TB

• Tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis, a small rod-shaped bacteria
• M. tuberculosis has a waxy coating that makes gram staining ineffective. They are resistant to the acids used in the staining procedure, described as “acid-fast”, making them acid-fast bacilli. Special staining is required, using the Zeihl-Neelsen stain
• M. tuberculosis bacteria are very slow dividing and have high oxygen requirements - most common site of infection is the lungs
• When the immune system cannot control the infection, disseminated and severe disease can develop, referred to as miliary tuberculosis. Disseminated miliary tuberculosis gives an appearance of millet seeds uniformly distributed across the lung fields
• Latent tuberculosis is present when the immune system encapsulates the bacteria and stops the progression of the disease. Patients with latent tuberculosis have no symptoms and cannot spread the bacteria. Most otherwise healthy patients with latent tuberculosis never develop an active infection. When latent tuberculosis reactivates, and an infection develops, usually due to immunosuppression, this is called secondary tuberculosis.
• Risk Factors

  • Close contact with active tuberculosis (e.g., a household member)
  • Immigrants from areas with high tuberculosis prevalence
  • People with relatives or close contacts from countries with a high rate of TB
  • Immunocompromised (e.g., HIV or immunosuppressant medications)
  • Malnutrition, homelessness, drug users, smokers and alcoholics
• The BGC vaccine is a live attenuated vaccine that is given to children aged 12-18 months. Patients are tested for a TB infection with a Mantoux test first. The BCG vaccine is only given to high risk individuals.
• Typical signs and symptoms of tuberculosis include:

  • Cough
  • Haemoptysis (coughing up blood)
  • Lethargy
  • Fever or night sweats
  • Weight loss
  • Lymphadenopathy
  • Erythema nodosum
  • Spinal pain in spinal tuberculosis
• There are two tests for an immune response to tuberculosis caused by previous infection, latent TB or active TB:

  • Mantoux test
  • Interferon‑gamma release assay (IGRA)

  In patients where active disease is suspected, investigations to support the diagnosis include:

  • Chest x-ray
  • Cultures
• Chest X-ray:

  Primary tuberculosis may show patchy consolidation, pleural effusions and hilar lymphadenopathy.

  Reactivated tuberculosis may show patchy or nodular consolidation with cavitation (gas-filled spaces), typically in the upper zones.

  Disseminated miliary tuberculosis gives an appearance of millet seeds uniformly distributed across the lung fields.
• There are several ways to collect cultures:

  • Sputum cultures (3 separate sputum samples are collected)
  • Mycobacterium blood cultures (require special blood culture bottle)
  • Lymph node aspiration or biopsy
• The NICE guidelines on tuberculosis (2016) describe “deep cough” sputum samples. If they are not producing enough sputum, the options are:

  • Sputum induction with nebulised hypertonic saline
  • Bronchoscopy and bronchoalveolar lavage (saline is used to wash the airways and collect a sample)
• Latent tuberculosis is treated with either:

  • Isoniazid and rifampicin for 3 months
  • Isoniazid for 6 months
• The treatment for active tuberculosis can be remembered with the RIPE mnemonic:

  • R – Rifampicin for 6 months
  • I – Isoniazid for 6 months
  • P – Pyrazinamide for 2 months
  • E – Ethambutol for 2 months
• Patients with active TB should be isolated for at least the first 2 weeks of treatment to prevent spread
• Rifampicin can cause red/orange discolouration of secretions, such as urine and tears.
• Isoniazid can cause peripheral neuropathy. vitamin B6 is co-prescribed to reduce the risk.
• Pyrazinamide can cause hyperuricaemia (high uric acid levels), resulting in gout and kidney stones.
• Ethambutol can cause colour blindness and reduced visual acuity.
• Rifampicin, isoniazid and pyrazinamide are all associated with hepatotoxicity.
• Mycobacterium tuberculosis complex:

  • M. tuberculosis (majority of cases)
  • M. bovis
  • M. africanum
• Main sites of extra-pulmonary TB:

  • Central nervous system - meningitis
  • Lymphatics
  • Genitourinary - sterile pyuria, pyelonephritis
  • Cardiac - pericarditis
  • Spinal vertebrae - Pott's disease
• Miliary TB is a disseminated form of TB commonly found in the lungs, which can develop from primary progressive or post-primary TB.
• Pulmonary symptoms of TB:

  • Cough - usually chronic
  • Sputum - initially dry and then purulent or blood stained
  • Breathlessness
  • Pleuritic chest pain
• Analysing samples for TB:

  • Stained with Ziehl-Neelson stain for acid-fast bacilli
  • Samples are cultures on Lowestein-Jensen media for 4-8 weeks
  • Sensitivities are complete after a further 3-4 weeks