Respiratory Tract Infections: Tuberculosis

Question 0

Mycobacteria

Answer 0

Mycobacterium tuberculosis causes TB,
Less commonly causes by M.bovis, M.africanum, M. Microts
M.tb most probably human adapted m.bovis
Have a resistant cell wall containing Mycotic acids-> resist destruction
Many harmless species in the environment
Obligate aerobes-> like tissues with high air conc
Facultative intra cellular pathogens-> infect mononuclear phagocytes
Slow growing-> 12-18h
Hydrophobic
Acid fast-> once stained resist decolorisation

Question 1

M. Tuberculosis

Answer 1

Infects 1/3 of global pop
8 million new cases each year
>2 million deaths per year
HIV has caused much of the recent increase

Question 2

Leprosy

Answer 2

Caused by mycobacterium leprae 
Tuberculoid leprsoy-> common
Strong cellular immune response
Few bacilli lesions
Depigmented anaesthetic lesions 
Lepromatous leprosy-> uncommon
Weak cellular immune response
Many bacilli in lesions
Thick granulomatous infections

Question 3

Pathophysiology of m. Tuberculosis

Answer 3

Spread via airborne droplet nuclei-> can remain airborne for several hours-> inhaled-> lodge in alveoli->engulfed by macrophages
Slow replication and spread via lymph
In most individuals cell mediated immunity develops after 2-8 weeks
Activated T cells and macrophages form granulomas that limit infection spread-> bacterial cells which are sometimes viable remain here
Most individuals are asymptomatic and never develop the disease

Question 4

Clinical features of TB

Answer 4

Non specific symptoms-> night sweats, fever, weight loss
Respiratory symptoms-> cough, shortness of breath, haemostatsis, chest pain
Mainly pulmonary-> extra pulmonary tends to be in HIV +

Question 5

Other infection sites of TB

Answer 5

CNS-> meningitis
Skin/soft tissue-> most common-> cervical lymphadenitis
Bone and joints
Genitourinary tract-> prostatic, arcnitis, renal lesions, infertility in women, sterile pyuria (WBC in urine)
Disseminated disease-> many organs involved simultaneously, primary progressive or re activation of latent infection, millet seed lesions on CXR

Question 6

Diagnosis of TB

Answer 6

Early diagnosis increases survival and prevents spread
Category 3 pathogen-> potential for lab acquired infection-> category 3 lab
Sputum 
Broncho alveolar lavage 
Pus/tissue
Urine
CSF
Use Ziehl-Neelson stain

Question 7

TB microscopy

Answer 7

Culture in lowenstein-Jensen-> slow 2-8 weeks-> beige, dry, rough calories
Rapid culture-> 1-2 weeks-> mycobacteria growth indicator tube

Question 8

Antibiotic sensitive tests

Answer 8

Performed in a reference lab
-> Rifampincin, isoniazid, pyazinamide, ethambutol
Important to detect resistance especially in multi drug resistant (Rifampincin and isoniazid)
Extensively drug resistance -> tuberculosis that is resistant to Rifampincin, isoniazid and to any quinolone and at least one second line anti TB agent

Question 9

Genomic tests for mycobacteria

Answer 9

PCR
DNA probes
Rapid detection of Rifampincin resistance
Typing methods to confirm outbreaks

Question 10

Treatment of pulmonary TB

Answer 10

Initial phase-> 2 months of 4 drugs-> Rifampicin, isoniazid, pyrazinamide, ethambutol
Continuation phase-> 4 months of Rifampicin and isoniazid
If resistance suspected 5 drugs may be used initially with longer courses

Question 11

Prevention of TB

Answer 11

Early diagnosis and treatment
-> usually non infectious after 2 weeks of therapy
Contact tracing and detection of latent infection
-> tuberculin skin test, chest radiograph, invitro interferon gamma release tests
Contacts treated if evidence of infection
Notifiable disease

Question 12

Vaccination

Answer 12

Bacillus calmette-guerin
Live attenuated M.bovis
Main role is protecting children from severe disease
Efficacy is 70% in Uk