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Multiple drug resistant tuberculosis:
aetiology, diagnosis and outcome
I J Eltr ingham and F Drobniewski
Mycobacterial Reference Unit, PHL Dulwich, Dulwich Hospital, London, UK
Tuberculosis is an increasing problem worldwide both in terms of disease
burden and resistance to conventional antibiotic therapy. Studies of outbreaks
involving resistant strains have h ighligh ted th e need for b oth improved
infection control and the rapid provision of accurate susceptibility data. Each
pa tien t should undergo a risk assessment for possible resistance and those in
whom risk factors exist should be investigated by means of rapid molecular
techniques or other phenotypic methods, so tha t appropriate management can
be instituted with minimal delay. The ultimate outcome will vary according to
whether the patient is immunosuppressed, the tim e taken t o make a diagnosis,
the severity o f disease as well as the degree of resistance. The prognosis can be
improved w hen adequate a ntib iotic therap y is started as soon as resistance is
suspected. Adjuncts to conventional treatment such as surgery and perhaps
immuno therapy may be considered when response to antimicrob ial
chemotherapy has been subop timal.
Corresponding to
Dr IJ Eltringham
Mycobacterial Reference
Unit PHL Dulwich
Dulwich Hospital East
Dulwich Grove London
S
8QF UK
Tuberculosis (TB) is the leading cause of death due to an infectious
agent. It affects one-third of the world's population and 95% of the
disease burden is borne by developing countries whose economic and
healthcare infrastructures are often ill equipped to meet the demands
placed upon them
1
. Superimposed on this is the growing burden of HIV
infection, currently estimated to be over 22 million people and the
potential for re-activation of latent tuberculous infection in these
patients. Currently, the World Health Organization (WHO) estimates
that 6 million people are co-infected worldwide
2
. Not only are HTV
infected patients more likely to acquire TB after exposure, but the
annual rate of progression to disease approximates to the total lifetime
risk in non-HTV infected individuals
3
. In the US and Europe, annual
notification rates have increased since the mid 1980s and, although a
proportion of cases have co-incided with the development of the AIDS
pandemic, most of the excess cases have been seen in HTV negative
individuals suggesting other factors - such as increased mobility across
international boundaries, a declining public health infrastructure, and
poverty - are also responsible. The resurgence of what was considered
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to be a disease in decline prompted the WHO to declare TB to be a
global emergency in 1993 and subsequently to outline a strategy for
worldwide control which includes improved case detection and the
supervised administration of effective short course chemotherapy
4
.
Clearly, in order to improve control, these efforts must be directed
primarily at smear positive cases globally in order to reduce the pool of
infectious cases. Smear negativity, however, does not preclude trans-
mission, particularly in immunocompromised patients. With increasing
burden of disease comes the risk of antibiotic resistance. In patients
receiving inad equate therapy for T B, either as a result of no ncompliance
or ineffective regimens, the opportunity for overgrowth by resistant
organisms occurs with the potential for transmission to others. This
article focuses on drug resistance, as other aspects of TB are covered by
the article in this issue by Glynn.
Resistance to first line anti-tuberculosis drugs
The growth characteristics of
M ycobacterium tuberculosis,
the causative
organism of tuberculosis, are unusual in that it divides slowly both
in
vitro
and
in vivo,
is inherently resistant to many conven tional an tibiotics
and adapts to a number of different ecological niches
in vivo
5
.
The anti-
biotics selected to treat TB should penetrate macrophages and caseous
material and be effective against dormant subpopulations of organisms
which could be responsible for re-activation at a later date
5
. Rifampicin,
isoniazid and pyrazinamide are bactericidal agents which have good
sterilizing activity and when used in com bination p revent the emergence
of resistant mutants. Ethambutol and streptomycin are less effective
than other first line agents, although the addition of either to a regimen
is recommended until resistance to isoniazid can be excluded
6
. In vitro
studies show that resistant mutants arise spontaneously at a rate of
10~
5
-10~
8
organisms depending upon the antibiotic concerned
7
. The
total organism load in a cavitating lung lesion is likely to be in the region
of 10
9
, so if a single agent is used to treat clinical infection overgrowth
by resistant organisms is likely. The probability of resistance occurring
simultaneously to three agents in a fully compliant individual is
extremely small (circa 10~
20
). Resistance becomes clinically significant
when selective pressure is applied to spontaneous mutants by a com-
bination of patient and medical non compliance, incorrectly prescribed
drug regimens, or inadequate uptake of drugs.
