patterns of disease

Question 1

what does a case definition include?

Answer 1

• clinical signs
• history & signalment
• lab tests on biological sample
• level of confidence of certainty

Question 2

what are the categories we can can assign animals to based on the case definition?

Answer 2

• confirmed case - meets most or all of the case definition
• suspect case - meets some but not all rules
• non-case - very unlikely to be a case

Question 3

what is an index case?

Answer 3

• an index case is the first case in an outbreak

Question 4

what is a secondary case?

Answer 4

• a case resulting from contact or transmission from index case

Question 5

what is the basic reproduction number (R0)?

Answer 5

• is the number of secondary cases that one case would produce in a completely susceptible population
• mostly estimated retrospectively

Question 6

what does the basic reproduction number depend on?

Answer 6

• frequency of contacts between animals
• how long an infected animal is infective to others
• probability of each contact resulting in transmission of infection

Question 7

what is the effective reproduction number (Re)?

Answer 7

• is the number of secondary cases from a single infectious primary case with adjustment to account for immunity in the population
• other control measures may also drive R down

Question 8

if R>1 what is happening to the outbreak?

Answer 8

• if R>1, outbreak is growing

Question 9

if R<1 what is happening to outbreak?

Answer 9

if R<1 outbreak is shrinking

Question 10

what is herd immunity?

Answer 10

• is the level of immunity within the population

Question 11

how does herd immunity influence spread of infection through a population?

Answer 11

• if one infected individual enters a completely susceptible population -> there will be rapid spread of infection through population
• if one infected individual enters a population that has reached herd immunity threshold -> infection will fail to spread & persist in population

Question 12

what is the herd immunity threshold?

Answer 12

• is the level of immunity in the population at which R drops below one

Question 13

what is the Reed-Frost SIR model?

Answer 13

• is a modelling approach to explain dynamics of infectious disease spread in a population
• looks at susceptible, immune & recovered animals in a population
• uses a closed population
• more advanced modelling approaches are more commonly used

Question 14

define endemic disease level?

Answer 14

• endemic disease level is the normal ongoing occurrence of disease

Question 15

define epidemic

Answer 15

• refers to when disease frequency rises above expected level

Question 16

what factors may contribute to occurrence of an epidemic?

Answer 16

• introduction of a disease into a susceptible population or area
• change in determinants that favour increased frequency

Question 17

what do epidemic curves show?

Answer 17

• epidemic curves show distribution of cases over time
• show outliers -> show index case, secondary spread into another population
• provide a general sense of outbreak magnitude
• allow inferences about outbreak’s pattern of spread
• can tell us more likely time of exposure

Question 18

secular trends vs seasonal trends in epidemic curves

Answer 18

secular trends
- show a pattern of disease over long periods of time

seasonal trends
- show disease occurrence in the seasons within a year

Question 19

what is a time series analysis used for?

Answer 19

to split raw data into different components:

• secular trend
• seasonal pattern
• unexplained residues

Question 20

what are spatial patterns?

Answer 20

• distribution of disease occurrence is shown as dots on a map
• space & time can impact disease occurrence

Question 21

what ares spatio-temporal patterns?

Answer 21

• map can have the addition of space-time clusters of disease & show change of disease over time
• used to inform understanding of risk & control methods