Taking Control of Tuberculosis

Current diagnostic tools take weeks to provide accurate results allowing time for co-infection and spread of disease.  Here, Dr Ellen Jo Baron sets the scene for a new diagnostic test able to revolutionise TB management on a global scale

World map showing reported cases of tuberculosis per 100,000 citizens. Red = >300, orange = 200-300; yellow = 100-200; green 50-100; blue = <50 and grey = n/a.

Tuberculosis (TB) has taken a backseat over the last few decades as it has not received the funding appropriate to such a major health threat to the developing world. The World Health Organisation (WHO) published statistics last month showing that worldwide TB rates are stabilising at approximately 14 million cases, comprising one-third of the world’s population. 

There are, however, a few countries where TB rates are increasing. The Health Protection Agency (HPA) has released statistics showing that the UK has seen a 2% increase (the only European country where TB is actually on the rise) and in African countries where HIV/TB co-infection is rife, TB deaths have quadrupled over the last 15 years. Clearly, tuberculosis is making a comeback - the disease is stronger now than ever before, and is rapidly becoming more drug-resistant, usually by gene mutations. In fact, much of the antibiotic resistance observed among TB strains is multi-drug resistance (MDR), showing resistance toward at least two first-line drugs. TB is second only to HIV as the world’s deadliest infectious disease, claiming a life every 20 seconds. 

One of the key obstacles in controlling TB infection is the lack of fast and accurate diagnostic methods, particularly in areas of the world with the greatest disease burden. This article will examine those current diagnostics and reveal details of a new test that is set to revolutionise the way that TB is diagnosed and, therefore, controlled throughout the world.

Over the last year, the World Health Organisation has expressed concern that they may not be able to reach the targets for TB control as defined in the UN’s Millennium Development Goals (MDG’s). Those targets for 2015 included minimum requirements for cases detected and treated and required falling rates of TB incidence. Additionally, as defined in 2006 by the Stop TB Partnership, a global consortium of public health, non-government organisations, World Bank, pharmaceutical and other corporate stakeholders, governments, and academic institutions, prevalence and mortality rates should be decreased to one-half of the 1990 rates.  This would essentially mean that 50 million people would receive treatment and 14 million lives would be saved. 

As we know, early detection and proper treatment are essential to combat TB.  UN Secretary-General, Ban Ki-Moon, argued that although “the epidemic is continuing its decline, the [global] rate of decline is far too slow, Unless we accelerate action, the numbers of those falling ill will continue to grow.” A 2008 report stated that “the detection rate increased only marginally to 61%, short of the 65% benchmark for 2006 contained in the Stop TB Partnership Global Plan and the ultimate target of 70%.”

One-third of TB cases come from Sub-Saharan Africa and Southeast Asia, where the main attributable factors are widespread poverty and a lack of adequate public healthcare. 

People who have TB, or live in poverty, can often become mired in a vicious

“Tuberculosis is making a comeback — the disease is stronger now than ever before, and is rapidly becoming more drug-resistant, usually by gene mutations.”

circle of continuous proliferation of both. TB most commonly attacks people in their working years crippling their ability to earn a living. Additionally, the costs associated with healthcare for TB can put families of sufferers under tremendous financial strain. This has a particularly significant impact on children who often become malnourished and have to leave school to take care of their family. In some cases, these children ultimately become orphans. 

In turn, people living in poverty who cannot afford or access healthcare often share living space with many people, some of whom are already infected with TB. Spread of infection becomes much more likely in such settings. 

There is a strong connection between HIV and TB.  A third of all HIV/AIDS sufferers are co-infected with TB. The WHO Global TB Report stated that approximately 1.37 million HIV/AIDS sufferers were diagnosed with TB in 2008 - indeed TB is the leading cause of death amongst HIV sufferers. Shockingly, new data show that 25% of all TB deaths are HIV-related, double the amount previously thought. 

Optimal TB management requires rapid detection, treatment, and prevention - but current technologies are outdated and impractical. The most commonly used methods have not changed in more than 125 years. According to the WHO, “only 2% of multi-drug resistant TB (MDR-TB) cases worldwide are being diagnosed and treated appropriately.”

The current testing method begins with spreading the patient’s sputum on a glass slide, treating the material with a special stain, and observing the slide under a microscope to detect the stained bacteria. The stain does not wash off the bacteria on the slide even when an acid solution is used, hence the organisms are called acid-fast bacilli (AFB). But this method, as practiced in high-burden parts of the world, fails to detect half of all active cases; a failure rate that is even higher in those co-infected with HIV. Tests that are more accurate exact a price, both monetarily and in the prolonged length of time it takes to achieve accurate detection of the TB bacteria by cultivating them in the laboratory. Due to the slow growth rate of the TB organism (the bacterium has a doubling time of 24 hours), growing the organism in culture can take weeks before results are available. It then takes even longer to determine if there is drug resistance, because the positive culture requires a second cultivation in the presence and absence of antibiotics in order to determine which antimicrobial agents are effective in inhibiting its growth. Employing all these procedures currently required to diagnose drug-resistant TB means that results may not be available for months. 

