|Classification and external resources|
Multi-drug-resistant tuberculosis (MDR-TB) is defined as tuberculosis that is resistant to at least isoniazid (INH) and rifampicin (RMP), the two most powerful first-line treatment anti-TB drugs.Isolates that are multiply resistant to any other combination of anti-TB drugs but not to INH and RMP are not classed as MDR-TB.
MDR-TB develops in otherwise treatable TB when the course of antibiotics is interrupted and the levels of drug in the body are insufficient to kill 100% of bacteria. This can happen for a number of reasons: Patients may feel better and halt their antibiotic course, drug supplies may run out or become scarce, patients may forget to take their medication from time to time or patients do not receive effective therapy. Most tuberculosis therapy consists of short-course chemotherapy which is only curing a small percentage of patients with multi-drug resistant tuberculosis. Delays in second line drugs make multi-drug resistant tuberculosis more difficult to treat. MDR-TB is spread from person to person as readily as drug-sensitive TB and in the same manner. Even with the patent off second line antituberculosis medication the price is still high and therefore a big problem for patients living in poor countries to be treated. With patients not treated, the spread of Tuberculosis would be problematic in poor countries. In order to fully cure infectious diseases, such as Tuberculosis, we need a plan to ensure equal access to health care.
Cases of MDR tuberculosis have been reported in every country surveyed. MDR-TB most commonly develops in the course of TB treatment, and is most commonly due to doctors giving inappropriate treatment, or patients missing doses or failing to complete their treatment. Because MDR tuberculosis is an airborne pathogen, persons with active, pulmonary tuberculosis caused by a multidrug-resistant strain can transmit the disease if they are alive and coughing. TB strains are often less fit and less transmissible, and outbreaks occur more readily in people with weakened immune systems (e.g., patients with HIV). Outbreaks among non immunocompromised healthy people do occur, but are less common.
A 1997 survey of 35 countries found rates above 2% in about a third of the countries surveyed. The highest rates were in the former USSR, the Baltic states, Argentina, and China, and was associated with poor or failing national tuberculosis control programmes. In 2013, the conditions in India, China and Eastern Europe were characterized as being "at epidemic proportions". In Moldova, the crumbling health system has led to the rise of MDR-TB. In 2013, the Mexico–United States border was noted to be "a very hot region for drug resistant TB", though the number of cases remained small.
It has been known for many years that INH-resistant TB is less virulent in guinea pigs, and the epidemiological evidence is that MDR strains of TB do not dominate naturally. A study in Los Angeles, California found that only 6% of cases of MDR-TB were clustered. Likewise, the appearance of high rates of MDR-TB in New York City in the early 1990s was associated with the explosion of AIDS in that area. In New York City, a report issued by city health authorities states that fully 80 percent of all MDR-TB cases could be traced back to prisons and homeless shelters. When patients have MDR-TB, they require longer periods of treatment—about two years of multidrug regimen. Several of the less powerful second-line drugs, which are required to treat MDR-TB, are also more toxic, with side effects such as nausea, abdominal pain, and even psychosis. The Partners in Health team had treated patients in Peru who were sick with strains that were resistant to ten and even twelve drugs. Most such patients require adjuvant surgery for any hope of a cure.
Mechanisms of M. tuberculosis drug resistance
Some of the ways the tubercle bacillus acquires drug resistance are:
1) Cell wall: The cell wall of M. tuberculosis consists of complex lipids, and it acts as a permeability barrier from drugs.
2) Drug modifying & inactivating enzymes: The M. tuberculosis genome codes for certain enzymes that make it drug resistant. The enzymes usually phosphorylate, acetylate, or adenylate the drug compounds.
3) Drug efflux systems
4) Mutations: Spontaneous mutations in the M. tuberculosis genome can give rise to proteins that make the bacterium drug resistant, depending on the drug action.
Examples of mutations that make M. tuberculosis drug resistant: An example of this is the mutation in the rpoB gene, which encodes the beta subunit of the bacteria's RNA Polymerase. This mutation makes the bacillus resistant to Rifampicin. Non-resistant TB is sensitive to Rifampicin because this drug binds to the beta subunit of the RNA Polymerase, and hence disrupts translation and elongation. When the rpoB gene is mutated, the resulting beta subunit protein has different amino acids, and thus a different conformation. Rifampicin can no longer bind to the beta subunit and prevent translation.
