NUMBERS AND RATES IN EPIDEMIOLOGY

1. NUMBERS AND RATES IN EPIDEMIOLOGY A simple count of cases (numbers) not provide all the information a health professional may need. To compare the occurrence of a disease at different times, a health professional converts the case counts into rates, which relate the number of cases to the size of the population in which they occurred.

2. EPIDEMIOLOGY Prevalence and incidence The prevalence of a disease is the number of cases in a defined population at a specified point in time, while its incidence is the number of new cases arising in a given period in a specified population – these are fundamentally different ways of measuring occurrence and the relation between prevalence and incidence varies between diseases.  High prevalence and low incidence  Diabetes • Low prevalence and high incidence  Common cold. Measuring prevalence and incidence basically involves the counting of cases in defined populations at risk. Population at risk: That part of a population which is susceptible to a disease.

3. Data from institutions represent an unknown proportion, of the cases in the community. Institutional cases are often no more than the tip of the iceberg; the nature and extent of the larger mass beneath the surface can only be discovered by well-designed epidemiological studies.

4. EPIDEMIOLOGIC RATES Prevalence rate: P = No of current cases at specified time x 10ⁿ Population at risk Incidence rate: I = No of new cases in a stated period x 10ⁿ Population at risk Crude birth = No of live births in a year_ rate Mid – year population Crude death = No of deaths in a year_ rate Mid – year population Natural increase = No of live births minus No. of deaths in a year Mid – year population Fertility rate = Total No. of births in a year____ No. of women aged 15-49 years Maternal Mortality = Maternal pregnancy-related deaths in one year_ rate Total births in same years

5. EPIDEMIOLOGIC RATES Still birth = Annual No. of foetal deaths after 28 weeks gestation__ rate Total No. of births in a year Perinatal Mortality = Annual No. of stillbirths and deaths in the first 7 days rate Total No. of births in a year Neonatal Mortality = Annual No. of deaths in the first 28 days_ rate No. of live births in a year Postneonatal = Annual No. of deaths between 28 days and 1 year Mortality rate No. of live births in a year Infant Mortality = Annual No. of deaths in the first year_ rate No. of live births in a year Under five mortality = Annual No. of deaths under 5 years_ rate No. of live births in a year Under weight at born= No. of live births minus 2500 g_ No. of live births

6. NATURAL HISTORY AND SPECTRUM OF DISEASE USUAL TIME OF DIAGNOSIS PATHOLOGIC CHANGES ONSET OF SYMPTOMS EXPOSURE STAGE OF SUBCLINICAL DISEASE STAGE OF RECORVERY, DISABILITY OR DEATH STAGE OFCLINICAL DISEASE STAGE OF SUSCEPTIBILITY

7. NATURAL HISTORY AND SPECTRUM OF DISEASE • Is progress of a disease process in an individual over time, without intervention. • The process begins with exposure. • Without medical intervention the process end with recovery, disability or death. • The process begins to accumulation of factors sufficient in a susceptible host. • A period of subclinical follows exposure ending with the onset of symptoms, called in infectious diseases: Incubation period; for chronic diseases: Latency period. • The onset of symptoms mark the transition from subclinical to clinical disease. • Some pathologic changes may be detectable with laboratory, radiographic or others. • Identify the disease during this phase since early intervention may be more effective. • Most diagnoses are made during of clinical disease.

8. CASE DEFINITION IN EPIDEMIOLOGY Is a set of standard criteria for deciding whether a person has a particular disease or other health – related condition. Consist of clinical criteria limitations on time, place and person. Usually include confirmatory laboratory tests or combinations of symptoms (subjective complains), signs (objective physical findings) and other findings.

9. BEHAVIOR IN INFECCION FOCUS Chemoprophylaxis: The protection of persons who are exposed to the risk of infection (Malaria). Disinfestations: Destruction of insects, rodents or pests presents on the person or the clothes or in the surroundings, and which may transmit disease. Disinfection: The act of disinfecting to destroy microorganisms, but not usually bacterial spores, reducing the number of microorganisms to a level which is not harmful to health. Terminal disinfection: Disinfection of a sick room and its contents at the termination of a disease. Fumigation: Disinfection by exposure to the fumes of a vaporized germicide. Quarantine: The period of isolation of an infections or suspected case, to prevent the spread of disease. For contacts, this is the longest incubation period known for the specific disease.

10. PATTERNS OF EXPECTED OCCURRENCE Endemic: Persistent level of occurrence with a Low-to-moderate disease level. Hyperendemic: Persistent high level of occurrence. Sporadic: Irregular pattern of occurrence, with occasional cases occurring at irregular intervals. Epidemic: Occurrence (outbreak) of a disease within an area is clearly in excess of the expected level for a given time period. Pandemic: Epidemic spreads over several countries or continents and affects a large number of people. HIV/AIDS Pandemic. Epidemic occur when an agent and susceptible hosts are present in adequate number, and the agent can effectively be conveyed from a source to the susceptible hosts.

11. OTHER INDICATORS OF THE HEALTH STATUS Life expectancy: Average number of years an individual of a given age is expected to live if current mortality rates continue. USA: 71.6 Japan: 75.8 Mauritius: 65.0 Gambia: 54 Years of potential life lost: Is based on the years of life lost through premature death Life expectancy free from disability More complex measures take into account not only the duration of life but also some notion of its quality

12. BASIC CONCEPT OF EPIDEMIOLOGY Carrier: A person without apparent disease who is nonetheless capable of transmitting the agent to others. Asymptomatic Carrier Carrier who never shows symptoms of disease while infected. Chronic Carrier A person who continues to harbour an infection agent for an extended period of time (months or years) following the initial infection. Convalescent Carrier Someone who is capable of transmitting an infections disease after they are clinically ill (during convalescence). Direct Contact Transmission of an infections agent through kissing, skin-to-skin contact, and sexual intercourse. Direct Transmission Immediate transfer of an agent from a reservoir to a susceptible host by direct contact or droplet spread.

13. NOTIFICATION OF DISEASES National Notification: Is a list of certain diseases, cases of which must be reported to the appropriate health authority. It includes communicable and chronic and non-epidemic infections. Factors may limit the usefulness of notifications: • Concealment of cases – (Leprosy, HIV/AIDS). • Errors of diagnosis. • Incomplete reporting (ignorance or negligence of health workers). International Notification: Few diseases are subject to notification on the basic of international agreement. • Plague • Cholera • Yellow fever WHO is putting emphasis the capabilities of countries to carry out surveillance of major communicable diseases which tend to spread from country to country (Influenza, HIV Infection).