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Tuberculosis

الكلية كلية الطب     القسم  الاحياء المجهرية     المرحلة 3
أستاذ المادة حبيب صاحب نهر المزيداوي       4/19/2011 10:31:08 PM

Tuberculosis:

 

Mycobacterium tuberculosis complex:

 

General concept:

 

Tuberculosis primarily affects the lower respiratory system and is characterized by a chronic productive cough, low-grade fever, night sweats, and weight loss.

 

Structure:

 

Mycobacteria are slender, curved rods that are acid fast and resistant to acids, alkalis, and dehydration. The cell wall contains complex waxes and glycolipids. Multiplication on enriched media is very slow, with doubling times of 18 to 24 hours; clinical isolates may require 4 to 6 weeks to grow.

 

Classification and Antigenic Types:

 

 On the basis of growth rate, catalase and niacin production, and pigmentation in light or dark, mycobacteria are classified into members of the Mycobacterium tuberculosis complex (M tuberculosis, M bovis, M africanum, M microtii) and nontuberculous species. Gene probe technology now facilitates this distinction.

 

Pathogenesis:

 

Tuberculous mycobacteria enter the alveoli by airborne transmission. They resist destruction by alveolar macrophages and multiply, forming the primary lesion or tubercle; they then spread to regional lymph nodes, enter the circulation, and reseed the lungs. Tissue destruction results from cell-mediated hypersensitivity.

 

Host Defenses:

 

Susceptibility is influenced by genetic and ethnic factors. Acquired resistance is mediated by T lymphocytes, which lyse infected macrophages directly or activate them via soluble mediators (e.g., gamma interferon) to destroy intracellular bacilli; antibodies play no protective role.

 

Epidemiology:

 

 M tuberculosis is contagious, but only 5-10 percent of infected normal individuals develop active disease. Tuberculosis is most common among the elderly, poor, malnourished, or immunocompromised, especially persons infected with human immunodeficiency virus (HIV). Persistent infection may reactivate after decades owing to deterioration of immune status; exogenous reinfection also occurs.

 

Diagnosis:

 

Recent infection with M tuberculosis results in conversion to a positive Mantoux skin test with purified protein derivative (PPD). A diagnosis of active disease is based on clinical manifestations, an abnormal chest radiograph, acid-fast bacilli in sputum or bronchoscopic specimens and recovery of the organism. Assays based upon amplification of mycobacterial genes in clinical specimens are currently being tested.

 

Treatment and Control:

 

Therapy consists of a 6 to 9 month course of isoniazid, rifampin, pyrazinamide and ethambutol. Additional drugs may be used if drug resistance is suspected (e.g., infection in Southeast Asian immigrants or in areas where outbreaks of drug-resistant tuberculosis have been documented). If the patient is HIV-positive, treatment for longer periods (9-12 months) is recommended. PPD conversion without other signs or symptoms may warrant prophylactic isoniazid therapy for 6 months. M bovis BCG vaccine is used in more than 120 countries, but its efficacy is controversial. Although BCG has not been used routinely in the U.S., the current epidemic has prompted a reevaluation of its use, especially in high-risk populations.

 


المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .