EPI 1 Flashcards
what is veterinary epidemiology
The study of the distribution and determinants of health-related states or events in specified population and the application of this study to the control of health problems
what are the two broad types of epidemiology and describe them
1) Descriptive epidemiology
- Documents the distribution of disease in a population and how it varies across animal, place and time
- Allows us to develop hypothesises then go onto the analytic branch
2) Analytic epidemiology
- Tests the relationship between exposure (‘risk factors’) and disease outcomes
why is veterinary epidemiology important
Potential to make substantial impacts on overall health and productivity by targeting interventions at the population level
What are the 3 factors that describe risk in an individual and a population
INDIVIDUAL 1) host 2) pathogen 3) environment POPULATION 1) individual 2) place 3) time
what are individual-level factors that influence patterns of disease in a population
- Age structure of population - different risk factors
- Distribution of genotypes
- Immunity of population
- Population dynamics
what are the place (spatial) factors that influence patterns of disease in a population
- Proximity to pollutants
- Proximity to infectious agents
- Proximity to disease risks
what are the 3 different types of temporal factors influencing the pattern of disease in a population
1) Subject-referent time
○ Number of days since calving
○ Examples: milk fever in dairy cattle, gestational diabetes in human
2) Calendar time
○ Exact calendar date
○ Examples: influenza in humans, bovine ephemeral fever in cattle - often vary seasonally
3) Epidemic curves are useful way to visualise the temporal pattern of disease
○ When things vary overtime
What are the 4 temporal patterns of diseases and describe
1) Endemic - disease occurs at expected frequency
2) Epidemic - disease occurs at greater than expected frequency
3) Pandemic - huge epidemic (international)
4) Sporadic - single case or clusters of cases
Endemic and sporadic patterns of disease features and examples
Endemic - Disease occurs at expected frequency - Disease present in population or region at all times - Level of disease usually low and predictable - Examples: lameness in dairy cattle Sporadic - Single case or cluster of cases - Infrequent disease occurrence - Irregular unpredictable - Examples - food poisoning
What are the two types of epidemics and define
1) common source
- subjects are exposed to a common noxious influence
2) propagated epidemics
- occurs when the agent is transmitted through the population from host to host
what are the two types of common source epidemics and describe and examples
1) common point source
○ Group is exposed over a relatively short period then disease cases will emerge over one incubation period
○ Curve rises rapidly and contains a definite peck at the top, followed by a gradual decline
○ Example: foodborne disease outbreaks - salmonellosis
2) Common continuous source epidemics
○ Group is exposed continuously and cases emerge over more than one incubation period
○ Curve rises rapidly, no definite peck, plateau of case numbers over time
○ Example: anthrax in cattle
why measure health?
quantify the occurrence of disease
- to determine the importance of different diseases, set priorities, implement control activities, compare level of disease among groups of individuals
define closed population
no additions or removals during a defined follow-up period
define open population
- individuals enter (e.g. births and purchases) and individuals leave (e.g. sales and deaths) during the follow-up period
- The denominator (population at risk) is changing
define morbidity
- describes the amount of disease within a population
- expressed as either incidence or prevalence
What 3 things do you need for a risk (rate) and describe
- a numerator: the number of individuals diseased or dead
- a denominator: the total number of animals (or animal time) in the study group or population; and
- a referent time period - longer time period higher the frequency of the disease
what is the difference between incidence risk and incidence rate and when is each better
rate involves time while risk does not
rate is better for an open population with individuals coming and going however need good record keeping to ensure know when individuals come and leave
define prevalence and the equation
P𝑟𝑒𝑣𝑎𝑙𝑒𝑛𝑐𝑒 = 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑒𝑥𝑖𝑠𝑡𝑖𝑛𝑔 𝑐𝑎𝑠𝑒𝑠 of disease/𝑆𝑖𝑧𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑎𝑡 𝑟𝑖𝑠𝑘
- equals the probability of an individual being diseased at a given point in time
define incidence and list the 2 types
Incidence measures how frequently susceptible individuals become disease cases as they are observed over time
- an incident case occurs when an individual changes from being susceptible to being diseased
1) incidence risk
2) incidence rate
define incidence risk, equation and example
- the proportion of initially susceptible individuals in a population who become cases over a defined follow-up period
- also called cumulative incidence
𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑐𝑒 𝑟𝑖𝑠𝑘 = 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑐𝑎𝑠𝑒s/𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑖𝑛𝑑𝑖𝑣𝑖𝑑𝑢𝑎𝑙𝑠 𝑖𝑛𝑖𝑡𝑖𝑎𝑙𝑙𝑦 𝑎𝑡 𝑟𝑖𝑠𝑘
follow-up period may be fixed eg 5 year incidence risk of arthritis or arbitrarily (lifetime risk)
Incidence rate define, what also called, equation
- Number of new cases of disease that occur per unit of individual time at risk, over a defined follow-up period
- Also called incidence density
Incidence rate = number of incident cases/ (individuals at risk X t)
What is the difference between relative and absolute comparisons and example
Relative comparisons are made by division $5 ÷ $1 = 5 ‘you have five times as much money as me’ [$ units cancel out]
• Absolute comparisons are made by subtraction $5 – $1 = $4 ‘you have $4 more than me’
When to use a 2 x 2 table
- when comparing two groups of animals with different exposure and disease outcomes
- is there an association between exposure and diseases?
- if so how strong is it?
Once you have used the 2 x 2 table to find R+, R- and RT what are the 3 measures of association and types within
(1) measures strength
1. incidence ratio 2. odds ratio
(2) measures of effect
1. attributable risk
(3) measures of total effect
1. population attributable fraction