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A APPENDIX FPPENDIX F

Section B of Introduction to mouth or nasal passages, upper respiratory tract,
CDC Guidelines Pages 4-6 and bronchi to reach the alveoli of the lungs. Once
in the alveoli, the organisms are taken up by alveo-
B. Epidemiology, Transmission, and Pathogen- lar macrophages and spread throughout the body.
esis of TB
Usually within 2-10 weeks after initial infection
The prevalence of TB is not distributed evenly with M. tuberculosis, the immune response limits fur-
throughout all segments of the U.S. population. ther multiplication and spread of the tubercle ba-
Some subgroups or persons have a higher risk for cilli; however, some of the bacilli remain dormant
TB either because they are more likely than other and viable for many years. This condition is re-
persons in the general population to have been ex- ferred to as latent TB infection. Persons with latent
posed to and infected with M. tuberculosis or be- TB infection usually have positive purified protein
cause their infection is more likely to progress to ac- derivative (PPD)-tuberculin skin-test results, but
tive TB after they have been infected (5). In some they do not have symptoms of active TB, and they
cases, both of these factors may be present. Groups are not infectious.
of persons known to have a higher prevalence of TB
infection include contacts of persons who have ac- In general, persons who become infected with M. tu-
tive TB, foreign-born persons from areas of the berculosis have approximately a 10% risk for devel-
world with a high prevalence of TB (e.g., Asia, Af- oping active TB during their lifetimes. This risk is
rica, the Caribbean, and Latin America), medically greatest during the first 2 years after infection.
underserved populations (e.g., some African- Immunocompromised persons have a greater risk
Americans, Hispanics, Asians and Pacific Islanders, for the progression of latent TB infection to active TB
American Indians, and Alaskan Natives), homeless disease; HIV infection is the strongest known risk
persons, current or former correctional-facility in- factor for this progression. Persons with latent TB
mates, alcoholics, injecting-drug users, and the eld- infection who become coinfected with HIV have ap-
erly. Groups with a higher risk for progression proximately an 8%-10% risk per year for develop-
from latent TB infection to active disease include ing active TB (8). HIV- infected persons who are al-
persons who have been infected recently (i.e., ready severely immunosuppressed and who become
within the previous 2 years), children less than 4 newly infected with M. tuberculosis have an even
years of age, persons with fibrotic lesions on chest greater risk for developing active TB (9-12).
radiographs, and persons with certain medical con-
ditions (i.e., human immunodeficiency virus {HIV} The probability that a person who is exposed to M.
infection, silicosis, gastrectomy or jejuno-ileal by- tuberculosis will become infected depends primarily
pass, being greater than or equal to 10% below on the concentration of infectious droplet nuclei in
ideal body weight, chronic renal failure with renal the air and the duration of exposure. Characteristics
dialysis, diabetes mellitus, immunosuppression re- of the TB patient that enhance transmission include
sulting fro receipt of high-dose corticosteroid or a) disease in the lungs, airways, or larynx; b) pres-
other immunosuppressive therapy, and some ma- ence of cough or other forceful expiratory measures;
lignancies) (5). M. tuberculosis is carried in air- c) presence of acid-fast bacilli (AFB) in the sputum;
borne particles, or droplet nuclei, that can be gener- d) failure of the patient to cover the mouth and nose
ated when persons who have pulmonary or when coughing or sneezing ; e) presence of cavita-
laryngeal TB sneeze, cough, speak, or sing (6). The tion on chest radiograph; f) inappropriate or short
particles are an estimated 1-5 mm in size, and nor- duration of chemotherapy; and g) administration of
mal air currents can keep them airborne for pro- procedures that can induce coughing or cause aero-
longed time periods and spread them throughout a solization of M. tuberculosis (e.g., sputum induction).
room or building (7). Infection occurs when a sus- Environmental factors that enhance the likelihood
ceptible person inhales droplet nuclei containing M. of transmission include a) exposure in relatively
tuberculosis, and these droplet nuclei traverse the small, enclosed spaces; b) inadequate local or gen-

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