Other mutations make the bacterium resistant to other drugs. For example, there are many mutations that can make M. tuberculosis resistant to Isoniazid. Mutations leading to INH resistance have been identified in different gene targets including katG, inhA, ahpC and other genes that remain to be established. Amino acid replacements in the NADH binding site of InhA apparently result in INH resistance by preventing the inhibition of mycolic acid biosynthesis, which the bacterium uses in its cell wall. Mutations in the katG gene causes the enzyme catalase peroxidase unable to convert INH to its biologically active form. Hence, INH is not able to affect M. tuberculosis.
Extensively drug-resistant TB
MDR-TB can become resistant to the major second-line drug groups: fluoroquinolones and injectable drugs. When MDR-TB is resistant to at least one drug from each group, it's defined as extensively drug-resistant tuberculosis (XDR-TB).
In a study of MDR-TB patients from 2005 to 2008 in various countries, 43.7% had resistance to at least one second-line drug.
Prevention of MDR TB
1) Rapid diagnosis & treatment of TB: One of the greatest risk factors for drug resistant TB is problems in treatment and diagnosis, especially in developing countries. If TB is identified and treated soon, drug resistance can be avoided.
2) Completion of treatment: Previous treatment of TB is an indicator of MDR TB. If the patient does not complete his/her antibiotic treatment, or if the physician does not prescribe the proper antibiotic regimen, resistance can develop. Also, drugs that are of poor quality or less in quantity, especially in developing countries, contribute to MDR TB.
3) Patients with HIV/AIDS should be identified and diagnosed as soon as possible. They lack the immunity to fight the TB infection and are at great risk of developing drug resistance.
4) Identify contacts who could have contracted TB: i.e. family members, people in close contact, etc.
5) Research: Much research and funding is needed in the diagnosis, prevention and treatment of TB and MDR TB.
"Opponents of a universal tuberculosis treatment, reasoning from misguided notions of cost-effectiveness, fail to acknowledge that MDRTB is not a disease of poor people in distant places. The disease is infectious and airborne. Treating only one group of patients looks inexpensive in the short run, but will prove disastrous for all in the long run."
Community-based treatment programs such as DOTS-Plus, a MDR-TB-specialized treatment using the popular Directly Observed Therapy – Short Course (DOTS) initiative, have shown considerable success in the treatment of MDR-TB in some parts of the world. In these locales, these programs have proven to be a good option for proper treatment of MDR-TB in poor, rural areas. A successful example has been in Lima, Peru, where the program has seen cure rates of over 80%.
However, TB clinicians[who?] have expressed concern in the DOTS program administered in the Republic of Georgia because it is anchored in a passive case finding. This means that the system is dependent on patients coming to health care providers rather than conducting compulsory screenings. As medical anthropologists, like Erin Koch have shown; this form of implementation does not suit all cultural structures and urges that the DOTS protocol be constantly reformed in the context of local practices, forms of knowledge and everyday life.
Furthermore, Erin Koch who has utilized Paul Farmer’s concept of “structural” violence; a perspective for understanding how “institutions, environment, poverty, and power reproduce, solidify, and naturalize the uneven distribution of disease and access to resources”, has also studied the effectiveness of the DOTS protocol in the widespread disease of tuberculosis in the Georgian prison system. Unlike the DOTS passive case finding utilized for the general Georgian public, the prison system combined multiple levels of surveillance which has proven more successful in reducing the spread of tuberculosis while increasing rates of cure.
In Koch’s final remark, she critically notes that it is because the DOTS protocol is aimed towards changing the individual’s behavior and not addressing the need to change the institutional, political, and economic contexts that certain limitations like MDR tuberculosis arise.
Paul Farmer believes that DOTS should be the cornerstone of tuberculosis control around the world.
