Tuberculosis Flashcards
Describe the aetiology of TB
Aetiology: Mycobacterium tuberculosis - a complex of organisms. An obligate aerobe, acid fast rod bacillus, non-spore forming. Likes to colonise in highly oxygenated tissue
Describe the risk factors of TB
Recently infected individuals (latent), those with weakened immune systems, homelessness, injecting drug users, persons from high risk areas, those in contact with people with TB, diabetes mellitus
Describe the diagnosis of TB
Sputum analysis. Acid fast stain of the bacterium, difficult to stain due to waxy outer layer. MDR-TB 50 is used to check for drug susceptibility.
Transmission of TB
Transmitted via aerosol droplets, such as when people cough, speak, sing.
Describe the signs and symptoms of TB
Usually develops slowly, it may take several weeks for the individual to become unwell.
Persistent cough with sputum
Weight loss
Loss of appetite
Fever with sweating ,particularly at night.
Describe granulomatous inflammation as a specific type of chronic inflammation.
Mycobacterium tuberculosis is capable of evading the immune system by disrupting endosomes within the phagocytes in which they are engulfed (macrophages). This prevents the formation of phagolysosomes. The macrophage is then unable to digest the bacteria. However, cytokines released from T helper cells are able to super activate the macrophages and aid destruction.
A granulomatous forms. At the centre is necrotic tissue (caseous). This is surrounded by macrophages, which have differentiated to epithelioid macrophages and some form multinucleate cells. Surrounding the macrophages are T lymphocytes, they act by secreting cytokines to stimulate inflammation. Fibroblasts lie at the exterior, trying to contain the infection. Formation of a granuloma helps to limit the extent of inflammation.
Describe the pathogenesis and sequelae of primary and secondary TB
PRIMARY INFECTION
Sensitisation/primary infection occurs via infection with mycobacterium tuberculosis that cannot be digested by macrophages, leads to bacterial replication in the macrophage. Due to immune defences individuals don’t normally enter active disease.
Ghon focus: Area of necrotic tissue where bacterial infection has occurred. Parenchymal lesion.
Ghon complex: The Ghon focus and infected lymph nodes.
Infected individuals with no active disease are said to have latent infection.
Latent TB: Tb is contained with no signs or symptoms seen. Remains latent unless secondary exposure occurs ( in this situation a much more overwhelming immune reaction is seen, due to sensitisation of the body) or if the individual becomes immunocompromised.
SECONDARY INFECTION
Active TB: Sees activation of latent TB. The patient feels in well and has B (systemic) symptoms: Weight loss, lack of appetite, fever, night sweats, persistent cough - red flags.
Often in apical regions
Cavitation: The granuloma may spread into airways, allowing the bacteria to spread via the airway, as the individual speaks etc. It may also cavitate into vessels, allowing the bacteria to spread via the blood to different organs.
TB as a complication of HIV
A leading cause of death amongst those with HIV
1/3 of people living with HIV are infected by TB
CD4 count > 300 cell/ml = Secondary TB
CD4 count < 200 cells/ml = Primary progressive TB
TB in the immunocompromised
PRIMARY PROGRESSIVE TB
- Primary exposure for the immunocompromised = Secondary exposure for immunocompetent
- TB colonises the lungs and spreads around the body
- More often seen in the lower lobes
- Primary exposure may occur whilst immunocompetent and then the individual becomes immunocompromised. Secondary exposure then sees very rapid progression.
- The lesions expand very quickly in apical areas and then quickly spreads to airways and vasculature.
Describe the global burden of TB
Global: Second to AIDS/HIV as the greatest killer worldwide. Mostly in low and middle-income countries. Leading killer in those with HIV.
Amongst the top 3 causes of death for women aged 15 - 44
Describe the burden of TB in the UK
London = Highest incidence
Greatest number of cases seen in those of Indian ethnicity. Lowest amongst those from Afghanistan and Poland.
MDR strains seen, most commonly amongst those from Eastern Europe.
2012: 8751 cases, 7.4 % resistant to any first line drug, 1.6 % MDR
Describe the key methods for the diagnosis of TB and MDR strains
Microbiological sampling: Normal via pulmonary samples but non-pulmonary samples can be used. Molecular tests are performed.
Describe the key methods for the diagnosis of TB and MDR strains
- Microbiology testing
Microscopy: Smear test with acid fast stain. Detects 50 % of cases, less sensitive in HIV patients and children.
THEN:
Culture for mycobacterium: Can take several weeks for cultures to become positive
THEN:
Antimicrobial susceptibility: The cultures are checked for antimicrobial susceptibility
THEN:
Nucleic acid amplification methods (molecular tests): Used on cultures to detect the presence of M. tuberculosis DNA
Describe the key methods for the diagnosis of TB and MDR strains
- Molecular testing
Detect presence –> Detect resistance –> Determine species
Describe the key methods for the diagnosis of TB
- Skin test
TST (Tuberculin Skin Test) aka Mantoux test.
- 0.1 ml of tuberculin derived protein injected into skin of forearm. A 6- 10 mm wheal is formed in the skin.
Diameter classified is dependent on associated risk factors.
Induration of 5 mm or less: Positive for those with HIV, recent contact with those with TB, has TB previously, organ transplant patients, immunosuppressed
Induration of 10 mm or less: Immigrants from high-prevalence countries, injection drug users, residents and employees of high risk congregate areas, children under 4
Induration of 15 mm or less: Positive in any person
