Lecture 11 - Epidemiology

Question 1

What does the transmission of infectious diseases depend on?

Answer 1

• location of pathogen’s replication
• stability of pathogen in the environment
• mode of exit from the original host

Question 2

Common routes of transmission

Answer 2

• fecal-oral transmission (pathogens of intestinal tract)
• airborne transmission (pathogens of the respiratory tract)
• vector-borne / mechanical transmission (via insect vectors etc.)
• sexual transmission (pathogens of sexual organs)

Question 3

Vertical Transmission

Answer 3

passing from parent to child

Question 4

Horizontal Transmission

Answer 4

transmission of pathogen between members of a species other than parent to offspring

Question 5

Zoonotic transfer

Answer 5

Transmitted between species from Animals to Humans

Question 6

Natural hosts vs incidental hosts

Answer 6

• pathogen normally infects its reservoir host (natural host)
• pathogen is sometimes transmitted from its reservoir host to another species (incidental hosts) • incidental hosts are are often dead-end hosts; pathogens may replicate and transmit in the host species but can not maintain their population over time

the case with Ebola.

Question 7

What is Epidemiology?

Answer 7

Epidemiology studies how disease spread in populations

• not limited to infectious diseases
• can also study cancer, chemical poisoning, obesity, genetic diseases, etc.

Epidemiology studies diseases in populations,
not necessarily in individuals

Question 8

What are the 4 aspects of epidemiology? (didp)

Answer 8

• determining the source/cause of diseases
• identifying disease risk factors
• designing and evaluating infection control policies
• predicting future spread of disease

Question 9

What is a case?

Answer 9

• individual occurrence of a specific disease is counted as
  a case
• for all diseases, a ‘case’ is specifically defined to unambiguously diagnose and count affected individuals

Question 10

What are 2 ways to define a case depending on the disease or situation

Answer 10

• ‘an individual who shows signs of the disease’
• ‘an individual who is infected by the pathogen (but may
  or may not show signs of the disease)’

Question 11

What are the number of cases in a population counted to measure…?

Answer 11

Number of cases in a population is counted to

measure the spatial and temporal spread of disease

Question 12

What is an incidence?

Answer 12

Incidence= the new cases / the time period
* number of NEW cases appearing in a population during a specified time period
* usually presented as a proportion, such as “Incidence rate per 100,000 people”

Question 13

What is prevalence?

Answer 13

Prevalence = (people from the last day + the new cases) / the time period

• TOTAL sum of cases in a population at a particular point in time or a time-period
• again, usually presented as a proportion per population

Prevalence will always be higher than incidence

Question 14

2 reasons why incidence is one of the best indicators of disease risk?

Answer 14

• can indicate how fast a disease is spreading
• can detect new and re-emerging diseases

Question 15

Explain incidence rate

Answer 15

High incidence does not automatically equate to increasing rate of disease

• incidence is influenced by population size
• its better to refer to incidence rate per population to evaluate if a disease is spreading more

Question 16

What indicates the disease burden within a population?

Answer 16

Prevalence

knowing the total number of diseased
individuals at a given time-point allows
governing bodies to appropriately set up
healthcare systems and policies

Question 17

What is HAART?

Answer 17

Incidence and prevalence are also useful to monitor the effectiveness of disease control measures

• development of HAART (highly active
  antiretroviral therapy) around 1995 caused the
  decline in death rate of HIV

Question 18

What is an endemic disease?

Answer 18

• disease which is habitually present in a population
• disease occur in cyclical patterns of increased and decreased incidences

Question 19

What causes the cyclical patterns of an endemic disease?

Answer 19

Seasonal changes and variations in population immune status causes the cyclical pattern

• at the beginning of disease-spread, incidence is high since the population lacks immunity towards the disease
• incidence drops as population gains immunity
• decreased incidence eventually causes immunity to drop, enabling the disease to spread again

Question 20

Can severe disease be endemic?

Answer 20

A severe disease still can be an endemic

• periodicity of incidences can be clearly seen in diseases such as chickenpox, influenza, pertussis and measles

Question 21

What is an outbreak?

