-
10-27-c: Tuberculosis and other mycobacteria: Mycobacteria
- “Fungus-bacteria”:
- Waxy, hydrophobic cell wall: high lipid content (mycolic acid-long chain, branched; acyl lipids – short chain)
- Unusual staining pattern:
- Mycolic acid retains carbolfuschin, a phenolic red pigment, even after destaining with acid-alcohol:
- So is “acid-fast bacillus” or AFB
-
Mycobacteria: bacteriology:
- Slender bacilli, beaded appearance
- Non-spore forming, nonmotile, aerobic
- Do not gram stain well:
- Need to use Ziehl-Neelsen Acid Fast Stain!: know this name
- Carbolfuchsin applied with heat
- Decolorize with hydrochloric acid – ethanol
- Counterstain with methylene blue
- Kinyoun Stain:
- Modified reagents, “cold”
- Useful on tissue
-
Mycobactera: bacteriology 2
- Do not grow on routine lab media
- Lowenstein-Jensen agar: egg, glycerol; malachite green (antibacterial)
- Very slow growth: 12-18 hrs. generation time for most: 4-8 weeks for colonies to appear
- Even the rapid growers are relatively slow: >/= 7 days for colonies
-
Mycobacterium tuberculosis Colonies
- “Buff, rough, and tough”
- Produce niacin
-
Dx of mycobacterium TB
- Liquid media growth systems: Mycobacterial Growth Indicator Tube (MyGIT) – 4-14 days
- Species identification: NAAT (rapid hours vs 6-12wks)
- These have greatly shortened the turnaround time for TB diagnosis (or exclusion):
- Check out p 1322 for mycobacterial species and how they fit together
-
Mycobacterial Ecology
- M. tuberculosis (MTB) id a human pathogen: M. bovis may also infect humans
- M. leprae is a human pathogen
- Nontuberculous mycobacteria (NTM): also called mycobacteria other than tuberculosis (MOTT) or “atypical” mycobacteria
- Found in water and soil – cell wall structure is adapted
- No human-human transmission:
-
TB Pathogenesis
- Respiratory droplets containing MTB are inhaled by a susceptible person:
- Small droplet nuclei (1-5um) can reach the alveoli, where infection begins: most larger droplets deposit in the upper respiratory tract
- MTB that reach the alveoli are ingested by alveolar macrophages:
- Most are killed or inhibited
- A small number multiply intracellularly and are released when the macrophages die
-
If viable, these bacilli can spread through:
- Lymphatic channels to regional lymph nodes
- The bloodstream to other tissues and organs, including areas in which TB disease is most likely to develop:
- Lungs
- Kidneys
- Brain
- Bone
- Liver
-
Extracellular MTB attract bloodstream macrophages:
- Most bacilli killed, granuloma forms
- Specific T-cell immune responses develop:
- MTB growth controlled, no progression
- Generally develops 2-20 weeks after injection
- At this point of TB infection, the tuberculin skin test (TST) will turn (+):
- Asymptomatic and non-infectious
- “Latent TB infection” – LTBI
-
TB bacilli may overcome the immune system
- Begin to multiply
- Progression from TB infection to TB disease
- May occur soon after, or many years after infection
-
In the U.S., unless treated (people with positive PPD Tb test)
- 7-10% of people will develop clinical Tb following infection
- 5% within the first 1-2 years after infection
- 5% later in life
-
Increased risk of Reactivation
- Recent TB infection (within the past 2 years)
- Chest radiograph findings suggestive of previous tB
- Immunosuppression: HIV infection, prolonged corticosteroids, other antilymphocyte Rx. (e.g. anti-rejection, anti-tumor), TNFalfa inhibitors
- Substance abuse
- Diabetes mellitus
- Silicosis
- Hematologic and lymphatic diseases
- Head and Neck cancer
- Stage 5 CKD (ESRD)
- Gastrectomy, intestinal bypass, malabsorption
- Low body weight, cachexia
-
Immunity – less than stellar
- Even with evidence of T cell response, ~10% of people develop active disease
- Genetic testing of isolates has shown that people can get reinfected
- Bacille Calmette-Gu’erin (BCG) Vaccine: may offer benefit in protection of disseminated disease in children, especially meningitis; range of efficacy from 0 to >90%
-
TB Transmission
- Droplet Nuclei:
- Generated when coughing & singing; also when talking or sneezing
- Can remain airborne for hours, even after an active case patient has left a room
- # of MTB expelled in droplet nuclei is key:
- Other factors:
- Duration and intensity of exposure
- Size and ventilation of “room”
- Host factors
- Closed spaces, recirculated air
-
Case patients with AFB (+) sputum smears are more infectious than those with AFB (-) smears:
- Among household contacts of smear (+) cases, rates of skin test (+) are 30-50% > than age-matched community controls
- Among household contacts of smear (-) cases, rates of skin test (+) are only 5% > than age-matched community controls
-
Higher risk for acquiring TB infection
- Close contacts of person known or suspected to have active TB
- Foreign-born persons immigrating from areas with high TB prevalence in last 5 yrs.:
- Residents and employees of high-risk congregate settings (e.g. prisons, NHs)
- **Health care workers (HCWs) who care for high-risk patients:
-
Medically underserved, low-income, minority populations including IVDAs
Infants and children exposed to high-risk adults
-
General TB Epidemiology
- Disease of antiquity
- Germ Theory: Koch 1882 (Nobel Prize 1905); Pasteur 1864 – beer and wine, then milk
-
Decline in TB incidence and mortality in the U.S. > 1900:
- Improved socioeconomic conditions – more people living in single-family dwellings
- Public health interventions:
- Identifying active cases and infected people
- Isolation of cases in sanatoria
- ** Rx: (streptomycin 1946, isoniazid 1952)
- Big change in the 1980s!!