Unfortunately for many patients in the developing world, this amounts to a post-mortem diagnosis, particularly as they are often co-infected with both TB and HIV - a rapidly fatal combination. Additionally, because diagnosis takes so long, these patients are sent back into the community, rather than isolated and treated appropriately, allowing for further transmission of the organism within the community in the local area.

In most of the developed world, isolation of patients with TB is implemented to

Public health campaigns have tried to halt the spread of TB

prevent transmission to surrounding family and communities. However, even in the UK there are currently no guidelines advocating isolation during the time that the laboratory testing is being conducted, making a rapid diagnosis even more necessary.

Until now, if a clinician were to request a quicker TB diagnostic method than culture, accuracy would suffer.  Sputum smear testing alone can theoretically produce results within a day. However 30% or more of patients whose sputum is smear-negative ultimately reveal positive cultures in follow-up tests, proving this diagnostic method to be highly insensitive and dangerously inaccurate. Missed diagnoses via smear testing are estimated to contribute to as much as 20% of newly transmitted TB infections. And in an unexpected twist of fate, patients with HIV/TB co-infection are actually more likely to be smear-negative but to progress rapidly to a fatal outcome.

As World TB Day arrived at the end of last month, Ki-Moon called for faster action to combat TB on a global scale. He highlighted the need for infection prevention, early detection, and worldwide availability of treatment, whilst calling for more effort to combat MDR-TB, XDR-TB (extensively drug-resistant TB), and the TB/HIV co-epidemic.

With this in mind, he called for organisations to come together and develop diagnostic solutions that will prevent the spread of the disease. Specifically, the WHO has called for faster and more accurate diagnostics based on the detection of specific DNA patterns in a gene called rpoB, found in all TB bacteria, that is considered to be a surrogate marker for MDR-TB.

In line with these announcements, a number of organisations have collaborated to develop a new diagnostic test for TB. The new test detects TB in less than two hours, is highly accurate, and has recently become available in Europe. The test is run on Cepheid’s GeneXpert System, a leading molecular testing platform, and was developed jointly by Cepheid, the Foundation for Innovative New Diagnostics (FIND), the University of Medicine and Dentistry of New Jersey (UMDNJ), and the US-run National Institute of Allergy and Infectious Diseases (NIAID). 

Not only is the advanced technology able to detect the TB organism directly from sputum, it can also simultaneously detect rifampicin resistant strains of the infection via the presence of  mutations in the rpoB gene, which is generally used as a surrogate marker for multi-drug resistance. And the test boasts unprecedented levels of sensitivity for detection of both smear-positive and smear-negative TB cases. Especially for HIV co-infected patients, the latter feature could be a life-saver.

This new test is one of the most important diagnostic developments to have occurred in many years. It is the most technologically advanced test for TB ever developed, yet it is simple enough to perform in all corners of the world, including resource-limited settings in which it is most needed.

When an individual is suspected of having a TB infection, the first step is often a chest X-ray.  If the chest X-ray is suspicious, the patient is asked to cough and produce a sputum specimen, which is mixed with a solution that kills the TB organisms and placed into a small plastic GeneXpert cartridge. The GeneXpert System carries out steps that would have, until recently, required a dedicated laboratory facility and highly trained molecular diagnostic specialists.  Within the cartridge, specimen processing and reagent mixing involving dozens of micro-pipetting steps are used to carry out nested real-time polymerase chain reaction (PCR), a method to multiply DNA in a test tube dramatically faster than occurs naturally in cultures. This reaction amplifies a DNA signature sequence that is specific to TB complex bacteria. DNA amplification and detection occur simultaneously within about 90 minutes. Because the test procedure is so simple, it is possible to run a STAT test on demand for patients with X-ray findings suspicious for TB, thus maximising the medical value of the test. At this time, all other PCR-based tests require multiple hands-on processing steps that necessitate performing the tests in batches.

Building on an ingenious protocol developed by Dr David Alland, M.D. and his colleagues at UMDNJ, the team engineered a protocol inside the GeneXpert cartridge that detects the presence of 5 different mutations in the core of the rpoB gene, now proven to detect virtually all rifampicin resistant strains of TB. Because rifampicin resistance is almost always present when there is resistance to other drugs, this result can be used to predict MDR-TB. Still more important is that the presence of rifampicin resistance predicts failure of first-line therapy, guiding the clinician to the most effective choice of second-line treatment and potentially avoiding costly treatment failures that lead to progressive disease and continued spread to others.

Tuberculosis is not an infection that is going to vanish overnight and, contrary to popular belief, it is not a disease from the 19th century that we no longer need to worry about, although it should be. It is a tragedy that we have not yet managed to eradicate this devastating infection from the world, both in developing countries and in our own supposedly developed country.  Its close links with poverty and the HIV virus make it a much more widespread and serious issue in Africa and Southeast Asia, but it is also a concern in the UK and Europe.  With over 8,000 new cases of TB reported every year in the UK, we are seeing the number of cases increase, bucking the global trend.  It is my sincere hope that this new diagnostic test, which will soon be available throughout the world, will mark the tipping point when we finally start to win the battle against TB.