The risk of primary resistance varies according to a patient's geo-
graphic location, ethnic group, HTV status and history of contact with
an individual infected with drug resistant TB. In general terms, higher
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Mu ltiple drug resistant tuberculosis
rates of primary resistance reflect a breakdown of TB treatment
programmes, although in practice this is not always clear because in many
individuals it is difficult to be certain that no previous therapy has been
given. In this context, the term initial resistance is used to include both
primary and undisclosed acquired resistance. Initial drug resistance is less
than 2% in UK born Caucasians without a history of contact with
resistant TB. Initial resistance to isoniazid is about 5% in black Africans
and individuals from the Indian subcontinent
6
. The inclusion of a fourth
agent is advisable when treating patients from areas with high initial
resistance rates for isoniazid. Single resistance to rifampicin, pyrazinamide
(excluding Mycobactertum bovts) and ethambutol are uncommon, but
may compromise successful treatment. Streptomycin resistance is quite
common in some countries, but the agent is not used on a regular basis in
the UK.
Multiple drug resistant
M. tuberculosis
(MDRTB) is resistance to at
least rifampicin and isoniazid. Of rifampicin resistant isolates seen at the
PHLS Mycobacteria Reference Unit (MRU), 90% are also resistant to
isoniazid. Rates of M DRTB in the UK have been increasing slowly over
the past two decades from approximately 0.6% of primary isolates in
the decade up to 1 991 to 1-2% of current isolates
8
-
9
. There is also wide
geographical variation, with the highest rates recorded in the North
Thames region
2
.
MDRTB strains referred to the MRU show considerable variation in
susceptibility to agents other than rifampicin and isoniazid and
frequently are isolated from patients who have lived outside the UK. At
least two major nosocomial outbreaks have occurred to date in the UK
and all but one of the patients involved were infected with HTV. This
situation differs from that in the US, where numerous outbreaks have
occurred in hospitals and rates of infection m hospital workers, assessed
by skin-test conversions have been high
10
.
Diagnosis of drug resistantM tuberculosis infection
The concept of clinical resistance is straightforward in that treatment is
likely to fail when an isolate is resistant to the agents used, hi micro-
biological terms, defining resistance is far more complex
7
. In practice,in
vitro
methods of measuring drug resistance fall into three groups: (i) the
absolute concentration method; (li) the proportion method; and (hi) the
resistance ratio method.
The technique currently used in the UK, and one of those adopted by
the WHO, is the resistance ratio method. It has been used in large
clinical trials and has proved to be a good predictor of clinical outcome
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Table 1 Known mutat ions conferring resistance in Mycobactenum tuberculosis
An t ib io t i c
Rifampic in
Isomazid
Pyrazinamide
Streptomycin
Eth ionamide
Ciprof loxacin
Target
RNA synthesis
Mycolic acid
synthesis
Protein synthesis
Cell wal l
DNA synthesis
M u t a t i o n
rpoB
katG
inhAJmabA
ahpC
oxyR
pncA
rrs
rpsL
inhA
gyrA
gyrB
Gene funct ion
DNA dependen t-
RNA polymerase
Catalase/peroxidase
Fatty acid biosynthesis
Alky lhydroperoxidase C
Oxidative stress regulator
Pyrazinamidase
16S rR NA
Ribosomal prote in SI2
Cross resistance with inhA
DNA gyrase
Molecular assay
PCR-SSCP, he ter od up lex
l ine probe, sequencing
PCR-SSCP, se qu en cin g
PCR-SSCP, se que nci ng
Sequencing
Sequencing
Sequencing
PCR-SSCP, se que nci ng
PCR-SSCP, se que nci ng
Sequencing
PCR-SSCP, se que nci ng
PCR = polymerase chain re actio n, SSCP = single strand co nfo rm atio nal polym orphis m
Adapted from Drobniewski and Pozniak
13
where resistance is demonstrated to one or more agents
11
. However,
most culture based techniques are relatively slow, although semi-
automated and automated liquid culture systems offer significant time
savings in generating drug susceptibility data. These rapid methods are
needed when a patient has significant risk factors for disease caused by
drug resistant organisms.