-
Unfunded public health TB programs:
- HIV/AIDS epidemic
- Immigration from countries where TB is highly prevalent
- Transmission in congregate settings: jails, nursing homes
-
TB back on decline in 1990s
- Improved methods of identification:
- More rapid and accurate better identification of resistant strains
- Better identification of resistant strains
- Improved follow-up by public health sector:
- Increased federal resources
- Decreased transmission in congregative settings, especially prisons:
- Expanded use of treatment of latent TB:
- Now most TB in U.S. is from foreign born people, up to 60%
-
HIV and TB Epidemiology
- People with TB infection who acquire HIV infection have a rate of TB reactivation of 7-10% per year vs <10% lifetime risk for a non-HIV infected person:
- HIV (+) people who acquire TB infection progress to active TB at a rate of ~35% within 6 months vs 2-5% in 2 years for HIV (-)
- High rates of extra pulmonary TB
- Imperatives for testing
-
TB Disease clinical features
- “The consumption”:
- Fever, weight loss/wasting
- Night sweats
- Malaise, anorexia
- Cough >2wks; classically with bloody sputum
- Chest pain, often pleuritic
- Variable and may be more subtle!
- Laboratory:
- Nonspecific
- Anemia (normochromic), hypoalbuminemia late
- Late evidence of SIADH, adrenal insufficiency
-
Extrapulmonary Disease
- Hematogenous phase early in infection
- “common sites”
- CNS – acute or chronic meningitis, brain abscess
- Kidney – one cause of “sterile” pyuria
- Adrenal glands – Addison’s Disease
- Liver – granulomatous hepatitis
- Bone – spondylitis – classically described Pott’s Disease****!; long bones
- Serosa – pleura, pericardium, peritoneum
- Most often without active pulmonary disease:
-
TB Disease Diagnosis
- Index of suspicion!!!:
- Growth of MTB is definitive
- AFB (or fluorochrome) stained sputum smear is the most rapid method to detect presumptive MTB:
- Sensitivity is only 50-70%
- Half of people with active pulmonary TB may have (-) smears but (+) cultures
- Sensitivity of AFB smears of “fluids” such as CSF, urine, etc. are <30#
-
TB Diagnosis – caveats
- (-) AFB smear does not exclude TB!!**: !
- (+) Sputum AFB smear in a patient with suggestive illness has PPV of 90%
- Yield of culture varies with AFB load: 3 specimens over 24-72 hrs
- Growing role for NAAT: especially CSF and other “fluids”; direct test on sputum, best if AFB (+)
- No role for skin testing in active disease!
