Tuberculosis Flashcards
What is Tuberculosis? (4)
a contagious, debilitating (consuming) bacterial disease spread by airborne droplets from an infected person
- Caused by a bacterium - Mycobacterium tuberculosis
- slow growing, difficult to kill, waxy coat
- coughing, speaking, sneezing
Pulmonary Tuberculosis Symptoms (7)
Early in disease, non-specific and insidious:
- Cough(won’t go away)
- Feeling tired all the time
- Weight loss
-Loss of Appetite
-Fever
-Night Sweats
Cough - initially non-productive later is productive haemoptysis (Tuberculous pneumonia, Bronchopneumonia)
- approx 75%: 10yr mortality pre-antibiotics - clinical illness directly following infection = primary TB
The great imitator (3)
TB can infect any part of the body, mimicking other diseases
Extra-pulmonary TB
England 2016
85% pulmonary TB
25% also had extra-pulmonary TB
HIV+ve - only 30% pulmonary
Scrofula
Miliary TB
Potts disease
Natural History of Tuberculosis
Tuberculosis in the world -WHO TB Report 2022
In 2021
* 10.6 million new TB cases (4.5% ↑ from 2020)
* 1.4 million deaths – (plus 0.187 million HIV+ve)
* 1/3rd world population latently infected
* Multiple drug resistant TB (MDR-TB) = ~450,000
* Extensively drug resistant (XDR-TB) = ~6% of MDR-TB
Tuberculosis in the world
Biggest killer for a single infectious agent (prior to SARSCoV2)
TB is the 13th leading cause of death worldwide
TB Granuloma (3)
containment of infection = tissue damage
Spherical collection of lymphocytes, macrophages and epithelioid cells with a small area of central caseation necrosis.
The centre of the granuloma undergoes a combination of liquefaction and coagulative necrosis producing caseous necrosis, which is unique to TB and causes considerable tissue destruction to the host = allows transmission
Tuberculosis: cellular pathology (3)
Cell-mediated delayed type hypersensitivity response (Type IV)
Normal/healthy lung: alveolar spaces
TB: TB granuloma, epithelioid like cells (activated macrophages)
Lung specimen
Caseous necrotic tissue - constitutes the granulomas in this gross appearance of a Ghon complex of primary TB
Most patients with primary tuberculosis are asymptomatic
- Granulomatous lymph node
- Ghon focus
Diagnosis of Pulmonary TB (4)
-Chest X-ray
-CT Scanning
-Sputum - cough or induced, smear, culture, PCR: TB sputum – ZN stain - called Acid Fast Bacilli
- Bronchoscopy, Lavage, biopsy
Immuno-diagnosis of latent M.tuberculosis infection
Skin tests for TB antigens
IFN-γ assays – blood tests
Reactivity ≠ disease
exposure
Old TB Treatment
sanatorium: fresh air, good nutrition and lung exercise
>65% mortality – didn’t work!
TB Treatment Now (3)
antibiotics
1943 - Selman Waxman - Streptomycin
- TB rapidly became resistant to 1 drug
- 10,000,000,000 bacteria mutating DNA
Post- 1973 - 4 drug combination trials – Prof Denny Mitchison: Standard short !! Course - 6 months
Isoniazid
rifampicin
pyrazinamide
ethambutol
- >95% effective – relapse rates
- prevented resistance
Treatment of TB – a challenge to medicine and science (3)
Post- 1956:
isoniazid (H)
rifampicin (R)
pyrazinamide (Z)
ethambutol (E)
Intensive phase RHZE for 2 months
Continuation phase RH for 4 months
in some cases, treatment lasts longer e.g.
