Mycobacterium Tuberculosis (TB)

 

Tuberculosis

Etiology

  • Caused by M. tuberculosis
  • can infect any organ of the body, yet most often the lungs
  • Strains specific to human infection
  1. M. tuberculosis hominis
  2. M. tuberculosis bovis
  3. M. avium intracellular (this strain infects only immunocompromised persons)

 

Modes of Transmission

  • airborne transmission – respiratory secretions
  1. coughing
  2. sneezing
  3. talking
  • consumption of contaminated dairy products – M. tuberculosis bovis

 

TB and HIV infection association

  • antibiotics in the 1950’s decreased TB infection rates in western countries
  • Rise of HIV in the 1980’s resulted in increased infection rates
  • Largest caseload of TB infections occurred between 1985-1993
  • after 1993 infection rates again declined with increased resources to screening, prevention and support for AIDS patients.
  • TB continues to be prevalent in developing countries with high HIV rates
  • antibiotic resistant strains have now emerged

 

Pathophysiology

Cell-mediated immune response results in tissue hypersensitivity and resistance to TB antigens.

Hypersensitivity immune response to the infection results in the destructive aspects of the disease.

  1. Cavitation
  2. Caseating necrosis

 

Macrophages: the primary cell infected by the bacterium.

Mode of infection:

  1. infected airborne droplets pass down bronchial tree
  2. settle in alveoli
  3. alveolar macrophages phagocytize, yet do not kill
  4. macrophages initiate the cell-mediated immune response
  5. TB has no surface antigens to promote early immunoglobulin response

 

Primary TB

  • Develops in previously unexposed individuals
  • most develop latent infection – non-active, non-transmitable, infection walled off in granulomas
  • progressive primary TB develops in 5% of cases
  • results in lung tissue destruction and spread to multiple sites
  • immunocompromised individuals are likely to develop progressive primary TB

 

Ghon’s complex: granulomatous lesion where infection has been walled off by the immune system.

Secondary TB. Discuss the pathophysiology of each form of the disease.

Mention how secondary infection might occur.

  • Indicative of reinfection from inhaled droplets or reactivation of previously healed primary lesions
  • occurs in cases of impaired immune defenses
  • partial immunity that follows primary TB affords some protection against reinfection
  • cell-mediated hypersensitivity can be an aggravating factor (cavitation and bronchial dissemination)
  • pleural effusion and tuberculosis empyema are common progressions

 

Manifestations

Primary TB:

  1. fever
  2. weight loss
  3. fatigue
  4. night sweats

 

infection onset may present with:

  1. high fever
  2. pleuritis
  3. lymphadermitis

 

Secondary TB

  1. low-grade fever
  2. night sweats
  3. fatigue, low endurance
  4. anorexia
  5. weight-loss
  6. initial dry cough progressing to purulent and sanguinous sputum
  7. Dyspnea (advanced)
  8. Orthopnea (advanced)

 

Diagnostics

  1. Tuberculose skin test
  • indicates delayed hypersensitivity rxn (cell-mediated, type IV) following exposure
  • a positive skin reaction is not indicative of active TB
  • false-positives may result in cases of cross rxn with non-tuberculosis mycobacterium such as M. avium-intracelluar
  • false-negatives may occurs in immunocompromised individuals where low or no immune response is mounted against the pathogen
  1. Chest X-ray
  2. Culture
  • for definitive diagnosis of active TB
  • individuals who test positive usually remain so for the remainder of their lives

 

Treatment and Management

  • Multi-antibiotic regime to prevent further development of resistant strains
  • chemotherapy is required for prolonged duration
  • prophylactic treatment is used for infected individuals without active TB
  1. those who have come in contact with active cases
  2. those who have converted from negative to positive skin test within the last 2 years
  3. those with a Hx of untreated or inadequately treated TB

 

Antibiotics:

  1. INH
  2. rifampin
  3. pyrazinamide – PZA
  4. ethambutol
  5. streptomycin – first drug used to treat, now some strains are resistant

 

Immunization

  1. Bacillus Calmette-Guerin (BCG) Vaccine
  • used to prevent infection in those at high risk
  • it is the only vaccine available

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: