Tuberculosis Flashcards
What are the main symptoms of tuberculosis?
Fever, Night sweats, persistent coughs, bloody phlegm, chest pain/shortness of breath
Fever and Night sweats are more specific to TB compared to other colds
Describe the immune response to TB exposure
TB has a long asymptomatic period - patients can be infected years before symptoms and then have a quick symptom onset
exposure: 4-6 weeks when dendritic cells interact causing an adaptive and innate immune response. macrophages take up TB trying to kill it off causing more and more macrophages and dendtrictic cells coming together- forms a granuloma in an attempt to contain TB which can lead to more infectious issues. The macrophages can destroy TB- becomes latent TB (asymptomatic) which never becomes active TB.
What are the 4 stages of TB?
Primary infection
Latent period
active TB - have symptomatic TB
Transmission - primarily by patients with active TB.
Describe the difficulties in diagnosing TB
difficult to diagnose: occurs mostly in resource limited settings.
molecular testing and whole genome sequencing used - difficult when there are millions of cases in a country every year. can be expensive.
Diagnosis of TB
Different tests preferred at different suspected stages of TB
Tuberculin skin test used to look for latent TB - antigen for TB injected under the skin and left for a few days, if a bump appears the patient usually has latent TB.
microscopy - nelson stain used ( N/A for gram stain). TB bacteria stains red, but they cant always be seen - high sensitivity but low specificity.
X-ray
CT Scans if TB is extra-pulmonary (outside of the lungs)
culture growth - TB bacteria wont grow on most things in the lab/ grows very slowly so TB bacteria has specific culture conditions which can be solid/liquid.
Gene expert machine: Found in a clinical lab for diagnosis using a PCR approach. Probes detect a specific gene from the sample and show if that gene is present. Primary way for diagnosis using sputum from patient, very easy machine to use for people not highly trained in molecular biology
Describe Mycobacterium Tuberculosis
Rod shaped bacterium, waxy cell wall, strict anerobe , genome of 4.4/4.5 mega bases in size, one single chromosome, no plasmids
Historical epidemiology of M.TB
started in africa about 73,000 yearsa go and spread as humans spread around the world
Modern epidemiology
9 lineages of TB that infect humans occurring in different parts of the world. Lineage 4 occurs in every country that has TB
lineage 2 also very prevalent, primarily in Asia but also in Europe too. Other lineages have a more restricted distribution geographically.
Also different in terms of their virulence/pathogenicity but primarily act in a similar way to each other.
Tuberculosis Incidence and Epidemiology
Poverty related disease- occurs primarily in poverty related and resource limited countries - in low poverty areas people generally have lower health and worse immune systems, live more closely together, share beds ect.
Incidence comparisons should be per 100,000- larger countries can have many infections but a lower incidence.
high surveillance results in more active cases being reported, but doesnt mean that there are actually more cases occuring - look at the number of tests taken and how many were positive
Countries with low indicence can have pockets of high incidence - TB is poverty related.
Treating TB
difficult to treat
takes a long time to treat
patient dependent - patients can be treated with more drugs if they are sensitive to them
first line of drugs: a combination of ioniazad/ rifampicin drugs. Isoniazid, Ethambuotal, Moxifloxacin, Rifamipicin, Rifapentine, pyrazinamide
second line of drugs: more side effects. floroquionolones.
TB first line of treatment
patients sensitive to all drugs take a minimum of 4 months to treat, normally 6
normally 2-3 pills every day for normally 4 months at least,
resistance-> second line drugs are used which are normally up to 99 months worth of treatment
adhernace to taking these drugs and effectiveness of these drugs over such a long period of time can be a challenge.
minimal side effects, main side affects are ioniazid turns urine bright orange
typical first line treatment for TB
isoniazid and rifampicin are the backbone.
Normally two months of isoniazid, rifampicin, pyrazinamide and ethambutol followed by four months of just isoniazid and rifampicin.
W.H.O: two months of all four tablets followed by two of just isoniazid and rifampicin.
moxifloxacin used if issues are encountered with these drugs or the patient cant use one of these drugs
resistance to primary isoniazid/ rifampicin: second line of treatment must be used.
