Measures of Disease Occurence

1. Measures of Disease Occurence Dr. Fortune Sithole

2. Epidemiology and Disease Epidemiology is concerned with: • The presence of existing health problems AND • The occurrence of new health events in a population And: their quantification or measurement

3. Measure Disease: what do we mean? Measure (or quantify): • The amount of disease at one point in time or in a period of time OR • Change in the amount of disease over a time period In a population, group or subgroup • Species, age, sex, production group, geographic location, season, exposure group etc.

4. Why Measure Disease? • Know what is going on in the population and how disease is changing in a population • Identify important dzes and changing dz patterns • Set priorities for disease control or management programs and also for research. • Quantify dz risk in animals/humans and their products • Essential for trade, food safety and public health planning • Compare disease in groups • Outbreak investigation • Risk factor studies, vaccine trials, drug efficacy trials “Risk Factor” is another word for “Determinant of Disease”

5. Types of Measures

6. Measures of Disease When measuring disease occurrence it is important todefine: • What is being measured? • The persons or animals included in the measurement: called the study group • Place or location of the study group • Time period of the study

7. Populations • Study sample/group • Subjects in the study • Often a “sample “ from a source population • The “n” of the study is the number of subjects in the study • Source population • Population that the subjects were drawn from • Target population • Population we want to generalize our results to

8. Considerations in Planning • Carefully define the study sample • Clarify relationships between the numerator and denominator for the calculation of proportions, rates and ratios • Identify potential associations between outcome and the factors being studied • Is one gender more likely to be affected? • Does the rate of disease differ by geographical location? • etc.

9. Types of Measures The most common types of measures are: • Count • Proportion • Rate • Ratio

10. 1. Count Count: absolute number of individuals/animals that have a particular disease • ex., 26 people from St. Kitts and Nevis had evidence of exposure to Chikungunya virus All other measures of disease occurrence are based on counts

11. 2. Proportion Proportion: is the count of diseased individuals expressed as a fraction of the individuals that could be diseased • The number of diseased individuals • Divided by the number individuals in the study (study samples) • The animals in the numerator are included in the animals in the denomination • 26 / 83 horses had exposure to Babesiacaballi • Means: of 83 horses, 26 had exposure to B.caballi Portions are often calculated out and expressed as fractions (0 to 1) or percentages (O% to 100%): • The proportion of 83 horses exposed to B. caballiwas 0.31 • 31.3% of 83 horses had exposure to Babesiacaballi Most common proportion is Prevalence

12. 3. Rate Rate: an expression of the change in the amount of disease in a defined population per unit of time • The number of new occurrences of disease(cases) • Divided by total time of exposure in the study • Influenza occurred at the rate of 3.1 cases per 1,000 pig years (3100 pigs/year) in the study group • Note: some (non-epidemiologist) authors will use “rate” interchangeably with “proportion”, without reference to time : eg birth rate in cattle..THIS IS INCORRECT Most common rates are cumulative incidence and incidence density

13. 4. Ratio Ratio: a fraction in which the numerator is NOT part of the denominator • For every part of one thing how much is there of another thing • There were 6.1 ticks/1 sheep • There were 6.1 ticks per sheep • OFTEN ratios use a “:” rather than “/” • There were 6.1 ticks:1 sheep Odds is the ratio of the number diseased to the number not diseased

14. Prevalence and Incidence

15. Prevalence and Incidence Prevalence • Point Prevalence • Period Prevalence Incidence • Cumulative Incidence • Attack Rate (a specific type of cumulative incidence) • Incidence Density or Incidence Rate

16. Prevalence the concept The proportion of the study population that is diseased at any one time. • “Any one time” maybe a point or time interval • A measure of the amount of disease in the population at any one time • Interpret: the probability of being diseased

17. Point Prevalence The proportion of the study population that is diseased at a single point in time • The most common measure of prevalence • Is often determined by cross-sectional studies and surveys • Interpret: the probability that an individual in the study population is diseased at the time of the study

18. Calculating Point Prevalence Number of cases of disease present in the population at a particular time Total population at that specified time • Is expressed as: • Percentage (0% to 100%) • Proportion (0 to 1) • Fraction: # of diseased/1,000 population

19. Period Prevalence The proportion of the study group that is diseased during a specified period of time • Measures all the cases of disease in the study population over a period of time • Includes: • Old cases of disease that were present at the beginning of the time period • New cases of disease that occur over the time period. • Interpret: the probability that an individual in the study population is diseased during the period under study

20. Calculating Period Prevalence Number of cases of disease present in the population over a period of time Total population over that period of time • Is expressed as: • Percentage (0% to 100%) • Proportion (0 to 1) • # of diseased/1,000 population/time period

21. Incidence the concept The number of new cases of dz that occur in the study group over time • The rate of occurrence of new cases of dz in the study group • How frequently susceptible individuals become dzed over time • Only count new cases of dz • “Incident cases” are new cases of disease • Use to predict dz occurrence in a population • Interpret: the probability of a dz free individual becoming dzed = risk of becoming dzed

22. Cumulative Incidence The proportion of disease free (susceptible) individuals in a population who became diseased during a specified period of time • Include only susceptible (disease free) individuals in the study population at the beginning of the study • Include only new cases of disease that develop over the time period of the study • Interpret: the probability (risk) of a susceptible individual becoming diseased during the study period

23. Calculating Cumulative Incidence New cases of disease or events during the time period Total population at risk at the beginning of the time period • Expressed as: • Number with reference to time and the population at risk • There were 21 cases of TB per 100 cattle in a one year study • Dimensionless fraction (0 to 1 OR percent) with reference to time • The incidence of TB was 0.21 (or 21%) in one year