-
TB Rx
- Population of MTB in patient with active TB:
- Rapidly growing bacilli – usually in cavities
- Slowly or intermittently growing bacilli – usually in the acid environment of necrosis
- 2 phases of therapy:
- initial rapid killing of large number of bacilli
- longer phase during which bacilli with slower growth are killed – “sterilizing phase” or “continuation phase”
-
TB Rx Early Activity
- Rate of killing is fastest during the initial days of Rx with multiple agents
- Measure is # of AFB observed on repeat sputum smear – decreased infectiousness
-
TB Rx Sterilizing Activity:
- Kill viable bacilli that are growing slowly or intermittently to minimize the risk of relapse
- Measure is rate of positive sputum AFB cultures after 2 mo of Rx or relapse rate after Rx completed (<5%)
- Better sterilizing activity = shorter regimen
-
TB “First Line” Rx
- Drug Abbreviation Early Activity Sterilizing activity
- Isoniazid INH +++ +
- Rifampin RIF + +++
- Pyrazinamide PZA ++ +++
- Ethambutol EMB ++ 0
- Streptomcin SM + 0
-
TB Rx – resistance
- Development of resistance:
- Patient non-compliance – argues for directly observed therapy (DOT)
- Inappropriate prescription – wrong or too few drugs
- Spontaneous mutation - ~1 in 10^5 – 10^8 bugs for any given drug
- Resistance phenotypes:
- Single drug, e.g. INH
- Multiple drug resistance (MDR) – INH + RIF
- Extensively drug resistant (XDR) – INH + RIF + fluoroquinolone + 2nd line injectable
-
Isoniazid (INH)
- Mechanism of action: inhibition of mycolic acid synthesis
- Mechanism of Resistance: most likely enzyme inactivation; spontaneous mutation
- Antimycobacterial Activity: bactericidal against intracellular and extracellular organisms; active as both an early agent and sterilizing agent
-
Isonazid: Absorption, metabolism, excretion
- Absorption: well-absorbed orally; wide distribution; CSF levels ~20% of serum levels
- Metabolism: Metabolized in the liver by N-acetyl transferase
- Rate of acetylation is genetically determined
- Excretion: Drug (<10%) & metabolite excreted into urine
-
Isoniazid Adverse reactions
- Hepatitis:
- Any time, peak in first 4-8 weeks
- 15% develop elevated transaminases
- risk increases with age, esp. >35 y.o.
- Hepatic failure & death if drug is continued
- Neurotoxicity: Increased excretion of pyridoxine so B6 is given with INH
- Hypersensitivity reactions:
-
Rifampin (RIF): mechanism of action, resistance, and antimycobacterial activity
- Mechanism of action: .
- Inhibits MTB DNA dependent RNA polymerase
- Human enzyme insensitive to effects of the drug
-
Mechanism of resistance: .
- Presumably enzyme inactivation
- Spontaneous mutation (1 in 10^6)
- Antimycobacterial Activity: .
- Bactericidal for all populations of MTB
- Excellent nearly activity and sterilizing activity
- Useful against intermittently metabolizing bacteria
-
Rifampin: absorption, metabolism, excretion
- Absorption: .
- excellent oral absorption
- Wide disribution, including CNS
- Metabolism: .
- Hepatic via cytochrome P-450 (deacylation)
- Induces its own increased metabolism
- Excretion: Mainly GI tract
-
Rifampin Adverse Reactions
- Hepatotoxicity: .
- Cholestasis (increased ALP, increased bili)
- Hypersensitivity reactions: .
- Flu-like illness with intermittent use
- Marked drug interactions: Induction of CYP-450 system
- Brick red discoloration of body secretions: permanent discoloration of soft contacts
-
Pyrazinamide (PZA): MoA, MoR, Antimycobacterial Activity
- Mechanism of Action: Precise action unknown
- Mechanism of Resistance: Unknown but evolves more rapidly than INH and rifampin when used alone
- Antimycobacterial Activity:
- Bactericidal at acid pH, especially on intracellular organisms
- Good early & sterilizing activity
-
Pyrazinamide: absorption, metabolism, excretion?
- Absorption: .
- Excellent oral absorption
- Widespread distribution including CNS
- Metabolism: Hepatic
- Excretion: Renal
-
Pyrazinamide Adverse Reactions
- Hepatotoxicity
- Gout
- Photosensitivity
-
Ethambutol (EMB): MoA, MoR, Antymycobacterial Activity
- Mechanism of Action: an antimetabolite affecting RNA synthesis
- Mechanism of Resistance: Unknown
- Antimycobacterial activity: Bacteriostatic; ACTIVE EARLY, NO STERILIZING ACTIVITY
-
Ethambutol: Absorption, metabolism, excretion
- Absorption: excellent oral absorption; wide distribution including CNS
- Metabolism: hepatic
- Excretion: renal
-
Ethambutol Adverse Reactions
- Retrobulbar neuritis: (permanent) visual loss or color blindness; red-green color blindness, blurred vision
- Peripheral Neuropathy:
- Hyperuricemia:
- Hypersensitivity reactions:
-
Current TB Rx Scheme
- Basic Principles: .
