Epidemiology (23-28) Flashcards
What is epidemiology of infectious diseases?
The study of infectious disease in populations:
→ transmission
→ contact patterns
→ rate of spread and recovery
→ immunity
→ control of disease
→ population structure
→ genetic disposition
What causes infectious disease?
Microparasites - small, difficult to count, multiply in their host
→ viruses, bacteria, fungi, protozoa
Macroparasites - large, can be counted, multiply external of the host
→ endoparasites (worms), ectoparasites (ticks, flees, lice)
What are some transmission routes of disease?
One-to-one contact → direct, indirect, droplet
Non-contact → airborne, vehicle, vector borne
What is DALYs?
Disability Adjusted Life Year
→ the number of healthy years of life lost due to pemature death and disability
DALY = YLD (years lived with disability) + YLL (years of life lost)
→ most affected by lower respiratory infections, diarrhoea diseases, HIV/AIDS
What is the plague?
Ancient bacterial disease caused by Yersinia pestis
→ vector borne disease - fleas, rodents are hosts/carriers
bubonic plague → lymphatic system
septicaemia plague → blood
pneumonic → lungs
→ caused ‘Black death’: 14th venturing, 50mil deaths in Europe
high mortality without treatment, high survival with treatment
Whats the difference between the different types of plague symptoms?
Bubonic → painful lymph nodes, fever, headache, chills, weakness, mortality: 30%
Pneumonic → fever, headache, weakness, pneumonia - shortness of breath, chest pain. cough, blood/watery mucus, mortality: 80%
Septicaemic → fever, chills, abdominal pain, shock, bleeding into the skin and other organs, skin/tissue may turn black, mortality: 100%
What caused the incidence of plague in Madagascar 2017?
Pneumonic → human-human transmission possible, changes to dynamic of disease enabled outbreak
What are some examples of arthropod vector-borne protozoan micro parasites?
Trypanosoma cruzi → human Chagas disease in central and S. America
Trypanosoma brucei spp → African sleeping sickness in Africa
Plasmoodium spp → malaria on many continents
(vector: living organism that can transmit infectious disease between humans or from animals to humans)
What is Chagas Disease?
Caused by Trypanosoma cruzi
→ endemic in 21 countries
→ 6-7 million persons infected
→ 10,000-14,000 deaths per year
→ 100mil at risk
transmission → 80% vector-borne, 20% transfusion of infected blood, congenital: mother to foetus
Whats the difference between acute and chronic Chagas disease?
Acute → pseudocysts form (replication sites) - rapture release inflammatory mediators, localised cell damage and inflammation, cute myocarditis
Chronic → type III hypersensitivity - kidney disease, chronic myocarditis - fibrosis and necrotic damage
70-80% of people infected remain asymptomatic
What are some characteristics of macro parasites?
Chronic recurring infections
High morbidity, low mortality (most vectors evolve not to kill host)
Endemic in nature
Continual reinfection
Age-related exposure, burden, pathology
soil transmitted helminths: round worms, hook worms, tape worms
What is an epidemic?
An increase in incidence of disease in excess of that expected in a given population
→ incidence: number of new cases per unit time
What is the compartmental model for the sequence of host stages?
susceptible → infected → recovered
S → I - effected contact rate
I → R - recovery rate
What is R0?
Basic core reproduction number
→average number of secondary cases arising from one infected individual introduced into a population of wholly susceptible individuals
endemic occurs when R0 > 1
How can you estimate R0 for a pathogen?
R0 = p x c x D
p → probability that a contact results in transmission
c → the frequency of host contacts between infectious and susceptible individuals
p x c → effective contact rate
D → average amount of time the host is infectious
What is effective R (Re)?
Restrained growth rate
→ R0 for a ‘virgin’ population where all individuals susceptible
→ Re is the true reproductive rate = R0 x fraction od susceptible individuals
Why do endemics end?
The pool of susceptible is depleted
→ Re declines to < 1
→ Re cannot return above 1 until new susceptibles are generated
How do endemics continue?
Susceptibles
→ born, migration
No lasting immunity
Pathogen mutates and can re-infect/continually infected individuals
Immunity wanes
What is epidemic fade-out?
The elimination of infectious agent due to chance
→ in small populations
→ generation (birth) of threshold susceptibles is slow, numbered of infecteds is low
What is waning immunity?
Loss of immunity post recovery from infection
recovered → susceptible
What can patterns in epidemic data tell us?
→ infecteds through time: prevalence and incidence
→ origin of the outbreak
→ mode of spread through the population
potential incubation period through time of exposure
→ clues to identify the infectious agent (R0 value comparisons)
What is the difference between incubation and latent period?
incubation period → the period between infection and clinical onset of the disease
latent period → the time from infection to infectiousness
What is a point epidemic?