24. Cumulative Incidence Assumptions: • All individuals in the study group are at risk of getting the outcome (aka dz) of interest • Every individual is assessed at the beginning, to identify and remove “old” cases • Entire population at risk must be followed from the beginning of the study till the end

25. Attack Rate Cumulative incidence during an epidemic • Applied to a narrowly defined population: • Usually a group of exposed individuals • In foodborne outbreak it maybe the group of people that ate at a particular restaurant • In an outbreak of head lice among children it could be the children in a school experiencing an outbreak • Observed for a limited period of time • The time period of the epidemic • Interpret:the probability (risk) of becoming dzed during the course of an epidemic

26. Calculating Attack Rates # of new cases of dz during a specified time period (epidemic) Total population at risk (exposed) during that specified period (epidemic) • Expressed as: • Number with reference to time and the population at risk • The attack rate during an outbreak of Salmonella in a large dairy was 21 cases per 100 adult cows per week • Dimensionless fraction (0 to 1 OR percent) may not have a reference to time as it is assumed to be the time of the outbreak • The attack rate during the salmonella outbreak was 0.21 (or 21%) per week among adult dairy cattle

27. Incidence Density (Incidence Rate) Instantaneous rate of occurrence of new cases of dz among non-dzedindividuals in the population • Used when animals are entering and leaving the population during the study • Numerator is the number of new cases of disease during the study period • The denominator is calculated by adding together the number of time units (months, weeks, etc.) that each at risk individual in the population was observed • Interpretation: None at the individual animal level • Instantaneous potential for change in disease status per unit of disease free individual time • Rate of change of disease per unit of animal time

28. Incidence Density (Incidence Rate) # of new cases of disease during the study Sum of the length of time during which each individual in the population is at risk • Expressed as: • Number of cases per individual-time at risk • 15 cases per 100 individual-months at risk

29. Incidence Density: Holstein Cattle with Babesia in St KittsEach line is one cow’s time at risk (in months)before becoming dzed, sold or dyingThere were 9 cows in this study Sold to Nevis Babesia Babesia Babesia Babesia Died of Anthrax Babesia Time at risk (from susceptible to dzed, dead or not in the study)

30. Incidence Density: Holstein Cattle with Babesia in St Kitts5 cases in 67 cow-months at riskIncidence density is 0.07 cases per cow-month at risk Cow Time At Risk 8 cow-months Sold to Nevis Babesia 8 cow-months 6 cow-months Babesia 4 cow-months Babesia 7 cow-months Babesia 8 cow-months Died of Anthrax 11 cow-months 11 cow-months Babesia 4 cow-months 67 Cow-months at risk Time at risk (from susceptible to dzed, dead or not in the study)

31. Incidence Density • Can be presented in many different ways: • 7 cases/1000 cow-months at risk • 1 case/143 cow months at risk • 0.07 cases/cow-month at risk • These are all equivalent! Hint: ONLY the incidence density is reported as animal-timeat risk. Period prevalence and cumulative incidence are reported as cases / animals / time.

32. Prevalence vs Incidence Prevalence: a measure of the amount of dz in a population • Includes all cases, old and new, relative to the entire population • Used clinically to estimate the probability that an animal is diseased Incidence: a measure of the rate of dz occurrence • Includes only new cases, relative to the population at risk (individuals that do not already have the disease) always reported with a time component • Used for disease management, estimate the number of cases of disease that are expected to occur in the next time period

33. Relationship Between Incidence and Prevalence • A new case (incident case) becomes an old case (prevalent case) and remains so until recovery or death • Prevalence therefore depends upon incidence and duration of disease • The longer the “old cases” remain diseased and alive, the higher the prevalence can be • Thus prevalence of a disease may increase when incidence remains stable but survival of cases improves

34. Increased by: • longer duration of disease • prolongation of life without cure • increase in new cases • in-migration of cases • out migration of healthy animals • improved diagnosis • Decreased by: • short duration of disease • high case-fatality rate from disease • decrease in new cases • in-migration of healthy animals • out-migration of cases • improved cure rate Factors Influencing Prevalence

35. Prevalence vs. Incidence Prevalence Incidence Incidence is generally used for acutely acquired diseases Used to assess causes (risk factors) for disease Best measure to assess increasing or decreasing trends in disease frequency • Prevalence is used for more permanent states, or chronic conditions • Reflects the overall magnitude of a disease • Best measure to assess the overall burden of the disorder, including the costs and resources

36. Concept of disease “RISK” • Disease Risk is defined as the probability of becoming diseased • We use incidence as a measure for risk • Cumulative incidence is sometimes called incidence risk • At Risk means an individual is not diseased and is capable of becoming diseased • Risk Factors are factors that increase the probability of becoming diseased • Exposure To ARisk Factor means an individual has come into contact with a risk factor or has the risk factor before becoming diseased

37. Specific Measures: Mortality

38. Mortality Rates • Mortality rate: the incidence of death in a given time period # of deaths during the time period Total population during the same time • Cause-specific mortality rate: Mortality rate due to a specific disease or event # of deaths from a specific disease/event Total population during the same time

39. Cause-Specific Mortality Rate • There were 178 on-course fatalities of racehorses in the UK in one year, of an estimated population of 15,000 racehorses in training. • What was the cause (racing) specific mortality rate? = 178 deaths / 15,000 racehorses per year = 11.86 deaths / 1,000 racehorses per year etc.

40. Case Fatality Rate The number of cases that are fatal, within a specified time following disease onset or diagnosis # of individual cases of dz that die Total number of individuals with the dz Often expressed as a proportion (O-1) or percent Ex: 3 of 467 people (0.03%) with Bartonella henselae infections (cat scratch disease) died during the follow up period