Epidemiology

Question 1

What are parasites and describe the types.

Answer 1

They are pathogens that cause infectious disease. Microparasites are small and difficult to count (multiply in host). Eg. Viruses, bacteria, fungi and protozoa.
Macroparasites are large and can be counted. They multiple externally of the host Eg. Endoparasites like worms, ectoparasites like ticks.

Question 2

What is a DALY?

Answer 2

Disability Adjusted Life Year. It’s the number of healthy years of life lost due to premature death and disability.

Question 3

Describe the plague and its history.

Answer 3

Bacterial disease: Yersinia pestis.
Vector borne disease - fleas:
- Bubonic plague: infection of the lymphatic system
- Septicaemic plague: infection of the blood
- Pneumonic plague: infection of the lungs.
Originated in the 14th century causing 25-50 million deaths in Europe.
Untreated mortality ranges: 30%-100% but early antibiotic treatment is effective.

Question 4

Describe the 3 types of plague.

Answer 4

Bubonic: painful lymph nodes, fever, headache, chills, weakness. 30% mortality.
Pneumonic: fever, headache, weakness, pneumonia. 80% mortality.
Septicaemic: fever, chills, abdominal pain, shock. 100% mortality.

Question 5

What is a vector?

Answer 5

Living organisms that can transmit infectious disease between humans or from animal to humans.

Question 6

Describe 4 routes of transmission.

Answer 6

Vector-borne by ‘kissing bugs’
Transfusion of infected blood.
Congenital: mother to foetus.
Accidental ingestion of infected sources.

Question 7

Provide a summary of chagas.

Answer 7

Caused by trypanosoma cruzi. Transmitted via kissing bugs when taking a blood meal.
Acute disease characterised by inflammation and acute myocarditis.

Question 8

Describe macroparasites.

Answer 8

They are chronic recurring infections. There’s high morbidity but low mortality. Endemic in nature. Continual reinfection. Age-related exposure.

Question 9

Describe soil-transmitted helminths and schistosomiasis.

Answer 9

Predominantly oral/faecal transmission.
Most ‘adult’ macroparasites reside in GIT.
Eggs released in faecal matter.
Behavioural-related exposure.

Question 10

What’s an epidemic?

Answer 10

An increase in incidence of disease in excess of that expected. Incidence: number of new cases per unit time.

Question 11

What is R0?

Answer 11

It’s the basic case reproduction number. It’s defined as ‘the average number of new cases arising from one infectious case into a population of wholly susceptible individuals’.

Question 12

Describe the formula we use for estimating R0.

Answer 12

R0 = p X c X D
R0 –> The reproductive number of the infectious agent.
p –> Probability that a contact result in transmission.
c –> The frequency of host contacts between infectious and susceptible individuals.
p X c –> Effective contact rate.
D –> average number of time that the host is infectious.

Question 13

What is Re? What’s its formula?

Answer 13

It’s the restrained growth rate. It’s defined for a ‘virgin’ population with all individuals susceptible. It’s formula is R0 X fraction of susceptible individuals.

Question 14

What causes an epidemic to end?

Answer 14

The pool of susceptible individuals is depleted.
Re declines to <1.
Re cannot return to >1 until new susceptibles are generated.

Question 15

What causes an epidemic to continue?

Answer 15

Susceptibles increase via birth or migration into a population.
No immunity.
Pathogen mutates and can re-infect or continually infect individuals.
Immunity wanes.

Question 16

What is epidemic fade out?

Answer 16

It’s the elimination of the infectious agent due to chance.
Generation/Birth of susceptibles is slow.
Number of infected is low.

Question 17

What is the Incubation period?

Answer 17

The period between infection and clinical onset of the disease.

Question 18

What is the latent period?

Answer 18

The time from infection to infectiousness.

Question 19

What is point epidemic?

Answer 19

Single common exposure and incubation period. It doesn’t spread via host-host transmission. Eg. Food borne disease outbreaks.

Question 20

What is continuous common source epidemic?

Answer 20

Prolonged exposure to source over time. Cases don’t all occur within the span of a single incubation period. Curve decay might be sharp/gradual. Eg. water-borne cholera.

Question 21

What is propagated progressive source epidemic?

Answer 21

Spread between hosts. Larger curves until susceptibles are depleted, or intervation made. More likely to occur in a small population. Eg. measles.

Question 22

When did Snow’s “Grand Experiment” happen? What did he do?

Answer 22

In 1853-1854. He identified the source of a cholera epidemic and removed the pump handle proving that cholera is water borne.

Question 23

Who discovered that vibrio cholerae was the causative agent of cholera?

Answer 23

Robert Koch in 1884.

Question 24

When is an epidemic not considered an epidemic anymore?

Answer 24

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

Question 25

What is endemic equilibrium?

Answer 25

Stability in the incidence of infection (constant). It's the persistence of the parasite in the host population. Each infection produces 1 new infection on average (Re = 1).

Question 26

Describe endemicity and what it actually is. Mention what NTDs are and how they relate in your answer.

Answer 26

It's not overly common in the developed world. (Exception - Chicken pox, influenzas). In the less developed world, more severe diseases are endemic. NTDs (Neglected Tropical Diseases) are prioritised by the WHOs 20 due to their endemic nature in large populations. Endemic diseases are common in animal populations: - Wild --> no one is treating. - Managed --> It can be advantageous.

Question 27

What is the critical community size (CCS)?

Answer 27

It's the minimum host population size required for the pathogen to persist. It's more of a concern for microparasites. Macroparasites: - May survive outside of the host - Can/often aggregate --> high burdens in a small number of hosts.

Question 28

Describe the history of the Ebola Virus.