Single common exposure and incubation period
→ does not spread by host-to-host transmission
e.g. food-borne disease outbreaks
What is continuous source epidemic?
Prolonged exposure to source over time
→ cases do not all occur within the span of a single incubation period
→ curve decay may be sharp or gradual
e.g. water-borne cholera (inc period 1-3 days)
What is propagated progressive source epidemic?
Spread between hosts
→ larger curves until susceptibles are depleted or intervention
→ this pattern most likely in a small population
→ in a larger population it would all ‘merge’ together
When is an epidemic not an epidemic?
The successive epidemic waves await replenishment of susceptibles
→ host-parasite relationship may eventually dampen down to a stable equilibrium (endemic) state
What is endemic equilibrium?
Stability in the incidence of infection
→ persistence of the parasite in the host population
→ each infection produces 1 secondary (new) infection on average Re = 1
Re > 1 means epidemic
What does persistence depend on?
Critical community size → the minimum host population size required (particularly micro-parasites)
Rate of contact for transmission
Duration of infectious period
Survival of host
What happens when you increase infectious period but maintain R0?
→ likely low host mortality
→ eliminates cycles - improves persistence
→ increases prevalence
How does Ebola virus have continual incidence?
1976 - Sudan
1976 - Zaire
1995 - Multiple small outbreaks
2013-2016 - West Africa
2014 - Congo
→ many seemingly isolated outbreaks - suggests reservoir host
What are the features of Ebola virus?
→ zoonotic pathogen
→ transmitted animal to human
→ fruit bats primary reservoir host - not diseased
→ constant human cases in isolated communities
→ human to human transmission drives large outbreak
What are the characteristics of gastrointestinal helminth worms for endemic persistence?
→ host density (CCS) not limit factor to transmission
→ external ‘reservoir’ of transmission stages
→ long generation time and period of infectiousness
→ immunity is transient (short)
→ continual re-infection
→ mode of transmission - often contaminative, not requiring host-host
What is the aim of intervention?
Control → maintains parasite population to an acceptable level
Elimination → zero incidence in a defined geographical area
Eradication → zero incidence worldwide
Extinction → infectious agent no longer exists in nature or lab
How can transmission be prevented?
Mass (random) or targets vaccination → e.g. small pox
Vaccination of risk group → e.g. child MMR
Spatial vaccination → e.g. ring vaccination
Reduction in contact → hand washing, condoms, sanitation
How can intervention be done after transmission?
Infectious curtailment → tracing, isolation, or culling
→ e.g. SARS, hospital MSRA (humans)
→ FMD, BSE, avian influenza (animals)
What is the logic behind interventions?
Re = S x c x p x D = S x R0
→ reduce number susceptibles
→ reduce time infectious
→ reduces contact
How many people should be vaccinated?
Pc = the minimum proportion of individuals you need to vaccinate for herd immunity to be effective
S = 1 - Pc
Pc = 1 - 1 / R0
What is herd immunity?
Large proportion of population immune - spread of disease unlikely
→ remaining unvaccinated people gain protection by reducing the infectious fraction - prevents virus reaching susceptibles
Is herd immunity reached without intervention?
No
→ active immunisation is required
What is the first ever eradicated disease?
Smallpox 1980
→ only human one
→ last case in 1975
→ first vaccine created by Edward Jenner
How can you estimate R0 from average age of infection and life expectancy (demography)?
R0 = 1 + L / A
→ A = average age of infection
→ L = life expectancy
A / L = proportion of lifetime before infection
Why is measles virus controlled not eradicated unlike small pox?
Measles and small pox share properties
→ no animal reservoir, safe cheap effective vaccine, high morbidity/mortality
Measles differs as:
→ transmitted more readily, R0 higher (requires higher Pc), highly infectious though not as virulent
How was ring culling used in avian influenza?
2003 Dutch avian influenza A virus epidemic
→ R0 = 5.8
→ 30mil birds slaughtered from 1145 farms
→ ring culling 1km zones around farms
→ movement restrictions in affected regions
What are some vector-borne infections?
Mosquitoes
→ Plasmodium spp - malaria, arboviruses - dengue, yellow fever
Black flies
→ Onchocerca volvulus - onchocerciasis
Tsetse flies
→ Trypanosome spp - african sleeping sickness
What does anthroponotic mean?
Transmission: human → arthropod → human
What is vectorial capacity (C)?
The average number of potentially infective bites that will be delivered by al the vectors feeding upon a single host in 1 day, units: per day
R0 = C x d
What are some vector controls?
Human bait traps → e.g. insecticide treated nets
Non-human bait traps
Urban breeding site source reduction
Rural drainage of breeding sites
Why does climate change have a big effect on vector-borne diseases?
Climate change brings extreme weather events
→ environmental parasites stages sensitive to climate, seasonally, breeding sites
→ white host environment relatively robust