Usually, multidrug-resistant tuberculosis can be cured with long treatments of second-line drugs, but these are more expensive than first-line drugs and have more adverse effects. The mantra that MDRTB drugs are too expensive to be cost effective has been repeated without honest investigation as to why these drugs, long off-patent, remain horribly overpriced. Thus has the notion of cost-effectiveness become one of the chief means by which we manage (and perpetuate) modern inequality. The treatment and prognosis of MDR-TB are much more akin to that for cancer than to that for infection. It has a mortality rate of up to 80%, which depends on a number of factors, including
- How many drugs the organism is resistant to (the fewer the better)
- How many drugs the patient is given (patients treated with five or more drugs do better)
- Whether an injectable drug is given or not (it should be given for the first three months at least)
- The expertise and experience of the physician responsible
- How co-operative the patient is with treatment (treatment is arduous and long, and requires persistence and determination on the part of the patient)
- Whether the patient is HIV positive or not (HIV co-infection is associated with an increased mortality).
The majority of patients suffering from multi-drug-resistant tuberculosis do not receive treatment,as they tend to live in underdeveloped countries or in a state of poverty. Denial of treatment remains a difficult human rights issue, as the high cost of second-line medications often precludes individuals unable to afford therapy.
A study concluded three major policies. First off, they strengthened the principle that the treatment of smear-positive cases in DOTS programs must be the foundation of any tuberculosis control approach, and should be a basic practice for all control programs. Secondly, there is a powerful economic case for treating smear-negative and extra-pulmonary cases in DOTS programs along with treating smear-negative and extra-pulmonary cases in DOTS programs as a new WHO “STOP TB” approach and the second global plan for tuberculosis control. Last but not least, the study shows that significant scaling up of all interventions is needed in the next 10 years if the millennium development goal and related goals for tuberculosis control are to be achieved. If the case detection rate will be improved, this is will guarantee people who gain access to treatment facilities are covered and that coverage is widely distributed to people who do not have access to do so.
In general, treatment courses are measured in months to years; MDR-TB may require surgery, and death rates remain high despite optimal treatment. However, good outcomes for the patient are still possible.
The treatment of MDR-TB must be undertaken by a physician experienced in the treatment of MDR-TB. Mortality and morbidity in patients treated in non-specialist centers are significantly higher to those of patients treated in specialist centers.
In addition to the obvious risks (i.e., known exposure to a patient with MDR-TB), risk factors for MDR-TB include HIV infection, previous incarceration, failed TB treatment, failure to respond to standard TB treatment, and relapse following standard TB treatment.
Treatment of MDR-TB must be done on the basis of sensitivity testing: It is impossible to treat such patients without this information. If treating a patient with suspected MDR-TB, the patient should be started on SHREZ (Streptomycin+isonicotinyl Hydrazine+Rifampicin+Ethambutol+pyraZinamide)+moxifloxacin+cycloserine pending the result of laboratory sensitivity testing. There is evidence that previous therapy with a drug for more than a month was associated with diminished efficacy of that drug regardless of in vitro tests indicating susceptibility, so, detailed knowledge of the treatment history of that patient is essential.
A gene probe for rpoB is available in some countries, which serves as a useful marker for MDR-TB, because isolated RMP resistance is rare (except when patients have a history of being treated with rifampicin alone). If the results of a gene probe (rpoB) are known to be positive, then it is reasonable to omit RMP and to use SHEZ+MXF+cycloserine. The reason for maintaining the patient on INH is that INH is so potent in treating TB that it is foolish to omit it until there is microbiological proof that it is ineffective (even though isoniazid resistance so commonly occurs with rifampicin resistance).
When sensitivities are known and the isolate is confirmed as resistant to both INH and RMP, five drugs should be chosen in the following order (based on known sensitivities):
- an aminoglycoside (e.g., amikacin, kanamycin) or polypeptide antibiotic (e.g., capreomycin)
- a fluoroquinolone: e.g., moxifloxacin (ciprofloxacin should no longer be used);
- a thioamide: prothionamide or ethionamide
- a macrolide: e.g., clarithromycin
- high-dose INH (if low-level resistance)
Note: Drugs placed nearer the top of the list are more effective and less toxic; drugs placed nearer the bottom of the list are less effective or more toxic, or more difficult to obtain.
