What is drug resistant TB?
A person 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, http://www.cancer.gov
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.
Is it possible to cure drug resistant TB?
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 it?
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 regimens. 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 it, it is necessary 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.
What other types are there?
A third type 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?
Globally, 160, 684 cases of MDR/RR TB were detected and notified in 2017. This was a small increase from 153,119 in 2016.
Between 2016 and 2017 the number of reported MDR/RR TB cases increased by more than 30% in six of the 30 high MDR TB burden countries (Angola, Democratic People's Republic of Korea, Indonesia, Nigeria, Somalia and Thailand).
Very few statistics are available for totally drug resistant TB.
Where do MDR TB and XDR TB occur?
The estimated cases of MDR TB, among notified cases of MDR TB, and equivalent cases of XDR TB by World Health Organisation (WHO) region are given below.6“Global Tuberculosis Report 2018”, WHO, Geneva, 2018
|Region||Cases of MDR/RR-TB TB||Cases of XDR-TB|
The total of 160,684 cases of MDR/RR TB detected was a small increase from 153,119 in 2016. Between 2016 and 2017 the number of reported MDR/RR TB cases increased by more than 30% in six of the 30 high MDR TB burden countries.
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.
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.