A person with active TB disease has drug resistant TB if the TB bacteria that the person is infected with, will not respond to, and are therefore resistant to, at least one of the main TB drugs.1“Drug resistance” National Cancer Institute,
Drug susceptible TB is the opposite of drug resistant TB. If someone is infected with TB bacteria that are fully susceptible, it means that all of the TB drugs will be effective so long as they are taken properly. It still means that several drugs need to be taken together to provide effective TB treatment.
Drug susceptibility testing is how you find out which drugs will be effective against certain TB bacteria.
It is more difficult to cure TB which is drug resistant than TB which is fully susceptible, but it is still possible.
How do you get drug resistant TB?
There are two ways that people get it.
- Firstly, people get acquired drug resistant TB when their TB treatment is inadequate. This can be for a number of reasons, including the fact that patients fail to keep to proper TB treatment regimes. It can also be that the wrong TB drugs are prescribed, or sub standard TB drugs are used for treatment.
- Secondly, transmitted or primary drug resistant TB, results from the direct transmission of drug resistant TB from one person to another. The occurrence and prevention of primary drug resistant TB has largely been neglected during the development of global programs to end TB. It had been believed that most development of drug resistant TB arose from acquired TB.
New tools have enabled researchers to investigate this further. They have found that primary resistance plays a much greater role than previously thought.
Our best estimates are that globally, at least 75% to 80% of drug-resistant TB is primarily transmitted, as opposed to acquired during treatment. Dr D Dowdy
So simply doing a better job of treating drug susceptible TB is no longer sufficient for controlling drug resistance. To control drug resistant TB we need to specifically diagnose and treat it.
What are the main types of drug resistant TB?
MDR (multi drug resistant) TB is the name given to TB when the bacteria that are causing it are resistant to at least isoniazid and rifampicin, two of the most effective TB drugs.
In May 2016 WHO issued guidance that people with TB resistant to rifampicin, with or without resistance to other drugs, should be treated with an MDR-TB treatment regimen.
This group of patients (effectively an expanded MDR-TB group), is sometimes referred to as MDR/RR-TB).2“WHO treatment guidelines for drug resistant tuberculosis (2016 update)”, WHO, Geneva, 2016,
When a person is described as having MDR-TB, it is not clear whether they may also be resistant to other drugs as well. So the World Health Organisation has now started to refer to “uncomplicated MRD-TB”. This is TB which is resistant to isoniazid and rifampicin (making it MDR TB) but it is known that the bacteria are not resistant to any of the second line TB drugs.
XDR-TB (extensively drug resistant TB) is defined as strains resistant to at least rifampicin and isoniazid. This is in addition to strains being resistant to one of the fluoroquinolones, as well as resistant to at least one of the second line injectable TB drugs amikacin, kanamycin or capreomycin.3“Extensively drug-resistant tuberculosis (XDR-TB): recommendations for prevention and control”, Weekly epidemiological record, WHO, Geneva, 2006, 81
MDR-TB and XDR-TB do not respond to the standard six months of TB treatment with “first line” anti TB drugs. Treatment for them can often take two years or more and requires treatment with other drugs that are less potent, more toxic and much more expensive. However, there are now starting to be some shorter regimens for treatment, based on the Bangladesh regimen. There is more about the treatment of drug resistant TB.
What other types of drug resistant TB are there?
A third type of drug resistant TB, variously referred to as totally drug resistant TB, XXDR-TB or TDR-TB has also now been detected.4Velayati, Ali “Emergence of New Forms of Totally Drug Resistant Tuberculosis Bacilli”, Chest, Vol 136, August 2009, no. 2 420-425
http://www.ncbi.nlm.nih.gov/pubmed/ 5“Migliori, G “125 years after Robert Koch’s discovery of the tubercle bacillus: the new XDR-TB threat. Is “science” enough to tackle the epidemic?”, European Respiratory Journal, March 1 2007
http://erj.ersjournals.com It is sometimes also referred to as extremely drug resistant TB. It is extremely difficult, although not always totally impossible to treat.