Direct susceptibility testing of clinical samples
In recent years, advances have been made in the detection of mutations
conferring resistance to a number of antibiotics. Telenti et at
12
demon-
strated the presence of point mutations in an 81 base pair segment of the
rpoB
gene, which codes for RNA polymerase. This has enabled several
genotypic methods for rifampicin resistance to be developed including a
commercial kit employing PCR amplification and solid phase
hybridisation to detect mutations in the product (see Table I)
13
. The test
has been applied to clinical samples and a high degree of correlation was
found with conventional methods
14
. Routine molecular determination of
resistance for other agents is less straightforward. For example, the
majority of pyrazinamide resistant isolates have mutations present in the
pncK
gene which codes for pyrazinamidase, an enzyme that converts
pyrazinamide into the bactericidal metabolite pyrazinoic acid. Although
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Mu ltiple drug resistant tuberculosis
pncA
mutations are a good predictor of high level resistance, they are
scattered throughout the gene so that detection is less straightforward
than for rifampicin
15
. Mutations can also be demonstrated in isolates
resistant to isoniazid
13
but these occur in several genes, as summarized in
Table 1. In a large study of ethambutol resistance, a substitution at codon
306 of
embB,
which is involved in the biosynthesis of arabinogalactan,
was found in only 62% of resistant isolates suggesting that, as with other
agents, resistance is multifactorial
16
.
The heterogeneity of mutations conferring resistance has hindered the
development of routine molecular diagnostic tests for agents other than
rifampicin and rapid phenotypic methods which are not dependent on an
understanding of specific resistance mechanisms are now being developed.
One such technique involves the transfection of mycobacteria by specific
phages engineered to express the lux (luciferase) gene
17
. Infection of
antibiotic-resistant strains with this phage, in the presence of antibiotic,
results in the production of light as the bacteria continue to grow;
conversely, antibiotic-susceptible strains stop growing or fail to express
lux
and, thus, do not produce light. More simply, the presence of living
organisms can be demonstrated by infecting isolates with the phage, in the
presence of antibiotic, and looking for phage production from resistant,
but not susceptible, isolates using a lawn of rapidly growing
mycobacteria as the indicator strain (Phab assay)
18
. This test has been
shown to discriminate between sensitive and resistant strains for both
rifampicin and isoniazid and this method is now being applied to smear
positive sputum samples
16
. An alternative approach has been to use
mRNA as a marker of viability. Unlike DNA, mRNA has a short
half
life and is present in multiple copies, so its demonstration should
provide a sensitive indicator of viable organisms. This could be applied
to both susceptibility testing and microbiological response to treatment
in serial sputum samples. However, before novel genotypic and
phenotypic methods can be applied outside clinical research
laboratories, efforts need to be directed not only to standardizing test
parameters but also to improving cost effectiveness. Although
amplification based methods for rapid detection of mycobacteria are
sensitive and specific, they are unlikely to supercede culture based
methods in the immediate future.
In addition, a careful process of selecting those patients and samples
appropriate to a molecular investigation is of paramount importance, as
these techniques are designed to confirm a diagnosis, not as a screening
test for populations in whom the likelihood of tuberculosis is rare. False
positive results would limit the usefulness of any data generated under
these conditions
19
. It is equally important that molecular diagnosis
should only be carried out by institutions with expertise and well
defined quality control procedures, as wide variations in sensitivity and
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specificity have been demonstrated between a number of laboratories
with apparent exper t ise in PCR diagnosis
2 0
.
Outcome of treatment of multiple drug resistant tuberculosis
In MDRTB patients, the exclusion of first line agents with proven
effectiveness in clinical trials means that regimens of more toxic and less
effective drugs are used. Therapy should be individualised and guided by
drug susceptibility data, using bactericidal agents whenever possible, and
taking due consideration of potential side effects and drug interactions.