In general, resistance to one drug within a class means resistance to all drugs within that class, but a notable exception is rifabutin: Rifampicin-resistance does not always mean rifabutin-resistance, and the laboratory should be asked to test for it. It is possible only to use one drug within each drug class. If it is difficult finding five drugs to treat then the clinician can request that high-level INH-resistance be looked for. If the strain has only low-level INH-resistance (resistance at 0.2 mg/l INH, but sensitive at 1.0 mg/l INH), then high dose INH can be used as part of the regimen. When counting drugs, PZA and interferon count as zero; that is to say, when adding PZA to a four-drug regimen, another drug must be chosen to make five. It is not possible to use more than one injectable (STM, capreomycin or amikacin), because the toxic effect of these drugs is additive: If possible, the aminoglycoside should be given daily for a minimum of three months (and perhaps thrice weekly thereafter). Ciprofloxacin should not be used in the treatment of tuberculosis if other fluoroquinolones are available.
There is no intermittent regimen validated for use in MDR-TB, but clinical experience is that giving injectable drugs for five days a week (because there is no-one available to give the drug at weekends) does not seem to result in inferior results. Directly observed therapy helps to improve outcomes in MDR-TB and should be considered an integral part of the treatment of MDR-TB.
Response to treatment must be obtained by repeated sputum cultures (monthly if possible). Treatment for MDR-TB must be given for a minimum of 18 months and cannot be stopped until the patient has been culture-negative for a minimum of nine months. It is not unusual for patients with MDR-TB to be on treatment for two years or more.
Patients with MDR-TB should be isolated in negative-pressure rooms, if possible. Patients with MDR-TB should not be accommodated on the same ward as immunosuppressed patients (HIV-infected patients, or patients on immunosuppressive drugs). Careful monitoring of compliance with treatment is crucial to the management of MDR-TB (and some physicians insist on hospitalisation if only for this reason). Some physicians will insist that these patients remain isolated until their sputum is smear-negative, or even culture-negative (which may take many months, or even years). Keeping these patients in hospital for weeks (or months) on end may be a practical or physical impossibility, and the final decision depends on the clinical judgement of the physician treating that patient. The attending physician should make full use of therapeutic drug monitoring (in particular, of the aminoglycosides) both to monitor compliance and to avoid toxic effects.
Some supplements may be useful as adjuncts in the treatment of tuberculosis, but, for the purposes of counting drugs for MDR-TB, they count as zero (if four drugs are already in the regimen, it may be beneficial to add arginine or vitamin D or both, but another drug will be needed to make five).
The drugs listed below have been used in desperation, and it is uncertain as to whether they are effective at all. They are used when it is not possible to find five drugs from the list above.
On December 28, 2012 the U.S. Food and Drug Administration (FDA) approved bedaquiline (marketed as Sirturo by Johnson & Johnson) to treat multi-drug resistant tuberculosis, the first new treatment in 40 years. Sirturo is to be used in a combination therapy for patients who have failed standard treatment and have no other options. Sirturo is an adenosine triphosphate synthase (ATP synthase) inhibitor.
The following drugs are experimental compounds that are not commercially available, but may be obtained from the manufacturer as part of a clinical trial or on a compassionate basis. Their efficacy and safety are unknown:
In cases of extremely resistant disease, surgery to remove infection portions of the lung is, in general, the final option. The center with the largest experience in this is the National Jewish Medical and Research Center in Denver, Colorado. In 17 years of experience, they have performed 180 operations; of these, 98 were lobectomies and 82 were pneumonectomies. There is a 3.3% operative mortality, with an additional 6.8% dying following the operation; 12% experienced significant morbidity (in particular, extreme breathlessness). Of 91 patients who were culture-positive before surgery, only 4 were culture-positive after surgery.
The resurgence of tuberculosis in the United States, the advent of HIV-related tuberculosis, and the development of strains of TB resistant to the first-line therapies developed in recent decades—serve to reinforce the thesis that Mycobacterium tuberculosis, the causative organism, makes its own preferential option for the poor. The simple truth is that almost all tuberculosis deaths result from a lack of access to existing effective therapy.
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- Video: Drug-Resistant TB in Russia July 24, 2007, Woodrow Wilson Center event featuring Salmaan Keshavjee and Murray Feshbach
- TB Drug Resistance Mutation Database
- MDR-TB : a story of Hope,Struggle & Triumph
- MDR-TB (DOTS Plus) protocol followed under RNTCP in India (PDF)