- patients with cavities on chest x-ray and positive sputum cultures at 2 months should have treatment extended to 9 months
- Meningeal TB
Treatment of TB Disease (4)
- Treatment must contain multiple drugs to which organisms are susceptible
- Treatment with a single drug or adding a single drug to failing regimen can lead to the development of drug-resistant TB
- High bacterial load in TB (unique)
- Selection of pre-existing resistant mutants
TB Antibiotics (4)
Isoniazid (INH) – inhibits synthesis of mycolic acid, required for mycobacterial cell wall
Rifampicin (RIF) – inhibits bacterial RNA polymerase
Pyrazinamide (PZA) – binds to the ribosomal protein S1 (RpsA) and inhibits translation + other possible mechanisms
Ethambutol (EMB) – inhibits arabinosyl transferases involved in cell wall biosynthesis of arabinogalactans
M. tuberculosis cell wall inhibition
Conversion of pyrazinamide
(Pro-drug) Pyrazinamide -> Pyrazinoic acid (active) + ammonia
Pz’ase only active at acid pH
POA - multiple targets – membrane e- potential; fatty acid synthesis for cell walls trans-translation
Why does TB therapy takes 6 months?
i) Actively replicating, rapidly killed by sterilising effect of isoniazid during days 1-4 (Log phase =Rapid killing)
ii) slowly killed, representing drug tolerant persister population (Stationary phase = Slow killing, stasis)
LTBI Treatment Regimens- Isoniazid or Rifampicin
Purpose: to prevent people with latent TB or exposed to TB from developing disease
- Preferred regimen - isoniazid (INH) daily for 9 months (min. 6 mo)
Rifampicin (RIF) – alternative, if:
* Cannot tolerate INH
* Have been exposed to INH-resistant TB
* RIF should be given daily for 4 months
* RIF should not be used with certain combinations of anti-retroviral (ARV) therapy
- 12-dose once-weekly regimen of INH and RPT(long lasting rif)
– not if on ARVs or < 12 years old - rifampicin and isoniazid daily for 3-4 months
Why a single drug? (2)
- low bacterial load,
- low chance of pre-existing mutants
TB is easy to treat with antibiotics, so what’s the problem? (3)
RESISTANCE
MDR TB is a manmade problem…..It is costly, deadly, debilitating, and the biggest threat to our current TB control strategies
Drug Mutation Rate:
Rifampin 10-8
Isoniazid 10-6
Pyrazinamide 10-6
Drug Resistance - factors increasing Drug-resistant TB (5)
- Patient has spent time with someone with active drug-resistant TB disease
- Patient does not take their medicine regularly
- Patient does not take all of their medicine
- Patient develops active TB disease after having taken TB medicine in the past
- Patient comes from area of the world where drug-resistant TB is common
Gandhi et al 2010
1) Drug-resistant mutants in large bacterial pop
2) Isoniazid - Monotherapy: Isoniazid-resistant bacteria proliferate
3) Isoniazid, Rifampicin =
Drug Resistance Testing
Conventional – Phenotypic
* Using solid and liquid media
* Absolute concentration
* Time consuming - >2 – 6 weeks
Genetic tests for mutations in target
* Rapid
* Predictive only
* Needs accurate Databases of mutations
Drug Resistance Mechanisms
- Barrier mechanisms (decreased permeability and efflux pump)
- degrading or inactivating enzymes (e.g. β- lactamases)
- modification of pathways involved in drug activation or drug metabolism (e.g. katG and isoniazid resistance)
- drug target modification (e.g. rpoB and rifampicin resistance)
- target amplification (e.g. inhA and isoniazid resistance)
Resistance in M.tb is mutations
Resistance Mechanisms
- Resistance mediated by DNA mutations in target genes
- Mutated targets are NOT inhibited by antibiotic
- Mutations predict antibiotic resistance - ?
M. tuberculosis rifampicin resistance testing:
- 95% RifampicinR rpoB SNPs in RRDR – codons 507-533
- 81bp mutation hotspot
- Cepheid GeneXpert Rif+ testing – realtime qPCR
- Mtb complex specific – use hemi-nested PCR approach
Not all mutations are born equal