The effectiveness of therapy should be judged by sputum smear and
culture conversion and, if sputum remains smear or culture positive
beyond 3-4 months in a patient whose clinical features are not resolving,
serious consideration should be given to repeating susceptibility tests and
making a therapeutic switch, substituting two, or preferably three, drugs
for agents not used previously. Anecdotal evidence suggests that relapse is
unacceptably high in MDRTB if less than 18-24 months of appropriate
therapy is used beyond culture conversion
11
. In HIV-positive patients, it
may be advisable to continue therapy for life, although bacteriological
and clinical cure can be demonstrated in some individuals, particularly
where effective agents are employed withm a month of diagnosis. When
choosing a regimen to treat a patient with MDRTB, it is sensible to
consider the following points
3
: (i) as many first line agents to which the
organ ism is sensitive should be used; (ii) the inclusion of paren teral agents
reduces problems with compliance and absorption; ( iii) administration of
oral agents shou ld be directly obs erved ; (iv) wh enev er po ssible
bactericidal agents should be used; (v) response and survival are more
likely if at least three agents to which the isolate is sensitive are used; and
(vi) single agents should never be added to a failing regimen.
In addition, combinations of agents whose side effect profiles are
similar should be avoided, monitoring of drug levels may be needed and,
in all instances, therapy should be monitored by physicians with
exper t ise in t rea t ing MDRTB cases.
Surgery is an option for some patients. In a large study, 57 patients with
MDRTB were treated surgically m addition to receiving a number of
chemotherapeutic regimens. Most were smear positive at the time of
surgery but, following pneumonectomy or lobectomy, over 80% of
patients remained smear negative at 36 months
11
. In another study,
however, artificial pneumoperitoneum was carried out in a series of end
stage MDRTB cases and no clear benefit could be demonstrated
2 1
. The
role of surgery is less clear in HIV-positive patients where disease is often
widespread. However, in some individuals with localized infection,
surgery may offer the only chance of surviving the disease.
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Studies of TB patients given immunotherapy with Mycobacterium
vacate
have yielded equivocal results. One poorly controlled study in
northern Nigeria showed improvement in both clinical and micro-
biological response in those givenM.vaccae 1-3 weeks after commence-
ment of quadruple therapy
22
. Follow-up of cases in M.
vaccae
treated
individuals in Romania failed to show any statistically significant
differences in terms of outcome compared to p lacebo
23
. Some benefit was
gained in patients with chronic, relapsing disease given immunotherapy,
where the chances of a successful outcome rose from 44% to 63% in
those given a single dose of M.vaccae at 1 month
24
. Recent preliminary
data from the South African study using
M.
vaccae suggest that results
have been disappointing, with no benefit shown over the application of
directly observed chemotherapy
25
.
Until more conclusive data of clinical efficacy are available from
ongoing double blind placebo controlled trials, it would be reasonable to
restrict immunotherapy to drug resistant cases where conventional
methods have failed.
Although published data on the use of immunomodulating cytokines
are limited in MDRTB pa tients, one small study showed that some benefit
may be afforded when appropriate chemotherapy is augmented w ith aero-
solised interferon gamma in patients with pulmonary disease
26
. However,
as with M. vaccae,more da ta from well controlled studies are needed
before cytokine therapy can be recommended outside clinical trials.
Outcome of treatment and prevention of transmission
Results from two large follow up studies in the US showed that a
favourable clinical outcome can be expected in the majority of HTV-
negative patients infected with MDRTB, although the prognosis was
worse when disease had been present for a number of years and thera-
peutic options were limited
3
. One crucial factor determining outcome is
the time taken to institute appropriate therapy. Risk factor assessment
must be carried out in all patients who are to receive treatment, and the
clinical history should identify any contact with a known resistant case,
previous TB therapy, and HIV status. An inadequate clinical response,
such as persistence of pyrexia beyond 2 weeks of appropriate first-line
therapy, is suggestive of treatment failure which may be due to M DR TB.