What are the statistics for drug resistant TB?
Globally in 2016 the World Health Organisation (WHO) estimated that 4.1% of new cases and 19% of previously treated cases of TB were of MDR/RR-TB.6“Global Tuberculosis Report 2017”, WHO, Geneva, 2016
There were an estimated 600,000 incident cases of MDR/RR-TB. Cases of MDR-TB (490,000) accounted for 82% of the total. The countries with the largest number of MDR/RR-TB cases (47% of the total) are China, India and the Russian Federation.
There were an estimated 240,000 deaths from MDR/RR-TB in 2016.
Some organisations believe that the current statistics greatly underestimate the extent of the problem.
Wherever we’re looking for drug resistant TB we’re finding it in very alarming numbers. And that suggests to us that the current statistics that are being published about the prevalence of multi drug resistant TB are really just scratching the surface of the problem."
Dr Leslie Shanks, Medical Director, MSF7DeCapua, J “MSF: Alarming scope of drug resistant TB”, Voice of America, March, 2012
By the end of 2016 extensively drug resistant (XDR) TB had been reported by 123 countries and territories. A total of 20% of patients with MDR-TB are estimated to have resistance to one of the fluoroquinolone drugs. An estimated 6.2% of people with MDR-TB have XDR-TB.
Very few statistics are available for totally drug resistant TB.8Velayati, Ali “Emergence of New Forms of Totally Drug Resistant Tuberculosis Bacilli”, Chest, Vol 136, August 2009, no. 2 420-425
http://www.ncbi.nlm.nih.gov/pubmed/ 9Migliori, G “125 years after Robert Koch’s discovery of the tubercle bacillus: the new XDR-TB threat. Is “science” enough to tackle the epidemic?”, European Respiratory Journal, March 1 2007
Where does MDR-TB occur?
The estimated cases of MDR-TB, among notified cases of MDR-TB, by World Health Organisation (WHO) region are given below.10“Global Tuberculosis Report 2017”, WHO, Geneva, 2017
|Region||Estimated cases of multi drug resistant TB|
|Global Total for multi drug resistant TB||601,000|
If there is a "plus sign" click on it for more columns
The highest prevalence of MDR-TB that had been documented by 2012 was in Minsk, Belarus. This was a prevalence of 47.8% reported in 2011. Among patients with infectious (smear positive) pulmonary disease, MDR-TB was seen in 35.3% of newly detected cases, and in a massive 76.5% of previously treated patients.11Hoffner, S. “Unexpected high levels of multidrug-resistant tuberculosis present new challenges for tuberculosis control”, The Lancet, 30 August 2012
High burden countries
The three lists of “high burden” countries have now been revised. There is now a new list of 30 high burden MDR-TB countries.
The 30 “high burden” countries are:
Angola, Azerbaijan, Bangladesh, Belarus, China, DPR Korea, DR Congo, Ethiopia, India, Indonesia, Kazakhstan, Kenya, Kyrgyzstan, Mozambique, Myanmar, Nigeria, Pakistan, Papua New Guinea, Peru, Philippines, Republic of Moldova, Russian Federation, Somalia, South Africa, Tajikistan, Thailand, Ukraine, Uzbekistan, Viet Nam, Zimbabwe.
There is more about high burden TB countries.
Diagnosing drug resistant TB
Globally there is very limited capacity to rapidly diagnose drug resistant TB. Some new TB tests are becoming available such as the Genexpert test, and the TrueNat test. However, point of care testing is still practically non existent in the areas with the highest TB burden. To overcome the problems of drug resistant TB there needs to be development of true point of care drug susceptibility tests, and their widespread implementation at affordable cost.
“Today MDR-TB spreads unchecked in most of the world. It is fueled by poverty at the individual and family levels, - limiting access to effective treatment - and at the regional and national level, where under resourced governments lack the capacity to tackle this disease.”12Keshavjee, S., Farmer, P.E. “Time to put boots on the ground: making universal access to MDR-TB treatment a reality”, Int J Tuberc Lung Dis, 14(10), October 2010, 1222-1225