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BIO371 > Lecture 11 - Epidemiology > Flashcards

Lecture 11 - Epidemiology Flashcards

1
Q

What does the transmission of infectious diseases depend on?

A

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

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2
Q

Common routes of transmission

A

• 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)

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3
Q

Vertical Transmission

A

passing from parent to child

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4
Q

Horizontal Transmission

A

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

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5
Q

Zoonotic transfer

A

Transmitted between species from Animals to Humans

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6
Q

Natural hosts vs incidental hosts

A

• 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.

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7
Q

What is Epidemiology?

A

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

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8
Q

What are the 4 aspects of epidemiology? (didp)

A
  • determining the source/cause of diseases
  • identifying disease risk factors
  • designing and evaluating infection control policies
  • predicting future spread of disease
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9
Q

What is a case?

A
  • 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
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10
Q

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

A
  • ‘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)’
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11
Q

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

A

Number of cases in a population is counted to

measure the spatial and temporal spread of disease

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12
Q

What is an incidence?

A

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”

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13
Q

What is prevalence?

A

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

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14
Q

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

A
  • can indicate how fast a disease is spreading
  • can detect new and re-emerging diseases
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15
Q

Explain incidence rate

A

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
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16
Q

What indicates the disease burden within a population?

A

Prevalence

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

17
Q

What is HAART?

A

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
18
Q

What is an endemic disease?

A
  • disease which is habitually present in a population
  • disease occur in cyclical patterns of increased and decreased incidences
19
Q

What causes the cyclical patterns of an endemic disease?

A

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
20
Q

Can severe disease be endemic?

A

A severe disease still can be an endemic

  • periodicity of incidences can be clearly seen in diseases such as chickenpox, influenza, pertussis and measles
21
Q

What is an outbreak?

A
  • unexpected cluster of cases appearing within a short period of time in a localized population
  • typically smaller than epidemic but the lines are blurry
22
Q

What is an epidemic?

A
  • significant rise in incidence of a disease above that normally expected in a population
23
Q

What is a pandemic?

A

Global epidemic, usually on more than one continent

24
Q

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

A

temporal and spatial

25
Q

What is a common source epidemic?

A
  • 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
26
Q

What is a propagated epidemic?

A
  • 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
27
Q

What is R0?

A
  • 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
28
Q

Why is R0 a range? (dapte)

A

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

29
Q

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

A

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)

30
Q

Why is R0 not the most reliable?

A

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
31
Q

What is Rt?

A

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
32
Q

What is the Triangle of epidemiology?

A
  • 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
33
Q

Explain the Host in the Triangle of epidemiology

A
  • the susceptibility of hosts in the population
  • age, sex, kinship, immigration status, immune status, presence of other diseases, etc.
34
Q

Explain the Agent in the Triangle of epidemiology

A
  • overall virulence of the pathogen
  • pathogen’s biology, habitat, virulence
    mechanisms, incubation period, mode of
    transmission, etc.
35
Q

Explain the Environmental factors in the Triangle of epidemiology

A
  • 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
36
Q

What do epidemic control strategies do? and how?

A

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
37
Q

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

A

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

BIO371 (5 decks)

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