Outcome is less favourable in patients co-infected with HTV. Median
survival rates of 78-315 days have been reported in HIV-positive
individuals
3
. Survival is significantly reduced when patients have AIDS
and sputum conversion is rare in these individuals
11
.
Recent nosocomial outbreaks have resulted from failures in simple
infection control. In the UK, experience with MDRTB in nosocomial
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outbreaks has been limited to two incidents involving relatively few
patients. In both instances, problems were subsequently found in the
implementation of infection control measures. In contrast, by 1994 the
US had already witnessed over 100 MDRTB infections in patients, and
16 cases in healthcare workers, occurring as a result of nosocomial
transmission
10
. In one outbreak, the results of intervention in the form
of tightening of infection control practices were analysed. Of new cases,
80% were attributable to nosocomial spread before intervention, but
this fell to zero when guidelines issued by the US Centers for Disease
Control were followed
10
. The release of MDRTB infected patients from
isolation is frequently a controversial issue. Patients whose sputum is
repeatedly culture negative may be treated on an outpatient basis, but
therapy must continue to be directly observed and the patient's clinical
condition closely monitored on follow-up, as up to 20 % of patients may
relapse following sputum conversion if therapy is not prolonged
11
.
Conclusions
The global resurgence of TB has been accompanied by increases in the
rates of antibiotic resistance. The majority of resistance is due to the
selection of small populations of naturally occurring mutants during
exposure to inappropriate regimens.
In 1995, it was estimated that less than one-quarter of the world's
population was covered by control programmes that followed the WHO
strategy and accurate figures for the number of patients being treated for
TB ,who have a favourable outcome, are not available. Estimates range
from 37% to 78
27
.
Com prehensive data on the existence and ex tent of
drug resistance around the globe have been lacking
28
, but a recent
monograph from the WHO quoted drug resistance data from 35
national surveillance programmes and median rates for primary
resistance in isolates ranged from 0% to 10.8%
29
.
Patients with suspected or proven MDRTB should be managed by a
multidisciplinary team involving chest, infectious disease, and HIV
physicians, as well as clinical microbiologists with experience in both
treatment, diagnosis and infection control
3
. Outbreaks of MDRTB
infection should not occur if current guidelines are followed but, when
they are suspected, clusters should be confirmed by molecular
techniques, such as restriction fragment length polymorphism analysis
or spoligotyping
9
. This enables the team to monitor the course and
extent of an outbreak as well as assessing the outcome of any
intervention.
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References
1 Watson JM , Maguire HC. PHLS work on the surveillance and epidemiology of tuberculosis.
CDRRev
1997; 8 110-2
2 Coker R, Miller R. HIV associated tubercu losis.B MJ1997, 314: 1847
3 Drobniewski F. Is death inevitable with mulnresistant TB plus HIV infection? Lancet 1997;
349:
71-2
4 Handysides S Tuberculosis remains the captain of all these men of death '. CDR Rev1997; 8:
R105
5 Mitchjson D .Understanding the chemotherapy of tuberculosis-current problems. / Antimtcrob
Cbemother 1992; 29: 477-93
6 Ormerod LP. Chemotherapy and management of tuberculosis in the United Kingdom
recommendations of the Jomt Tuberculosis Subcommittee of the British Thoracic Society.
Thorax
1990, 45 403-8
7 Gangadharam PRJ.
Drugresistance inmycobacterta
Baton Rouge, FL: CRC, 1984, 5375
8 Warburton A RE, Jenkins PA, Waight PA, Watson JM . Drug resistance m initial isolates of
Mycobactenumtuberculosis m England and Wales 1982-1991 Commun Dis Rep 1993; 13:
175-9
9 Drobniewski F, Yates M D. Multiple drug resistant tuberculosis. / Clm Micro1997; 50: 8 9-2
10 Wenger PN, Often J, Breeden A, Orfas D , Beck-Sague CM , Jarvis WR Control of nosocomial
transmission of mulndrug-resistant Mycobactenum tuberculosisamong healthcare w orkers
and HIV-infected patients.Lancet1995; 345: 23 5^ 0
11 Iseman MD . Treatment of multidrug-resistant tuberculosis.
N
Engl
J Med
1993; 329: 784-91
12 Telenti A, Imboden P, Marchasi F
et al.
