Principles of Epidemiology

Epidemiology is an aspect of medical science that deals with frequency, distribution, determinants and deterrents of diseases and health-related events in human population with the hope of taking appropriate measures to address the problems

Questions asked in epidemiology— what, why, when, how, where and who are affected by disease and health-related problems

It is measured by Rates and Ratio

Proportion

• A fraction, in which the numerator includes only individuals who meet the case definition and the denominator includes individual in the study population who do & do not meet the case definition but are at risk i.e. A/(A+B) where A is a target event of your case definition; A+B = total no. of events (target + Non-target)
• E.g. proportion of females with HTN in clinic today = [females with HTN]/ [All females]

Rates

• Essential to compare disease burden in different populations, to give clue to the aetiology of the disease and this initiates strategies for control of the disease
• It is same as Proportion BUT with Time dimension
• It is = A / (A+B), but event occurring over a period of time

Ratio

• A/B
• A special fraction in which the numerator includes only individuals who meet the case definition and the denominator includes only individual in the study population who do not meet the case definition but are at risk
• NB: Difference btw Rate and Ratio: In Rates, numerator is part of denominator but for Ratio, it is not like that

Disease occur in patterns and these patterns are used to generate hypothesis about the possible aetiology

Disease may be distributed in form of

1. Person
2. Place
3. Time

Time distribution

• Seasonal trends/changes in disease occurrence. E.g. Malaria occurrence in rainy season
• Secular trends/changes
• Disease does not increase in the community but appears to do so because of interventions put in place. E.g. If, in a community, HTN is defined as ≥125/95 mmHg (instead of ≥ 140/90 mmHg), then you would have an increase in cases of people with HTN, who were not initially defined as HTNsive
• Periodic changes
• E.g. comparing disease outbreaks of previous years with that of present year

Person distribution (who is affected?)

Here disease is characterized based on

• Age group
• Extremes of ages OR
• Working age group
• Gender
• Marital status
• Occupation
• Ethnic group
• Religion
• Socioeconomic
• Personal habits e.g. alcoholic intake, cigarette smoking

Place distribution (where is the disease occurring?)

• Variation in climatic zones
• Variations btw states in a country/nation (National variation)
• Variations btw LGAs in a state (Rural-urban variation)

Biological agents

• Viruses
• Bacteria
• Fungi
• Parasites

Physical agents

• Temperature = extremes of temp
• Radiation
• Noise
• Vibration

Chemical agents

• Pesticides
• Caustic agents

Mechanical agents

• Trauma
• Falling from height
• RTAs
• Cuts

Behavioural factors (Lifestyles)

• Cigarette smoking
• Alcohol intake
• Drug abuse
• Indiscriminate sexual activities

Socioeconomic practices

• Traditional practices
• Beliefs
• Religious practices

To prevent disease one or more of the following should be taken care of:

• Reservoir
• Route of transmission (ROT)
• Susceptible host

Reservoir

Elimination of reservoir via chemicals, Radiation, sanitation

For Human reservoir

• Early diagnosis and prompt treatment by introducing screening
• Mass Chemotherapy (depending on the disease condition)
• Segregation/Isolation/Quarantine

Route of transmission

Interrupting ROT by:

• Medical method
• Chemical agents
• Proper sewage disposal
• Food sanitation
• Environmental sanitation

Susceptible host

Build-up Herd immunity of suspecting population

• Chemoprophylaxis
• Vaccination
• Improving lifestyle e.g. stop smoking cigarette, etc.

Relocation of settlements, if interruption of ROT and building immunity does not work

To identify Aetiology of a disease condition— in terms of Host factors, Risk factors, Genetic factors and Social factors

For disease classification

• Classify disease in relation to organism involved or body systems affected

To study the Natural History of disease (i.e. the broad spectrum of disease) i.e. Stages of exposure, infection, latent period of disease manifestation, Full recovery, Recovery with disability

To Access the Health status of the population

• To know the trend of disease occurrence
• To observe disease occurring over a period of time

Used in Health Planning

To study the impact of control strategies

For community diagnosis

To assess health status of a community, the population of the community should be assessed holistically by looking at:

• Hospitals
• Laboratory
• Health establishment
• Morbidity and Mortality data
• Etc.