Answer 28

1976 Sudan --> First known outbreak killing 151. 1976 Zaire --> 280 deaths 1995 - 2018 --> Multiple small outbreaks 2013 - 2016 --> West Africa: 28616 cases, 11310 deaths. 2014 --> Small outbreak in Congo with 66 cases and 49 deaths. 2018 - 2020 --> 3481 cases and 2299 deaths.

Question 29

What does the history of the timeline of Ebola suggest about its nature?

Answer 29

It suggests a reservoir host. It is a zoonotic pathogen. It can be transmitted from animal to human. Fruit bats are the primary reservoir host.

Question 30

What are reservoirs and carriers.

Answer 30

Reservoir host --> referring to a population of species. Carrier --> referring to an individual. Clinically normal but infectious.

Question 31

Are macroparasites highly persistent? Give 3 examples of it.

Answer 31

They are, examples: hookworm, ancylostoma duodenale.

Question 32

What measurements of macroparasite infections.

Answer 32

Infection intensity/mean burden: it's related to R0. Prevalence: is determined by the mean worm burden and degree of parasite aggregation.

Question 33

What is the aim of intervention of disease?

Answer 33

Control - maintains the parasite population to an acceptable level. Elimination - zero incidence in a defined geographical area (local eradication). Eradication - zero incidence worldwide. Extinction - infectious agent no longer exists in nature or in lab.

Question 34

What can be done to prevent transmission of pathogens?

Answer 34

Mass or targeted vaccination. By risk group Spatial vaccination Reduction in contact.

Question 35

How do we intervene after transmission of pathogens?

Answer 35

Tracing and isolating.

Question 36

Describe herd immunity and its relation to intervention.

Answer 36

An infection spreads in a population and has sufficient replenishment of susceptibles - endemicity. There is 'partial' immunity that may be referred to as 'herd immunity'. Partial immunity may be considered at the individual level - a partial immune response. A pathogen that generated total herd immunity would go extinct. To achieve herd immunity, active immunisation is required.

Question 37

Describe smallpox disease.

Answer 37

Variola major (90%) Variola minor (10%) Transmission: 'prolonged face to face contact' - essentially from nose and mouth droplets via sneezing and coughing. R0. 3 - 6. Mortality rate: Major 35%, minor 1%.

Question 38

What is isolation by ring culling?

Answer 38

It removes susceptible individuals from the community.

Question 39

What is an anthroponotic, zoonotic?

Answer 39

Anthroponotic: Human-Arthropod-human. Zoonotic: Animal-arthropod-human.

Question 40

Describe the transmission of west nile virus.

Answer 40

A complex host-virus relationship. Birds are primary amplifier hosts. Migratory birds have a role in distribution.

Question 41

What is vectorial capacity (C)?

Answer 41

The average number of potentially infective bites that will be delivered by all the vectors feeding upon a single host in 1 day. R0 = c X d

Question 42

Describe a few examples of vector control.

Answer 42

Human bait traps. Eg. Insecticide treated net, African malaria vector. Non-human bait traps. Eg. Asian malaria vector. Urban breeding site source reduction. Eg. yellow fever & dengue vector.

Question 43

Describe wolbachia. Describe what it does to mosquitoes.

Answer 43

It's one of the most common endosymbiotic bacteria in arthropods and some nematodes, present in an estimated 40-60% of insect species globally. Wolbachia infested mosquitoes are less able to transmit viruses because Wolbachia prevents viral replication in the host.

Question 44

What is the equation for vectorial capacity?

Answer 44

C = [(V/N)(ah)^2(p^n)] / -ln(p) V = n.o. of vectors N = n.o. of hosts a^2 = The vector biting rate per day h^2 = The proportion of bloodmeals taken on host. p = The daily vector survival rate of the vector. n = Extrinsic incubation period EIP

Question 45

Describe if El Nino is responsible for periodic epidemic cycles.

Answer 45

El Nino of the Southern Oscillation (ENSO) El Nino surface pressure: high (warm) over W. Pacific (Peru and Ecuador); low over SE Pacific. Inverse cold phase: La Nina. Strong determinant of inter-annual variation in sea-level pressure across the Pacific Ocean.

Question 46

Describe what Dengue is.

Answer 46

Dengue is a mosquito-borne viral infection. The infection causes flu-like illness, and occasionally develops into the potentially lethal complication called sever dengue. Dengue is found in tropical and sub-tropical climates worldwide, mostly in urban and semi-urban areas. There's no specific treatment for dengue/severe dengue, but early detection and access to proper medical care lowers fatality rates below 1%.

Question 47

Describe pathogen invasions and its relation to climate change.

Answer 47

Bluetongue virus (BTV): - double stranded RNA virus of livestock. - Subclinical chronic infection in cattle --> reservoir host - Severe disease in some sheep/deer species. - Vector-borne by midges. - Competent vectors traditionally subtropical

Question 48

Describe the origin of SARS-CoV-2

Answer 48

Wet market in Wuhan, China, December 2019. Infection occurred due to a 'spillover' event. Reservoir host.

Question 49

Describe the co-morbidity in the 2 infants example.

Answer 49

The 2 infants had severe co-morbidities. Age structured co-morbidities data is unavailable. Increased risk of death with co-morbidities is known.

Question 50

Describe the different types of disease states.

Answer 50

No clinical disease. Pneumonia/Viral disease. Autoimmune. Deaths by each disease.

Question 51

What is Waning immunity and what does it represent?

Answer 51

A reduction in the host's immune response or a specificity of immune response that doesn't respond to a mutated strain. For the sake of epidemiology, it doesn't matter which. Important for long term Public Health strategy and planning.