Second line of treatment
Consists of core drugs which do most of the killing, support drugs to reduce resistance
also dependent on which drugs have the least amount of side effects for the patient. Side effects include deafness/ psychotic episodes - balance between treatment and life changing side effects. As many group A drugs should be taken as possible (highly effective, well-tolerated, generally fewer side effects). Group B taken with constant monitoring, and group c added based on drug resistance patterns and side effects. Minimum of 9 months of treatment - symptoms go away within about a week
Issues with second line of treatment
patient adherence to drug treatment stops after a week: dormant TB reactivates once drug pressure goes away causing a secondary infection and secondary reactivation typically with drug resistance.
Drug Resistance Mechanisms
Intrinsic methods: Impermeable cell wall, efflux pumps, enzymes
acquisition via mutations in genes: no plasmid. No horizontal gene transfer, all resistance for TB drugs occur from mutations in the targets of those drugs.
Rifampicin Drug Restisance
Rifampicin Resistance: primarily due to mutations in rpoB but can also be rpcoC and A genomic mechanisms.
Rifampicin resistance primarily used as a marker. Isoniazid resistance: rarer.
Two routes of drug resistance
accquired drug resistance and transmitted drug resistance. accquired drug resistance - when soieone is infected with susceptible strain and then gets a restiant strain during treatment. can be due to ineffective drug treatment/ drug taking by patients.
different drugs have different levels of penetration/ diffusion into bacteria which
can lead to resistane
transmitted resistance_ someone infected with a drug resistant strain
XDR- TB
XDR- TB: Extremely resistant TB strain, resistant to basically every drug used for it and spreads well
two primary categories of Drug Resistance testing
Phenotypic Testing -
Genotypic testing
Can tell us if resistance is accuquired or transmitted..?.
Phenotypic drug susceptibility testing
Sample grown on plate to tset for TB myobacteria presence. can use Lowenstein-Jensen (LJ) media (solid) or myobacterial growth indicator tube (MGIT) (liquid) ad look for grwoth in the media
LJ Media - Solid with egg, growth seen using CFU visualisation, takes 4-8 weeks to show growth/positivity
MGIT- shows growth via co2 production/o2 consumption, takes 10-14 days
1 plate/tube per drug is gold standard for Drug Susceptibility testing
looks for colonies
takes a very long tie as TB grows slowly
Rifampicin - Genotypic Drug Susceptibility Testing
primary GDST is for rifampicin using gene expert: shows if TB and rifampicin resistance - Indicates
rifampicin resistance determined by mutations in rpoB using RDR: specific rifampicin resistant determining region- screened for base pair mutations
How does the Gene Expert work?
used as a first line to see if someone has TB
has 5 probes covering different part of the region of interest. if the probe binds there is no mutation and if it doesnt bind then there is. the probes look for the wild type sequence.
it does PCR by itself
easy to use - liquify sputum sample and put it in the cartitridges which does all of the steps automatically
Whole Genome Sequencing for Drug Susceptibility Testing
WGS tells us if someone has resistance or helps to make sure they done have resistance to other things.
Whole Genome Sequences: Sample grown in culture for 2/3 weeks and entire genome sequenced: look for Mutations related to resistance are looked for to determine if someone is going to be resistant or not.
Genotypic Drug Suceptibility testing
Exepert/Xpert - PCR based, Rif resistance/diagnosis, no biosafety lab requirements, outputs RRDr mutations, no culturing required
LPA - PCR based - Rif/INH/FQ resistance, RIF/INHFQ resistance, no cultruting required, BSL2, outputs RRDR mutations
WGS - Sequencing, most expensive, BSL 3, no culturing, outputs all mutations
genotypic methods: must know the mutation relating to the drug resistance for detection - phenotypic methods used
TB Vaccine Development
DCB vaccine given to children - 60-70% effectiveness that goes away after teenage years. Adults around the world not very protected. New TB vaccine M72/AS01-subunit based vaccine using mycobacteria antigens to get immune response. In phase 3 testing testing its efficacy. Clinical trials will take time as TB has a long infection time.
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mRNA vacinations suggested but not much progress. difficult to ellicit long term immune responses to TB meaning vaccines dont often show promise