- Multiple drugs – avoid resistance
- Bactericidal
- Long course
- Start INH + RIF + PZA + EMB x 2 months
- If bug INH & RIF susceptible: continue x 4 more months; discontinue PZA & EMB
-
Current TB Rx Scheme Liver problems
- Problem – liver dysfunction: Jaundice, elefated LFTs
- Discontinue all Rx: .
- Re-add one at a time and monitor for toxicity
-
Identifying TB Infection
- To eliminate TB fron the U.S. requires identification and treatment of those with latent TB infection and high risk for developing active TB:
- High risk includes those persons:
- Recently infected
- With clinical conditions that increase the risk of developing active TB
- Working in places where transmission of TB is more likely
- Do not screen low risk persons or test for administrative purposes: .
-
Targeted Tuberculin Testing
Focuses tuberculin skin testing (TST) on high-risk groups rather than everyone
-
Latent TB infection (LTBI)
Person with a (+) TST with no evidence of active disease
-
Treatment of LTBI
Replaces the older terms “preventive therapy” and “chemoprophylaxis”
-
Tuberculin Skin Testing
- Sterilized broth from TB culture: .
- PPD – purified protein derivative
- 0.1 ml injected intradermally
- Amount of induration @ 48-72 hrs:
- Do not report results as “negative” or “positive”
- Report results as mm of INDURATION
- DO NOT measure amount of erythema
- Tuberculin test, Mantoux test, TST & PPD, frequently are used interchangeably
-
Interpretation of TST/PPD
- Palpate induration
- DO NOT measure erythema
- Requires skilled individual to place and read
-
Criteria for TST Positivity
- Some soil NTM immunologically cross-react with MTB: Larger reaction more likely to represent MTB infection
- Cut-offs for (+) TST range from 5-15 mm depending on TB risk
-
No/low TB risk use ________ mm for TB
>/= 15 mm
-
>/= 10 mm TB used for
- Recent immigrants (<5 yrs) from high prevalence country
- IVDA
- Residents/employees of high-risk congregate setting (e.g. prisons, hospitals)
- Persons with certain clinical conditions: DM, cancer, silicosis, etc.
- Children
-
>/= 5 mm TB used for
- PPD Reactor: (+) PPD of unknown duration; no prior test results not known
- PPD Converter: Person with “negative” PPD reaction who on repeat testing has an increase in reaction size of >/= 10 mm within a 2-year period; high risk of developing active TB
-
False Negative PPD
- Anergy
- Recent or overwhelming TB
- Recent immunization with live virus vaccine(s)
- Recent viral infection: measles; chickenpox
-
PPD Alternative
- Interferon Gamma Release Assay (IGRA): QuantiFERON Gold, Elispot
- Measure response of peripheral blood mononuclear cells to specific MTB antigens:
- Quantitative, no placement or reader error
- Expensive but single visit
- Not used in pregnancy
- Particularly useful in people who have received BCG (no cross reaction):
- Studies ongoing:
-
BCG Vaccine & TST
- PPD reactivity wanes over time after BCG vaccination
- PPD testing can boost the immune response
- Those with prior BCG vaccine should have a TST done if indicated
- No reliable method to distinguish (+) reactions cause by BCG vs natural infection
- HOWEVER, >/= 20 mm induration unusual with BCG
- IFG Release Assay an alternative: will not be (+) in BCG recipients
-
Treatment regimens for LTBI
- INH 300 mg daily for 9 months preferred
- INH 600 mg 2x/week – alternative
- RIF 300 g daily for 4 months alternative: especially if evidence of resistance
-
NTM/MOTT
- Mycobacterium marinum: .
- Fish tank or pond granuloma
- Cutaneous lesions, typical history
- Mycobacterium ulcerans: .
- Buruli ulcer in the tropics
- Severe skin ulceration, bone involvement
- Mycobacterium kansasii: .
- The most TB like of all the NTM/MOTT
- BUT not transmitted person to person
- Soil origin
-
NTM/MOTT
- Mycobacterium scrofulaceum: Scofula – lymphadenitis in children
- Mycobacterium gordonae: generally a contaminant, “tap water bacillus”
- Mycobacterium avium complex (MAC): severe disease in HIV
- Mycobacterium Fortium complex: may cause local infection in normal or compromised host
-
Leprosy
- 4.2 million cases in the world: approx. 4200 cases in U.S. in LA and TX
- 95% of population MAY be immune
- Cardinal findings: infiltrative skin lesions; hypoesthesia; peripheral neuropathy
- Tuberculoid – paucibacillary
- Lepromatous – multibacillary
|
|
