Microbiology-Pulmonary Tb Flashcards
How is Tb transmitted? What percent of these people will become infected? Manifest active disease? Develop extra-pulmonary disease?
Aerosolized droplet nuclei. 30% of exposed are infected. 10% of infected develop disease. 10-20% of diseased develop extra-pulmonary disease. Most people form a granuloma around the infection that keeps it under control.
Who is at risk for Tb exposure?
Close contact with people who have Tb (foreign born, high-risk congregate settings and medical personnel).
What group of people are at risk for developing Tb disease?
Immunocompromised (HIV, diabetes, cancer), children and people using illicit drugs.
You are working at a health clinic in Haiti and someone with Tb coughs right into your open mouth. How does disease progression occur from here if you are going to develop active pulmonary symptoms?
Inhaled droplet implants bacilli in alveolar macrophages, inflammatory process causes granuloma and caseating necrosis, bacilli are released from infected cells and move to regional lymph nodes, bacilli spread in blood and lymph, bacilli are removed systemically, granuloma recedes and calcifies. Latent bacteria can re-activate years later.
What is the Ghon complex?
The primary Tb granuloma + the adjoining lymph node.
What are the triggers for development of secondary Tb and where do these re-activations normally occur?
Anything that diminishes the immune system (illness, malnutrition, aging). Re-activations usually occur in the upper lung if viable bacteria survives in the Ghon complex.
What happens to the patient when a latent Tb infection breaks out of the Ghon complex?
Miliary Tb throughout the lungs. The bacilli can also disseminate into the systemic circulation and cause meningitis and Pott’s disease (dissemination into bone/muscle)
Why does pasteurization of milk prevent Tb infection?
Unpasteurized milk contains M. bovis that seeds in the tonsils causing lymphadenitis, which can disseminate to the lungs and cause miliary Tb.
When do the whistles start going off in a patient with a positive PPD (purified protein derivative of M. Tb)?
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What is IGRA?
Interferon gamma release assay. You take a patient’s blood sample and expose it to PPD. You then measure IF-gamma levels to see if there is an immune response against the Tb antigen. This is more sensitive than a PPD skin test.
What is indicated by the arrows in these patients?
Left = latent Tb Ghon complex. Right = cavitary lesion in active Tb.
What is the gold standard for confirming active Tb infection?
Sputum culture.
Why do mycobacterium resist gram stain even though they have a gram + cell envelope? What test do you use to visualize mycobacterium?
They lack an outer membrane and have a high lipid content that does not absorb the gram stain. You visualize these in the lab with an acid-fast test.
How do the innate characteristics of mycobacterium’s cell envelope contribute to its virulence?
PGL-I (binds complement and promotes phagocytosis by macrophages) and LAM (toxic to macrophages)
A patient comes to your clinic because they had a positive PPD test. Chest x-ray shows a single Ghon complex. How do you treat this patient?
9 months of isoniazid (blocks mycobacterial cell wall synthesis).
A patient comes to your clinic because they had a positive PPD. CXR reveals a cavitary lesion in the left lung and the patient complains of hemptysis. How do you treat this patient?
9 months of multiple drug therapy to prevent development of resistance (RIPE = Rifampin (inhibits bacterial RNA polymerase), Isoniazid, Pyrazinamide, Ethambutol).
What Tb drugs block cell wall synthesis?
Isoniazid, Pyrazinamide and Ethambutol
Why don’t we use the BCG vaccine in the military?
It is a live attenuated M. bovis vaccine that will cause a positive PPD. Since incidence is so low in the US it is more cost-effective to do PPD and treat infection.
What types of mycobacterium cause skin lesions?
M. marinum and M. ulcerans
What types of mycobacterium cause pulmonary and systemic symptoms?
M. Tb, M. bovis, M. africanum and M. avium-intracellulare (HIV patients).
What causes Hansen disease?
This is another name for leprosy. Host immune reactions to M. leprae.
What is the difference between lepromatous and tuberculoid leprosy?
In tuberculoid leprosy, you have a strong Th1 cell-mediated response that forms a tuberculoid and keeps the infection confined. In lepromatous leprosy you have a Th2 antibody response that causes diffuse and systemic leprosy.
How do you that a patient with leprosy?
Dapsone (blocks bacterial folate synthesis), Rifampicin and Clofazimine.
