Lecture 11 - Epidemiology Flashcards
What does the transmission of infectious diseases depend on?
• location of pathogen’s replication
• stability of pathogen in the environment
• mode of exit from the original host
Common routes of transmission
• 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)
Vertical Transmission
passing from parent to child
Horizontal Transmission
transmission of pathogen between members of a species other than parent to offspring
Zoonotic transfer
Transmitted between species from Animals to Humans
Natural hosts vs incidental hosts
• 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.
What is Epidemiology?
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
What are the 4 aspects of epidemiology? (didp)
- determining the source/cause of diseases
- identifying disease risk factors
- designing and evaluating infection control policies
- predicting future spread of disease
What is a case?
- 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
What are 2 ways to define a case depending on the disease or situation
- ‘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)’
What are the number of cases in a population counted to measure…?
Number of cases in a population is counted to
measure the spatial and temporal spread of disease
What is an incidence?
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”
What is prevalence?
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
2 reasons why incidence is one of the best indicators of disease risk?
- can indicate how fast a disease is spreading
- can detect new and re-emerging diseases
Explain incidence rate
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
What indicates the disease burden within a population?
Prevalence
knowing the total number of diseased
individuals at a given time-point allows
governing bodies to appropriately set up
healthcare systems and policies
What is HAART?
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
What is an endemic disease?
- disease which is habitually present in a population
- disease occur in cyclical patterns of increased and decreased incidences
What causes the cyclical patterns of an endemic disease?
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
Can severe disease be endemic?
A severe disease still can be an endemic
- periodicity of incidences can be clearly seen in diseases such as chickenpox, influenza, pertussis and measles
What is an outbreak?
- unexpected cluster of cases appearing within a short period of time in a localized population
- typically smaller than epidemic but the lines are blurry
What is an epidemic?
- significant rise in incidence of a disease above that normally expected in a population
What is a pandemic?
Global epidemic, usually on more than one continent
What distributions of incidences show different patterns which reflect how the epidemic spread?
temporal and spatial
What is a common source epidemic?
- 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
What is a propagated epidemic?
- 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
What is R0?
- 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
Why is R0 a range? (dapte)
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
What is the epidemic threshold and what happens if R0 is less or greater than it?
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)
Why is R0 not the most reliable?
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
What is Rt?
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
What is the Triangle of epidemiology?
- 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
Explain the Host in the Triangle of epidemiology
- the susceptibility of hosts in the population
- age, sex, kinship, immigration status, immune status, presence of other diseases, etc.
Explain the Agent in the Triangle of epidemiology
- overall virulence of the pathogen
- pathogen’s biology, habitat, virulence
mechanisms, incubation period, mode of
transmission, etc.
Explain the Environmental factors in the Triangle of epidemiology
- 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
What do epidemic control strategies do? and how?
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
Go review the last slide on the lecture 11 pdf about COVID -19 and flattening the curve
Control measures ‘flatten’ the epidemic curve to help health care
* it’s not always about perfectly eliminating the disease
