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Phthisiology at the Dawn of the New Century^*

A Review of Tuberculosis and the Prospects for Its Elimination

^* From the Florida Department of Health, Bureau of Tuberculosis Control and Refugee Health (Drs. Lauzardo and Ashkin), Division of Pulmonary and Critical Care Medicine, University of Florida, College of Medicine, Gainesville (Dr. Lauzardo), and Division of Pulmonary and Critical Care Medicine, University of Miami, School of Medicine, Miami, FL (Dr. Ashkin).

Correspondence to: Michael Lauzardo, MD, Florida Department of Health, 730 NE Waldo Rd, Suite 600, Gainesville, FL 32641-3699; e-mail: Michael_Lauzardo{at}doh.state.fl.us

	Abstract

TOP Abstract Introduction Epidemiology TB Infection TB Disease Molecular Epidemiology Pathogenesis Diagnosis Clinical Aspects Vaccine Development New Therapies Conclusion References

 
Tuberculosis (TB) has been and continues to be one of the most significant pathogens in terms of human morbidity and mortality. Although the resurgence of TB has been held in check in most developed countries, the epidemic rages on in most developing countries of the world. The specter of drug resistance is becoming a more credible challenge in many parts of the world, dimming the prospects of eventual elimination. However, great opportunities are arising as well, with an unprecedented focus on the global aspects of TB control. This article will review the status of TB today and put into perspective the prospects for its elimination in the coming century.

Key Words: drug-resistance • epidemiology • Mycobacterium tuberculosis • phthisiology • review • therapy • tuberculosis

	Introduction

TOP Abstract Introduction Epidemiology TB Infection TB Disease Molecular Epidemiology Pathogenesis Diagnosis Clinical Aspects Vaccine Development New Therapies Conclusion References

 
The effects of this treatment on tubercular disease are simply astounding... If it should so happen... then would be achieved the complete triumph of the treatment of tuberculosis. And, for my part, I rejoice that we are permitted to look forward with hope to that glorious consummation.

Sir Joseph Lister, 1890¹

Let us all do something to restrict this disease, so that with the dawn of the new century we may hope to see the tuberculosis problem solved.

Dr. Homer M. Thomas, 1899²

At the end of the last century, the world was awash with great expectation that tuberculosis (TB), "the captain of all these men of death," would soon be vanquished. Recent advances, such as the discovery of the tubercle bacillus by Koch and the development of radiography by Roentgen, heralded an unprecedented air of optimism in medicine and, indeed, throughout all of society. The dawn of the 20th century held great promise for a cure for the disease that had claimed the lives of some of the most prominent writers, musicians, and statesmen of the previous century.

Phthisiology, derived from the Greek phthisis meaning wasting or consumption, is the historical term given to the study of TB. The fact that most health-care providers do not recognize this term attests to the many advances that were made during the 20th century. The advances were seen as so significant that even as late as the 1970s and most of the 1980s, TB was taught to medical students as a disease that was on the verge of elimination and deserved little attention except for its historical significance.

The tragic reality that has unfolded during the end of the 20th century has relieved medicine of its unbridled optimism. In most of the world, case numbers have been increasing relentlessly. HIV, the scourge of the late 20th century, has conspired with poor distribution of resources and poverty to fuel the fire of the TB epidemic. More people have died during the last decade from TB than perhaps any other decade in history.

With the resurgence of TB during the preceding decade, becoming a TB physician has been a calling for many. The decision to pursue TB as a career has broadened our personal perspectives on how disease impacts not just the patient but all of society and how societal conditions affect disease. More than anything else, it has taught us that blind confidence in a treatment without the sweat and vigilance of public health can quickly lead to disaster.

This article is not meant to be an in-depth, exhaustive review of TB, but rather a brief overview of the current status of TB from the perspective of two "phthisiologists" whose careers are solely dedicated to the clinical and public health aspects of TB. We will review TB as it pertains to the current situation worldwide, recent advances and trends, and the challenges and opportunities that lie in the years ahead to fulfill the promise of eradicating TB.

