2.5 Epidemiology of tuberculosis


Tuberculosis infection and disease patterns among different populations are heterogeneous. Understanding the epidemiology of this disease is, therefore, vitally important as an aid to diagnosis, prevention, and public health program development.

Tasks of tuberculosis epidemiology.

  1. Identifiable populations at risk.
  2. The scope and impact of the infections.
  3. Temporal trends in infection patterns.
  4. Geographical locations.
  5. Reservoirs and mechanisms of transmission.
  6. Risk factors – why do some become infected and/or diseased and others not?

Epidemiological definitions (terms).

Epidemiological situation concerning tuberculosis is characterized by the following statistical data: infection; incidence; prevalence; mortality.
Infection (infected/contaminated) – percent of persons with positive tuberculin reactions, if the reactions are not due to vaccination.

Incidence Rate (incidence of new cases).
The incidence rate is the number of new cases of active disease (events) occurring in an identified population over a time period. Generally, tuberculosis morbidity is referred to in events per 100,000 populations per year.

Prevalence of tuberculosis.
Prevalence is the number of patients with active forms of tuberculosis in a population at the end of the year per 100000 population, the special importance has the prevalence index of active (MBT positive cases of lung tuberculosis). Prevalence thus reflects the cumulative morbidity from tuberculosis. If all new cases were promptly “cured” by treatment, the incidence and prevalence of disease would be closely approximate. But if patients are lost from therapy or partially treated, cases of chronic tuberculosis will accumulate, causing gross disparities in the incidence and prevalence values.

Mortality from tuberculosis – number of patients died because of tuberculosis per 100000 populations.

Smear-Positive (Bacillary, MBT+) Case of lung tuberculosis.
This refers to a patient with tuberculosis of the respiratory tract whose airway secretions, when examined by special stains and microscopy, demonstrate tubercle bacilli. Some authorities refer to these as “bacillary” cases.

Smear-Negative Pulmonary Case (MBT-).
This refers to a patient with pulmonary disease whose sputum microscopy examination fails to demonstrate bacilli. The diagnosis of disease is established by symptomatology, positive cultures, progressive changes on chest radiograph deemed to reflect disease activity, and/or other supporting data such as tuberculin skin test reactivity, epidemiological features, and-for infants and children-a history of exposure to tuberculosis.

Extra pulmonary Disease.
This is the case of a patient whose clinical illness presents with active inflammatory tuberculosis in organs outside the lungs. Strictly speaking, even endobronchial or pleural disease may be regarded as “extrapulmonary.” (In the United States, pleural disease is categorized as “extra-pulmonary”; in Canada and the United Kingdom, Russian Federation it is classified as “pulmonary”). Depending on age, race, and immunological competency, 5% to 70% of patients who develop active tuberculosis will manifest it primarily in organs other than the lungs. Most patients have either pulmonary or extrapulmonary tuberculosis; a minority manifests simultaneous disease in both systems. Although extrapulmonary and smear-negative pulmonary cases are both clearly components of the overall morbidity of tuberculosis, they are less significant epidemiologically than sputum smear-positive cases, which act as the primary vectors of transmission to others.

Annual Rate of Infection (ARI).
The annual rate of infection (ARI) is yearly incidence of new tuberculous infections among “eligible” (tuberculin-negative, not previously infected) members of a population, manifested primarily by tuberculin skin test conversion rates. The ARI has been employed as indirect or inferential marker of the prevalence of sputum smear-positive (communicable) cases; within a population. By following a group individuals known to be non reactive to tuberculin and observing the frequency with which their skin tests become reactive through time authorities have attempted to estimate the total tuberculosis morbidity within that community by comparison with established data bases. This technique has been employed primarily in developing nations that lack the resources for consistent diagnosis and case tabulations.

Tuberculosis infected.
This is the state of harboring viable tubercle bacilli within one’s body without manifesting signs or symptoms of overt disease. The great majority of normal individuals who are exposed to and infected with MBT enjoy this status throughout their entire lives.

Diseased tuberculosis patients – is the state of suffering from active, progressive invasion of an organ or organs by MBT. This typically is manifested by constitutional symptoms or signs or symptoms that relate to a specific organ system. In most cases, a tuberculin skin test is reactive. The most important bacteriological confirmation is cultivation of M. tuberculosis from the sputum, or from tissues of different organs. Largely for public health communication and reporting, World Health Organization recommends a system to classify persons with known or suspected disease and individuals being evaluated in contact investigations surrounding new cases. Information gathered in this system forms the backbone of case reporting in the world. As a morbid case of tuberculosis – is accepted a disease confirmed by detection of mycobacterium of tuberculosis, allocated from the affected focus, (with sputum, urine etc.) or received from tissues by biopsy.

Group 0. No known exposure to TB and a negative tuberculin test. (Typically, in a contact investigation some persons are deemed not have been exposed or infected).

Group 1. Tuberculosis exposure, no evidence of infection. Person known to have been exposed but tuberculin test is negative. If infant, may also include negative chest x-ray. May require follow-up at 3 months to confirm.

Group 2. Tuberculous infection, no disease. Persons with significant tuberculin reaction but no clinical, radio-graphic, or bacteriologic evidence of disease.

Group 3. Includes all patients with clinically active tuberculosis whose diagnostic studies are adequate to confirm the diagnosis; if inconclusive, should list as class 5. Essential data include site(s) involved, culture results, and treatment status for patients with negative cultures but an inferential diagnosis; chest x-ray and tuberculin status must be included.

Group 4. Tuberculosis, not clinically active. A history of previous episode(s) of tuberculosis or abnormal but stable chest x-ray, positive tuberculin test, negative bacteriology (if done), and no clinical or radiographic signs of disease. [Must note if person has received treatment for disease, preventive chemotherapy, or none.] Until active disease is excluded, should list as class 5.

Group 5. Tuberculosis suspect (diagnosis pending). Persons in whom active tuberculosis is suspected on basis of clinical, radiographic, and/or epidemiologic factors. Use this status for up to 3 months while complete evaluation is pending.

Global and continental epidemiology.

The profile of tuberculosis in the world today has been developed by a mix of direct observations and inferential means such as the annual risk of infection (ARI), as detailed above. Because tuberculosis is most extensive in impoverished nations, which typically have inadequate health information systems, much of the information is indirect. According to calculations using the ARI model by employing data from both developed and developing nations to calculate a relationship between the incidence of sputum smear-positive cases within a population and the risk to other members of that population of acquiring infection with the tubercle bacillus.

Because of deficiencies in reporting in the areas where tuberculosis is most prolific, these numbers are far lower. But increasing cases and rates were demonstrable for much of the world. Looking at longitudinal trends for tuberculosis, it could be noted a clear divergence between the industrialized nations, where incident case numbers and rates steadily and substantially declined, and the developing world, where the number of reported cases remained stable or rose. In the developing nations, if incidence rates declined, they did so only as a result of the dilution effects of explosive population growth. In general infected and tuberculosis patient’s pool grows in the world.

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