -> Latent -> Infectious -> Treatment -> Immune
What is the mathematical formulation of the transmission rate of a directly-transmitted (person-to-person) infection?
R(t) = pcSI/N pc = probability of transmission and contact rate cI/N = Proportion of population infectious
What is R(0)?
Basic reproductive number
Measures how effectively infection spreads
The higher R(0) is the harder it is to control
Average number of secondary infections occurring from a typical infected individual in a totally susceptible population
What does R(0) depend on? x2
How long individuals are infectious for (on average)
How rapidly they transmit infection (on average)
These vary amongst populations and can be changed
What impact do interventions have on R(0)?
Interventions aim to reduce R0 < 1, to eliminate infection that is present +/- prevent an epidemic occurring if it should be reintroduced
What value is R(t) in an endemic situation?
1
