8-Tuberculosis Flashcards
how TB is transmitted?
- Spread by droplet nuclei
- Expelled when person with infectious TB coughs, sneezes, speaks, or sings
- Close contacts at highest risk of becoming infected
- Transmission occurs from person with infectious TB disease (not latent TB infection)
Patients with latent TB can transmit the disease. True/False
False
Only active TB is transmitted
what is the epidemiology of TB?
1) Sex: ♂ > ♀ (2:1)
2) United States
- -The incidence of TB infection in the US has been slowly declining.
- -Foreign-born individuals (especially Asians and Hispanics) account for two-thirds of new TB cases.
3) Globally
- -A leading infectious cause of death worldwide
- -Despite ∼ 1 in 3 individuals being infected with TB worldwide, the overall incidence and prevalence have been declining.
- -Countries with the highest incidence of TB: India, Indonesia, China, Nigeria, Pakistan, and South Africa
4) The incidence of multidrug-resistant tuberculosis is steadily rising.
what are the general microbiological features of mycobacteria?
- -Nonmotile, aerobic, gram-positive, acid-fast bacilli with a rich lipid cell wall
- -Resistance: survives in aerosols, even over long distances
- -Acid-fast: able to survive in gastric secretions
- -Complex waxy, resistant cell walls increase the resistance to antimicrobial medication → Antibiotics only have an effect if used in combination over a long period of time.
MTB is G+ or G-?
The mycobacterial cell wall, despite having gram-positive characteristics, does not stain on culture due to unique features in the cell wall structure. Nevertheless, it is classified as a gram-positive bacilli.
what are the risk factors of TB?
- Originating/living in a country where TB is endemic
- Previous history of TB or exposure to TB
- HIV/AIDS
- Socioeconomic issues
- Overcrowding, institutions, prisons
- Lifestyle/drug abuse/alcoholism
- Other medical conditions e.g. Diabetes, lung amage (COPD)
- Immunosuppressive medications
- TNF inhibitors
what are the TB symptoms?
- Cough
- Fever
- Night sweats
- Weight loss
- Haemoptysis
what is the classification of TB?
1) TB infection; no active disease:
- -Positive tuberculin skin test
- -No clinical, bacteriological, or radiographic evidence of TB
- -Consider chemoprophylaxis in certain patients in this group
2) TB infection; active disease, clinical evidence:
- -Positive tuberculin skin test OR positive M. tuberculosis culture (if done)
- -Clinical and radiographic evidence of TB
3) TB not clinically active
- -History of episodes of TB OR
- -Abnormal but stable radiological changes; positive tuberculin skin test; negative M. tuberculosis culture
- -May have completed chemoprophylaxis therapy; may never have received therapy before or may be completing chemoprophylaxis currently.
what is the latent TB?
- -Definition: primary infection without any pathological findings on radiological imaging; however, screening tests indicating previous infection with M. tuberculosis are positive.
- -The lifetime risk of reactivation TB for a person with LTBI is about 5–10%.
clinical history taking of patients with suspected TB should include?
1)Cough • Dry or productive – not specific • Sputum color – not specific • Duration – weeks to months - suspicion • Unresponsive/ partially responsive to standard antibiotics 2)Haemoptysis • Mild to moderate • Intermittent • Painless **Similar to lung ca 3)Fever • Spiking temperature • PUO – raises the suspicion of TB 4)Night sweats • Drenching sweats 5)Shortness of breath • Generally only if underlying respiratory disease e.g. COPD • Occurs if there is a pleural effusion
what are the possible physical examination signs of the patient with TB?
- None
- Weight loss
- Pleural effusion
- Lymphadenopathy
- Clubbing
- Monoarthropathy
- Erythema nodosum
what is digital clubbing?
A physical finding characterized by painless swelling of the distal phalanges. Often defined by an angle ≥ 180° between the base of the nail and its surrounding skin (Lovibond angle). The nailbed often feels spongy when pressed and springs back when released. Typically associated with chronic hypoxemia (e.g., cardiac shunts, interstitial lung disease, lung cancer, cystic fibrosis), though patients with COPD alone typically do not develop this finding. Thought to be due to fibrovascular proliferation in the region of the nail bed due to accumulation of megakaryocytes in digital vessels that are normally filtered in the lung.
what is the erythema nodosum?
An inflammation of subcutaneous fat caused by a delayed hypersensitivity reaction. Women in early adulthood are commonly affected. Most cases are idiopathic, but may be associated with infections and autoimmune disorders (e.g., ulcerative colitis).
what are the differential diagnoses of TB?
- Lung cancer – similar symptoms – persistent cough, haemoptysis, weight loss, cavitating lung lesion (on CXR)
- Sarcoidosis – radiological similarities – upper lobe infiltrates, lympadenopathy. Relatively asymptomatic.
- Lymphoma – spiking fever, night sweats. Lymphadenopathy on imaging
what is the spectrum of disease of TB?
- Pulmonary TB
- Pleural TB
- Extra-pulmonary TB – Bone, renal, skin, adenitis
- Meningeal TB
- Miliary TB
what is the pathophysiology of TB?
1) Alveolar macrophages (CD14+) phagocytose the TB bacteria but cannot eliminate them.
- -Cord factor (trehalose-6,6’-dimycolate): a glycolipid in the cell wall of M. tuberculosis that inhibits fusion of phagosome and lysosome, thereby preventing lysis of phagocytosed mycobacteria
- -By hiding inside the macrophages, TB bacteria do not provoke a humoral immune response (antibody production by B lymphocytes) in the host → therefore, antibody detection tests play no role in the diagnosis of TB.The mycobacteria replicate safely within the macrophages
- -Phagocytosed M. tuberculosis can only be destroyed by activated T lymphocytes.
2) The infection is usually contained in the lung by formation of caseating granulomas that limit damage to the lungs and bacterial dissemination.
- -Granuloma formation is caused by a delayed T cell-mediated reaction (i.e., type IV hypersensitivity reaction) that leads to the activation of alveolar macrophages (i.e., epithelioid cells) and their fusion into multinucleated giant cells.
- -The bacteria may remain dormant in the granulomas for many years without any active clinical disease manifesting (latent TB).
- -Immunosuppression (compromised T cell function) → dormant bacteria become active, replicate in the macrophages and spread into the lungs and other organs (active TB).
3) IFN-γ deficiency: impaired phagocytosis and impaired granuloma formation → disseminated TB
why phagocytes cannot digest MTB?
M. tuberculosis manipulates the host’s macrophages and arrests the normal progression of the phagosome, a compartment of the macrophage. Usually, phagosomal contents (e.g., intracellular bacteria) are destroyed when they are exposed to lysosomal hydrolases, reactive oxygen, and nitrogen.