Answer 21

• unexpected cluster of cases appearing within a short period of time in a localized population
• typically smaller than epidemic but the lines are blurry

Question 22

What is an epidemic?

Answer 22

• significant rise in incidence of a disease above that normally expected in a population

Question 23

What is a pandemic?

Answer 23

Global epidemic, usually on more than one continent

Question 24

What distributions of incidences show different patterns which reflect how the epidemic spread?

Answer 24

temporal and spatial

Question 25

What is a common source epidemic?

Answer 25

* an epidemic caused by all individuals getting the disease from a single, same source of infection (no horizontal transmission) ex. Shiga toxin producing E.Coli * typically observed for food and water poisoning * shows a sharp rise in incidence, followed by a decline

Question 26

What is a propagated epidemic?

Answer 26

* an epidemic where the disease spreads by horizontal transmission * observed for any horizontally transmittable diseases such as SARS-CoV-2, influenza, etc. * more gradual rise in incidence

Question 27

What is R0?

Answer 27

* Basic reproduction number, R0 * average number of new infections caused by an infected individual in a population * 'how many new people would an infected individual spread the disease to?' * R0 gives the potential of the infectious disease becoming an epidemic * R0 of measles is 12 – 18: a person infected with measles will spread the virus to 12 - 18 new individuals on average

Question 28

Why is R0 a range? (dapte)

Answer 28

R0 is presented as a range since it is affected by many variables * demographic of host's population such as age distribution * population density of susceptible individuals * transmissibility of the pathogen * environmental conditions such as temperature and humidity

Question 29

What is the epidemic threshold and what happens if R0 is less or greater than it?

Answer 29

Disease spreads less when R0 < 1 * an infected individual will pass the disease to less than one new individual, on average * pathogen population decreases as it gets transmitted and will eventually dwindle Disease spreads more when R0 > 1 * an infected individual will pass the disease to more than one new individual, on average * pathogen population keeps on increasing so long as there are more susceptible individuals in the host's population R0 = 1 is the epidemic threshold * infectious disease with R0 = 1 has the potential to persist and spread in the host's population * SARS-CoV-2 had R0 = 3 (early estimation)

Question 30

Why is R0 not the most reliable?

Answer 30

Accurately calculating R0 is difficult, especially during an epidemic * the value is affected by many variables which could change over time or are simply unknown * R0 also assumes that the whole population is completely susceptible, which is not always true

Question 31

What is Rt?

Answer 31

Effective reproduction number, Rt = # of new cases in a time period / # of cases at the beginning of that period ex. 19522 covid cases on March 20 and 48899 new cases from then to the 25th. Rt = 48899/19522 = 2.5 During this 5-day period, about 2.5 new cases of COVID-19 occurred for every case of COVID-19 which existed at the beginning * calculated during the epidemic as an estimate of R0 * Tracking Rt allows us to estimate how the epidemic is progressing

Question 32

What is the Triangle of epidemiology?

Answer 32

* three major, interconnected factors influence the development of epidemic - Host, Agent (pathogen), environmental factors * interaction of host and agent determines how severe the disease spreads * the environment influences both host and agent by making disease spread easier or harder

Question 33

Explain the Host in the Triangle of epidemiology

Answer 33

* the susceptibility of hosts in the population * age, sex, kinship, immigration status, immune status, presence of other diseases, etc.

Question 34

Explain the Agent in the Triangle of epidemiology

Answer 34

* overall virulence of the pathogen * pathogen's biology, habitat, virulence mechanisms, incubation period, mode of transmission, etc.

Question 35

Explain the Environmental factors in the Triangle of epidemiology

Answer 35

* physical aspects of environment which influence the host and agent * climate, sanitation, crowding, location (as in hospitals, schools, etc.) * cultural and social factors such as hygienic practices

Question 36

What do epidemic control strategies do? and how?

Answer 36

Epidemic control strategies target key factors to get Rt lower than 1 * protection of susceptible individuals * suppressing pathogen's virulence * altering the environment to help suppress disease spread

Question 37

Go review the last slide on the lecture 11 pdf about COVID -19 and flattening the curve

Answer 37

Control measures 'flatten' the epidemic curve to help health care * it's not always about perfectly eliminating the disease