Mycobacterium tuberculosis

• Mycobacterium tuberculosis causes tuberculosis
• Most commonly pulmonary TB but extrapulmonary tuberculosis is possible (including adenitis, gastrointestinal TB, pericarditis, meningitis)
• Standard treatment for susceptible TB is RIPE x2mo then RI x4mo

Background

Microbiology

• Fastidious, aerobic acid-fast bacillus
• Cell wall has high lipid content
• Generation time is very long (15 to 20 hours)
• M. tuberculosis is a complex that comprises seven species:
  • M. tuberculosis sensu stricto: most common causative organism worldwide
  • M. africanum: 50% of cases in West africa
  • M. canetti: rare cause in Eastern African
  • M. bovis: disease in cattle but can infect humans
  • M. caprae: disease in cattle
  • M. microti: disease in rodents
  • M. pinnipdeii: disease in seals, with rare human infection

Epidemiology

• Typically spread via airborne route
  • Droplets are expelled during coughing, sneezing, or talking, and are suspended in the air
  • They can remain for up to 30 minutes
  • Killed by ultraviolet light
  • Not transmitted via fomites
• About a third of the world is infected, mostly as latent tuberculosis
  • This progresses to active tuberculosis at about 3 or 4% in the first year and 5% over the rest of their life
• Reinfection accounts for ~40% of active tuberculosis in endemic countries
• Highest rates in sub-Saharan Africa and south/southeast Asia

Risk Factors

• Source factors, such as sputum smear positivity, cough, cavitations
• Exposure duration, closeness of contact
• Factors in the exposed person, such as immune compromise, HIV status

Clinical Presentation

Classification

• Primary vs. reactivation vs. reinfection
• Latent vs. active

Primary tuberculosis

• Primary tuberculosis is usually asymptomatic
• Possible presentations include mild URTI with cough and/or fever
• May be seen on CXR as infiltrate in mid-lung zones with hilar adenopathy
• Ghon complex, especially in children
• May progress in children and the immunocompromised patients
• Immunological phenomena
  • Erythema nodosum
  • Phlyctenular conjunctivitis
  • Erythema induratum

Pulmonary tuberculosis

• Most common presentation of active tuberculosis
• Refer to separate article on pulmonary tuberculosis

Extra-pulmonary tuberculosis

• Pleural tuberculosis is most common extrapulmonary site
• Scrofula (cervical lymph node infection) next-most common
• Tuberculous meningitis
• Tuberculous pericarditis
• Renal tuberculosis
• Abdominal tuberculosis
• Gastrointestinal tuberculosis

Latent tuberculosis

• Refers to chronic latent infection contained within granulomas that may reactivate in the future
• Refer to Latent tuberculosis infection

Investigations

• Radiography: chest x-ray with or without CT chest
  • Primary TB: consolidation, lymphadenopathy, pleural effusion, Ghon complex
  • Reactivation TB: patchy upper-lobe consolidation, cavitation, fibrosis, pleural disease
  • Miliary TB: uniform 1-3 mm diameter diffuse nodules

Diagnosis

• Latent tuberculosis testing
  • Tuberculin skin test (TST)
  • Interferon-gamma release assay (IGRA)
• Serology or immunologic testing
  • Urine lipoarabinomannan antigen
• Microbiology
  • Samples can include routine or induced sputum (x3) or bronchoscopy, or tissue sample
  • Spontaneous sputum should include at least one morning sputum, ideally, but can be done all in a row at least one hour apart if needed
  • Acid-fast bacillus culture of sputum x3 is about 70% sensitive, and PCR (ANTB) x1 is about 75% sensitive
• Molecular testing
  • PCR, including GeneXpert

Management

• Management of pulmonary tuberculosis
• Management of drug-resistant tuberculosis

Drug	Dose	Side effects
First-line medications
Isoniazid	5 mg/kg daily, max 300 mg daily, with pyridoxine 25 mg po daily	Rash, hepatitis, neuropathy, CNS toxicity, anemia
Rifampin	10 mg/kg daily	Drug interactions, rash, hepatitis, flu-like illness, neutropenia, thrombocytopenia
Pyrazinamide	25 mg/kg daily, max 2 g daily	Hepatitis, rash, arthralgia, gout
Ethambutol	20 mg/kg daily, max 1.2 g daily	Optic/retrobulbar neuritis, rash
Second-line medications
Streptomycin	15 mg/kg daily, max 1 g	Auditory and vestibular toxicity, renal toxocity, avoid in pregnancy
Amikacin, kanamycin, or capreomycin	15 mg/kg daily, man 1 g
Ethionamide	250 mg BID to TID, max 1 g	GI disturbance, hepatotoxicity, endocrine effects, neurotoxicity, avoid in pregnancy
Para-amino salicylic acid	4 g BID or TID, max 10 g	GI disturbance, hepatic dysfunction, hypothyroidism, avoid in aspirin allergy
Cycloserine	250 mg BID to TID, max 1 g	Avoid in epilepsy, psychiatric illness, and alcoholism
Levofloxacin	500 to 1000 mg po daily	GI disturbance, headache, anxiety, tremor, long QT, avoid in pregnancy and children
Moxifloxacin	400 to 600 mg daily
Rifabutin	300 mg daily	Hepatotoxicity, uveitis, thrombocytopenia, neutropenia, drug interactions
Clofazimine	100 to 300 mg daily	Skin discolouration, conjunctiva, cornea, body fluid discolouration, GI intolerance, photosensitivity
Linezolid	600 mg po daily
Adjunctive therapies
Corticosteroids for patients with tuberculous meningitis or tuberculous pericarditis	Prednisone 40 to 80 mg po daily for 6 to 12 weeks

Drug-induced liver injury (DILI)

• Most common complication leading to treatment interruption, with a mortality of 6-12% if drugs are not stopped
• Pyrazinamide, followed by isoniazid, then rifampin, are the most common causes of liver injury12
• Most patients can have the same TB drugs reintroduced without recurrence of DILI, though recurrence can be delayed
• Procedure
  • Hold if ALT >120 and symptoms, if ALT >200 even without symptoms, or bili >2x ULN
  • Switch to second-line meds
  • Reintroduce the original drugs once AST & ALT are <2x ULN
  • Only rechallenge with pyrazinamide if it was a mild case

Adherence to treatment

• Refer to Let's Talk TB

Further Reading

• Canadian Canadian Tuberculosis Standards, 7th Edition (2014)