Page 167 - Textbook of Pathology, 6th Edition

alcoholism and immunocompromised states like AIDS. 151
However, the exact incidence of disease cannot be
determined as all patients infected with M. tuberculosis may
not develop the clinical disease and many cases remain
reactive to tuberculin without developing symptomatic
disease. CHAPTER 6
HIV-ASSOCIATED TUBERCULOSIS. HIV-infected
individuals have very high incidence of tuberculosis all over
the world. Vice-versa, rate of HIV infection in patients of
tuberculosis is very high. Moreover, HIV-infected individual
on acquiring infection with tubercle bacilli develops active
disease rapidly (within few weeks) rather than after months
or years. Pulmonary tuberculosis in HIV presents in typical
manner. However, it is more often sputum smear negative
but often culture positive. Extra-pulmonary tuberculosis is
more common in HIV disease and manifests commonly by
involving lymph nodes, pleura, pericardium, and Inflammation and Healing
tuberculous meningitis. Infection with M. avium-intracellulare
Figure 6.20 Tuberculosis of the lymph nodes showing presence of (avian or bird strain) is common in patients with HIV/AIDS.
acid-fast bacilli in Ziehl-Neelsen staining.
MODE OF TRANSMISSION. Human beings acquire
infection with tubercle bacilli by one of the following routes:
Slow growers. These species grow mycobacteria on solid
media (in 2-3 weeks). Based on the colour of colony formed, 1. Inhalation of organisms present in fresh cough droplets
they are further divided into following: or in dried sputum from an open case of pulmonary
tuberculosis.
Photochromogens: These organisms produce yellow pigment 2. Ingestion of the organisms leads to development of tonsi-
in the culture grown in light.
llar or intestinal tuberculosis. This mode of infection of
Scotochromogens: Pigment is produced, whether the growth human tubercle bacilli is from self-swallowing of infected
is in light or in dark. sputum of an open case of pulmonary tuberculosis, or
Non-chromogens: No pigment is produced by the bacilli and ingestion of bovine tubercle bacilli from milk of diseased
the organism is closely related to avium bacillus. cows.
The examples of slow growers are M. avium-intracellulare, 3. Inoculation of the organisms into the skin may rarely occur
M. kansasii, M. ulcerans and M. fortuitum. from infected postmortem tissue.
The infection by atypical mycobacteria is acquired 4. Transplacental route results in development of congenital
directly from the environment, unlike person-to-person tuberculosis in foetus from infected mother and is a rare mode
transmission of classical tuberculosis. They produce human of transmission.
disease, atypical mycobacteriosis, similar to tuberculosis but
are much less virulent. The lesions produced may be SPREAD OF TUBERCULOSIS. The disease spreads in the
granulomas, nodular collection of foamy cells, or acute body by various routes:
inflammation. 1. Local spread. This takes place by macrophages carrying
Five patterns of the disease are recognised: the bacilli into the surrounding tissues.
i) Pulmonary disease produced by M. kansasii or M. avium- 2. Lymphatic spread. Tuberculosis is primarily an infection
intracellulare. of lymphoid tissues. The bacilli may pass into lymphoid
ii) Lymphadenitis caused by M. avium-intracellulare or M. follicles of pharynx, bronchi, intestines or regional lymph
scrofulaceum. nodes resulting in regional tuberculous lymphadenitis which
iii) Ulcerated skin lesions produced by M. ulcerans or M. is typical of childhood infections. Primary complex is primary
marinum. focus with lymphangitis and lymphadenitis.
iv) Abscesses caused by M.fortuitum or M. chelonae. 3. Haematogenous spread. This occurs either as a result of
v) Bacteraemias by M. avium-intracellulare as seen in tuberculous bacillaemia because of the drainage of
immunosuppressed patients of AIDS. lymphatics into the venous system or due to caseous mate-
rial escaping through ulcerated wall of a vein. This produces
INCIDENCE. In spite of great advances in chemotherapy millet seed-sized lesions in different organs of the body like
and immunology, tuberculosis still continues to be lungs, liver, kidneys, bones and other tissues and is known
worldwide in distribution, more common in developing as miliary tuberculosis.
countries of Africa, Latin America and Asia. Other factors
contributing to higher incidence of tuberculosis are 4. By the natural passages. Infection may spread from:
malnutrition, inadequate medical care, poverty, crowding, i) lung lesions into pleura (tuberculous pleurisy);
chronic debilitating conditions like uncontrolled diabetes, ii) transbronchial spread into the adjacent lung segments;