EPIDEMIOLOGY, TRANSMISSION, AND PATHOGENICITY OF TB. TB is not evenly distributed throughout all segments of the U.S. population. Some subgroups or individuals have a higher risk of TB either because they are more likely than the general population to have been exposed to and infected by TB or because they are more likely to progress to active TB once infected. In some cases, both of these factors may be present.
Suspect populations include: Native Americans, immigrants from countries with
high incidence of TB, homeless persons, past/current prison inmates, alcoholics,
intravenous drug users, the elderly, and contacts of persons with active TB.
Persons with higher incidence of progression from latent to active TB include persons with such medical conditions as: Human Immunodeficiency Virus (HIV) infection, silicosis, post gastrectomy, jejuno-ileal bypass surgery, being 10 percent less than ideal weight, chronic renal failure, diabetes mellitus, immunosuppression due to high doses of immunosuppressive therapy, and some malignancies. Also included are those: infected with TB within the last two years, children less than 3 years, and persons with fibrotic
lesions on chest radiographs. Symptoms of TB include: a cough lasting 2 weeks or
greater, bloody sputum, night sweats, weight loss, anorexia, and fever.
The TB organism is carried in airborne particles known as droplet nuclei, that can be generated when persons with pulmonary or laryngeal TB sneeze, cough, speak, or sing. The particles are estimated to be approximately l-5 microns in size, and normal air currents keep them airborne and can spread them throughout a room or building. Infection occurs when a susceptible person inhales droplet nuclei containing TB, and bacilli are able to traverse the mouth or nasal passages, upper respiratory tract, and bronchi to reach the alveoli of the lungs. Once in the alveoli, the organisms are taken up by a alveolar macrophage and spread throughout the body. Usually within 2 to 10 weeks after initial infection with TB, the immune response limits further multiplication and spread of the tubercle bacilli; however, some of the bacilli remain dormant and viable for many years. This is
known as latent TB infection. Persons with latent TB infection usually have a positive purified protein derivative, (PPD) skin test, no symptoms of active TB, and are not infectious. In general, persons with latent TB infection have approximately a 10% risk during their lifetime for the development of active TB. The risk is greatest in the first 2 years after infection, but some risk persists for decades.
Persons with immunocompromising conditions have a greater risk for the progression of latent TB infection to active disease. The HIV is the strongest known risk factor yet identified for the progression from latent TB infection to active TB disease. Persons with latent TB infection who become infected with HIV have approximately an 8 to 10 percent risk per year for the development of active TB. Persons who are infected with HIV and become newly infected with TB have an even greater risk for the development of active TB.
The probability that a susceptible person will become infected with TB depends primarily upon the concentration of infectious droplet nuclei in the air and the duration of exposure. Characteristics of the TB patient that enhance transmission include: (1) Disease in the lungs, airways, or larynx, (2) presence of cough or other forceful expiratory measures, (3) presence of acid-fast bacilli (AFB) in the sputum, (4) failure of the patient to cover the mouth and nose when coughing or sneezing, (5) presence of cavitation on chest radiograph, (6) short duration of adequate chemotherapy, 'and (7) administration of procedures that can induce coughing or cause aerosolization of TB (e.g., sputum induction, administration of aerosolized medication, etc.). Environmental factors that enhance the likelihood of transmission include: (1) exposure of susceptible persons to an infectious person in relatively small enclosed spaces, (2) inadequate local or general ventilation that results in insufficient dilution and/or removal of infectious droplet nuclei, and (3) recirculation of air containing infectious droplet nuclei.