RATES is used to assess health status:

• Crude Rate
• Specific rate

• Incidence Rate (IR)
• Prevalence Rate
• Attack Rate
• Secondary Attack rate

Incidence Rate (IR)

Incidence rate represents the rate of new cases of a condition observed within a given period- affected population- in relation to the total population within which these cases have arisen (in te same period - the target population)

Incidence rate is generally expressed in 'number of persons affected per 100,000 of the population, per year', in order to allow for comparisons between different populations and time periods.

It estimates the risk of developing a disease in specified population during a specified period of time

Prevalence Rate

Prevalence Rate = Incidence rate x duration of disease (years)

It is the proportion of a population with a disease or a particular condition at a specific point in time (point prevalence) or over a specified period of time (period prevalence).

It is usually expressed as a percentage

Attack Rate

It is the same as incidence rate, BUT it is expressed in percentage and is used especially in epidemics or outbreaks

Secondary Attack rate

Defined as the number of exposed person developing the disease within the range of incubation period following exposure to a primary case.

It measures the frequency of new cases of a disease among contacts of known/index/primary cases during the epidemic period

Example:

• 7 cases of Hepatitis occurred among 70 elderly patients in a place. The total no. of relations/family members + those that came in contact with these 7 infected patients were 32.
• In several weeks, 5 family members of the 7 infected patients also developed hepatitis
• Attack Rate = 7/70 X 100 = 10% [population at risk = 70]
• Secondary attack rate = 5/ (32-5) X100 = 20% [population at risk = 32-5]

• Crude Mortality Rate
• Case Fatality Rate
• Age-specific MR
• Perinatal Mortality rate
• Neonatal mortality rate
• Post-neonatal Mortality rate
• Infant MR
• Proportional Mortality rate
• Maternal Mortality rate

Crude Mortality Rate

• A measure of the frequency of occurrence of death in a defined population over a specified period of time
• E.g. In 2001, there were 12,000 deaths from all causes among 2 million population of Atlanta, Georgia.

Case Fatality Rate

• It is the proportion of persons with a particular condition (cases) that die from the condition.
• It is usually expressed in %

• Usually used in Epidemics

Age-specific MR

• It is the MR limited to a particular age group

Infant MR

• It is one of the most commonly used measures of comparing health service among nations

• Defined as no. of infant (< 1yr) deaths per 1000 live birth per year

Perinatal Mortality rate

• The number of stillbirths and deaths in the first week of life per 1,000 total births, the pernatal period commences at 22 completed weeks of gestation, and ends seven completed days after birth.

Neonatal mortality rate

• Neonatal period is defined as period from birth up to 28 days
• Defined as number of Neonatal death per 1000 live birth per year

Post-neonatal Mortality rate

• Post-neonatal period is the period from 28 days of age up to but not including 1 year of age
• Defined as number of death of children btw 28 days to 1 year per 1000 live births per year

Proportional Mortality rate

• It is the number of death due to a particular cause or in a specific age group per 100 or 1000 deaths

Maternal Mortality rate

• The annual number of female deaths from any cause related to or aggravated by pregnancy or its management (excluding accidental or incidental causes) during pregnancy and childbirth or within 42 days of termination of pregnancy, irrespective of the duration and site of the pregnancy.

It is a legal document, signed by a licensed doctor who saw the patient 24 hrs b4 death or by a coroner, after death, indicating the following information:

• Cause of death
• Direct cause, on the first line
• Antecedent cause
• Contributing cause— last line
• Name, Gender, Place of residence, Date of death,
• Other information
• Birth Date, birth place, Occupation