Detection of rifampicin-resistance mutations in
Mycobactenum tuberculosis. Lancet
1993, 431 647-50
13 Drobniewski FA, Pozniak AL Molecular diagnosis, detection of drug resistance and
epidemiology of tuberculosis. rJ Hosp Med1996, 56 204-8
14 De Beenhouwer H, Lhiang G, James Getal Rapid detection of nfampicm resistance in sputum
and biopsy specimens from tuberculosis patients by PCR and line probe assay.TubercleLung
Dis 1995; 76. 426-9
15 Scorpio A, Lindholm-Levy P, Heifets L et al Characterization of pyrazinamide resistant
Mycobactenum tuberculosis. Antimicrob Agents Cbemother 1997;
41:
540-3
16 Sreevatsan S, Stockbauer K, Pan X
et
al Ethambutol resistance in
Mycobactenum
tuberculosis;
critical role of
embB
m utations.
Antimicrob Agents Chemother
1997,4 1: 1677-81
17 Jacobs WR, Barletta RG, Udani R
et al.
Rapid assessment of drug susceptibilities of
Mycobactenumtuberculosis
by use of luciferase reporter phages
Science
1993; 260: 819-23
18 Wilson SM, Al-Suwaidi Z, McNerney et al. Evaluation of a new rapid bactenophage-based
method for the drug susceptibility testing of Mycobactenum tuberculosis Nat Med1997, 3:
465-8
19 Roth A, Schaberg T, Mau ch H . Molecular diagnosis of tuberculosis- curren t clinical validity
and future perspectives.EurRespir] 1997; 10 1877-90
20 Noordho ek GT, Kolk AHJ, B]une Get al. Sensitivity and specificity of PCR for detection of
Mycobactenum tuberculosis:a blind comparison study among seven laboratories. / Clm
Microbiol
1994, 32: 277-84
21 Nitta AT, Iseman M D, Newell JD, Madsen LA, Goble M. Artificial pneu mopen toneum in the
treatment of pulmonary tuberculosis.
C lm Infect Dis
1993, 16: 219-22
22 Onyebu]oh PC , Abdulmumim T, Robinson S, Rook GA, Stanford J L. Immunotherapy w ith
Mycobactenum vaccaeas an add ition to chemotherapy for the treatm ent of pu lmonary
tuberculosis under difficult conditions in Africa.RespirM ed1 995; 89 : 199207
23 Corlan E, Marcia C, Macavei C, Stanford JL, Stanford CA. Immunotherapv with
Mycobactenum vaccaein the treatment of tuberculosis in Rom ania. Respir Med 1997; 91
13-9
24 Corlan E, Marcia C, Macavei C, Stanford JL, Stanford CA. Immunotherapy with
Mycobactenum vaccae
in the treatment of tuberculosis in Romania. 2. Chronic or relapsed
disease.Respir
Med
1997;91: 21-9
ritish Medical ulletin
1998,54 No 3) 57 7
8/11/2019 Br Med Bull 1998 Eltringham 569 78
10/10
Resurgent/emergent infect ious diseases
25 Durman
P.
Stanford Rook's "miracle cure' for TB fails.
The Times (London)
1 October 1997
26 Condos R, Rom WN , Schluger NW Treatment of mulodrug resistant pulmonary tuberculosis
with interferon-gamma via aerosol Lancet1997; 349: 1513-5
27 Raviglione MC, Dye C, Schmidt
et a\
Assessment of worldwide tuberculosis c ontrol.
Lancet
1997; 350 624-9
28 Drobniewski FA, Pablos-Mendez A, Raviglione M C. Epidemiology of tuberculosis in the world
Semm R espir Cut
Care
Med
1997; 18 419-29
29 Cohn D L, Bustreo F, Raviglione MC Drug resistant tuberculosis: review of the worldwide
situation and the WHO/IUATLD Global Surveillance Project
Chn Infect Dis
1997; 24 (suppl
) S121-30
5 7 8 ritish Medical ulletin 1998,54 No 3)