Epidemiology (23-28)

Question 1

What is epidemiology of infectious diseases?

Answer 1

The study of infectious disease in populations:
→ transmission
→ contact patterns
→ rate of spread and recovery
→ immunity
→ control of disease
→ population structure
→ genetic disposition

Question 2

What causes infectious disease?

Answer 2

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)

Question 3

What are some transmission routes of disease?

Answer 3

One-to-one contact → direct, indirect, droplet

Non-contact → airborne, vehicle, vector borne

Question 4

What is DALYs?

Answer 4

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

Question 5

What is the plague?

Answer 5

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

Question 6

Whats the difference between the different types of plague symptoms?

Answer 6

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%

Question 7

What caused the incidence of plague in Madagascar 2017?

Answer 7

Pneumonic → human-human transmission possible, changes to dynamic of disease enabled outbreak

Question 8

What are some examples of arthropod vector-borne protozoan micro parasites?

Answer 8

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)

Question 9

What is Chagas Disease?

Answer 9

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

Question 10

Whats the difference between acute and chronic Chagas disease?

Answer 10

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

Question 11

What are some characteristics of macro parasites?

Answer 11

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

Question 12

What is an epidemic?

Answer 12

An increase in incidence of disease in excess of that expected in a given population
→ incidence: number of new cases per unit time

Question 13

What is the compartmental model for the sequence of host stages?

Answer 13

susceptible → infected → recovered

S → I - effected contact rate
I → R - recovery rate

Question 14

What is R0?

Answer 14

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

Question 15

How can you estimate R0 for a pathogen?

Answer 15

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

Question 16

What is effective R (Re)?

Answer 16

Restrained growth rate
→ R0 for a ‘virgin’ population where all individuals susceptible
→ Re is the true reproductive rate = R0 x fraction od susceptible individuals

Question 17

Why do endemics end?

Answer 17

The pool of susceptible is depleted
→ Re declines to < 1
→ Re cannot return above 1 until new susceptibles are generated

Question 18

How do endemics continue?

Answer 18

Susceptibles
→ born, migration
No lasting immunity
Pathogen mutates and can re-infect/continually infected individuals
Immunity wanes

Question 19

What is epidemic fade-out?

Answer 19

The elimination of infectious agent due to chance
→ in small populations
→ generation (birth) of threshold susceptibles is slow, numbered of infecteds is low

Question 20

What is waning immunity?

Answer 20

Loss of immunity post recovery from infection
recovered → susceptible

Question 21

What can patterns in epidemic data tell us?

Answer 21

→ 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)

Question 22

What is the difference between incubation and latent period?

Answer 22

incubation period → the period between infection and clinical onset of the disease
latent period → the time from infection to infectiousness

Question 23

What is a point epidemic?

Answer 23

Single common exposure and incubation period
→ does not spread by host-to-host transmission
e.g. food-borne disease outbreaks

Question 24

What is continuous source epidemic?

Answer 24

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)

Question 25

What is propagated progressive source epidemic?

Answer 25

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

Question 26

When is an epidemic not an epidemic?

Answer 26

The successive epidemic waves await replenishment of susceptibles
→ host-parasite relationship may eventually dampen down to a stable equilibrium (endemic) state

Question 27

What is endemic equilibrium?

Answer 27

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

Question 28

What does persistence depend on?

Answer 28

Critical community size → the minimum host population size required (particularly micro-parasites)
Rate of contact for transmission
Duration of infectious period
Survival of host

Question 29

What happens when you increase infectious period but maintain R0?

Answer 29

→ likely low host mortality
→ eliminates cycles - improves persistence
→ increases prevalence

Question 30

How does Ebola virus have continual incidence?

Answer 30

1976 - Sudan
1976 - Zaire
1995 - Multiple small outbreaks
2013-2016 - West Africa
2014 - Congo
→ many seemingly isolated outbreaks - suggests reservoir host

Question 31

What are the features of Ebola virus?

Answer 31

→ 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

Question 32

What are the characteristics of gastrointestinal helminth worms for endemic persistence?

Answer 32

→ 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

Question 33

What is the aim of intervention?

Answer 33

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

Question 34

How can transmission be prevented?

Answer 34

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

Question 35

How can intervention be done after transmission?

Answer 35

Infectious curtailment → tracing, isolation, or culling
→ e.g. SARS, hospital MSRA (humans)
→ FMD, BSE, avian influenza (animals)

Question 36

What is the logic behind interventions?

Answer 36

Re = S x c x p x D = S x R0

→ reduce number susceptibles
→ reduce time infectious
→ reduces contact

Question 37

How many people should be vaccinated?

Answer 37

Pc = the minimum proportion of individuals you need to vaccinate for herd immunity to be effective

S = 1 - Pc
Pc = 1 - 1 / R0

Question 38

What is herd immunity?

Answer 38

Large proportion of population immune - spread of disease unlikely
→ remaining unvaccinated people gain protection by reducing the infectious fraction - prevents virus reaching susceptibles

Question 39

Is herd immunity reached without intervention?

Answer 39

No
→ active immunisation is required

Question 40

What is the first ever eradicated disease?

Answer 40

Smallpox 1980
→ only human one
→ last case in 1975
→ first vaccine created by Edward Jenner

Question 41

How can you estimate R0 from average age of infection and life expectancy (demography)?

Answer 41

R0 = 1 + L / A

→ A = average age of infection
→ L = life expectancy

A / L = proportion of lifetime before infection

Question 42

Why is measles virus controlled not eradicated unlike small pox?

Answer 42

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

Question 43

How was ring culling used in avian influenza?

Answer 43

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

Question 44

What are some vector-borne infections?

Answer 44

Mosquitoes
→ Plasmodium spp - malaria, arboviruses - dengue, yellow fever
Black flies
→ Onchocerca volvulus - onchocerciasis
Tsetse flies
→ Trypanosome spp - african sleeping sickness

Question 45

What does anthroponotic mean?

Answer 45

Transmission: human → arthropod → human

Question 46

What is vectorial capacity (C)?

Answer 46

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

Question 47

What are some vector controls?

Answer 47

Human bait traps → e.g. insecticide treated nets
Non-human bait traps
Urban breeding site source reduction
Rural drainage of breeding sites

Question 48

Why does climate change have a big effect on vector-borne diseases?

Answer 48

Climate change brings extreme weather events
→ environmental parasites stages sensitive to climate, seasonally, breeding sites
→ white host environment relatively robust