Epidemiology of TBand its controlDr. V. K. ChadhaSr. EpidemiologistNational TB InstituteBangalore
I. General concepts in TB Epidemiology II. Epidemiological indicators of TB and their estimation III. Global epidemiological trends of TB IV. TB situation in South East Asia - presentations by Country participants V. Prospects of TB control
Aims of Epidemiology ? • To describe natural history of disease • Describe Distribution and relative importance • Measure frequency • To define risk groups • To evaluate interventions • To describe trends • To predict future trends and changes in disease presentation.
What is Epidemiology ? Epi - among ; Demos - People ; Logos - Study DEFINITION Epidemiology is the study of the - • Frequency • Distribution - time, place & person • Determinants - physical, biological, social, behavioural & cultural of health problems & health related events and application of this study to control health problems.
Risk of exposure ? • Incidence / prevalence of infectious TB in the community • Duration of infectiousness • opportunities for case - contact interactions -Urban/Rural -No. of individuals in the house holds
Risk of Infection ? • No. of infectious droplets produced • Volume of shared air space • Length of exposure • Ventilation • Climatic conditions
Household transmission of TB- important epidemiological factor • Case control study in Malawi
Each case leads to two cases -_-_- 1 Infectious case 20 contacts 2 cases of TB 1 Non-infectious
Risk factors for disease given that infection has occurred ? [Relative Risk of remotely acquired infection = 1] (0.2% per year)
Incidence of TB in South Africa per 1000 population IJTLD,3(9),1999,791-798
Other High Risk Groups • Populations in war / civil unrest • Refugees and migrants • Slum dwellers • Homeless people/Foot path dwellers • Smoking • Prisoners
TB in prisons Studies in Thailand • TB incidence 90 times higher in prisons • High HIV sero-positivity in TB cases • High levels of drug resistance • RFLP studies signify role of recent transmission
Determinants of death? • Severity of illness • Smear positivity • delay in diagnosis • quality of treatment • drug susceptibility pattern
Epidemiological indicators of tuberculosis ? • Prevalence of infection • Incidence (average annual risk) of infection (ARI) • Prevalence of disease • Incidence of disease • Tuberculosis mortality rates
Estimating prevalence of infection • Study population-sampling • Registration of eligible age group - house-to-house / school based. • Informed consent. • Examination for BCG scar. • Tuberculin testing with 1TU/2TU PPD RT23 with tween 80. • Reading of reaction sizes appx. 72 hours later.
What is the rationale behind tuberculin surveys in children ? • Extent or recent transmission • Study trends in TB epidemiology • (Ultimate aim of control programme is to replace older more infected cohorts with younger less infected cohorts)
Dual skin testing at two different periods -Conversion -Boosting Compute average annual risk of infection (ARTI) = 1-(1-P)1/A
A RT I • Key epidemiological indicator in developing countries. • It is the probability of acquiring new tuberculosis infection or re-infection over the course of one year.
A R I expresses the overall impact of various factors influencing the transmission of tubercle bacilli ! - Load of infectious cases - Efficiency of case finding -Efficiency of treatment programme
ARI identifies the regions of high transmission • It provides an indirect estimate of size of sources of infection • Any change in disease burden and programme implementation is first reflected in the change in ARI • It holds the key to the study of epidemiological trends which are more important than exact estimates of disease prevalence
DISEASE SURVEY METHODOLOGY • Sampling of representative population • House to house registration • Screening: - MMR X-ray of all above five years of age - Symptomatic screening • X-ray pictures read by two independent readers and by an umpire reader • Sputum specimens (2/3) collected from persons with abnormal X-ray shadows & / or chest symptomatics • Sputum examination by direct microscopy (and culture).
Styblo derived the following relationship from data of pre- chemotherapy • Every one percent of ARTI corresponds to 50 new smear positive cases per 100,000 population per year
Relationship between ARI & Incidence of smear positive cases of Pulmonary Tuberculosis(Indian studies)
Relation between ARI and Incidence ! • Situation : Disease incidence remains same but the risk of infection declines Q 1. When is this situation likely? Q 2. What is the impact on equation (relationship) ?
The equation is dependent more on number of infections generated per case and not merely on incidence
Disease mortality rates ! • Community based prospective studies • Death certification
ESTIMATION OF ANNUAL RISK OF TUBERCULOUS INFECTION IN DIFFERENT ZONES OF INDIA • A CROSS SECTIONAL STUDY • 2000-2003