	Epidemiology

TOP Abstract Introduction Epidemiology TB Infection TB Disease Molecular Epidemiology Pathogenesis Diagnosis Clinical Aspects Vaccine Development New Therapies Conclusion References

 
The burden of TB on mankind has been and continues to be enormous. Of the world’s population, approximately one third, or 1.7 billion people, are believed to be infected with Mycobacterium tuberculosis. During the 1990s, an estimated 30 million people will have died as a result of TB, making a strong argument for TB being the most important pathogen in the world today.³

Precise data on the true extent of the TB pandemic are not possible because inadequate surveillance and reporting exist in many countries. Therefore, data on the impact of TB are achieved indirectly by using the average annual risk of TB infection (ARTI), the estimated incidence of smear-positive TB, case notifications and notification rates, the estimated coverage of the population with health-care services, and the estimated case-fatality rates.⁴

	TB Infection

TOP Abstract Introduction Epidemiology TB Infection TB Disease Molecular Epidemiology Pathogenesis Diagnosis Clinical Aspects Vaccine Development New Therapies Conclusion References

 
The tuberculin skin test is the only test that detects the presence of infection with M tuberculosis. Tuberculin skin test surveys, when performed in the same non-BCG (bacille Calmette-Guérin) vaccinated population at different times, provide the basis for the ARTI. Although this method has its limitations, it is estimated that for every 1% ARTI, an average of 50 smear-positive cases of TB will occur per 100,000 population.⁵

The vast majority of individuals with TB infection reside in developing countries. Sub-Saharan Africa probably has the highest ARTI (range, 1.5 to 2.5%), followed closely by southern and eastern Asia (range, 1 to 2%).⁴ In North Africa, the Middle East, and Latin America, the ARTI is significantly lower, ranging between 0.5% and 1.5%.⁴ Most areas of developed countries have ARTI rates < 0.5%. At this rate, the ARTI has questionable applicability. In developed countries, 80% of infected individuals are 50 years of age, as opposed to those in developing countries where 77% are < 50 years of age.³ This disparity is the result of the higher ARTI in developing countries and the younger ages, on average, of those populations.

	TB Disease

TOP Abstract Introduction Epidemiology TB Infection TB Disease Molecular Epidemiology Pathogenesis Diagnosis Clinical Aspects Vaccine Development New Therapies Conclusion References

 
Developing Countries
Several epidemiologic indicators can be used to describe the extent of TB, including the prevalence of infection, notification rates, the predicted incidence of disease, and mortality from TB.⁶ In 1995, Raviglione et al⁷ published data on TB notifications available to the World Health Organization (WHO) in December 1993. Table 1 shows the TB case notifications and average notification rates by WHO region. Despite the drop in notifications in the developed countries, the overall trend was toward an increase in the global TB notification rate.⁴

Using notification data with ARTI-based calculations, Dolin et al³ calculated that there were 7.5 million cases of TB in 1990 and that this number was predicted to increase to 10.2 million cases in 2000. This increase also represented an increase in the rate of TB cases. Figure 1 shows the relative distribution of TB worldwide. As can be seen, the vast majority of cases occur in developing countries.

View larger version (29K): [in this window] [in a new window] [Download PPT slide]   Figure 1. World map showing estimated cumulative TB cases, 1990 to 1999. Reprinted with permission from Dolin et al.3

As the epidemic of coinfection continues to evolve in Africa, a more ominous danger looms on the horizon. HIV is advancing rapidly throughout Asia and the Indian subcontinent where > 60% of the world’s population resides.^{11 12} Most of the world’s TB resides here also, with 72% of the global TB burden.¹³ In Chiang Rai, Thailand, where TB case rates dropped steadily throughout the 1980s, a rapid increase in HIV-related TB was identified in a study showing HIV coinfection in 1.5% of all TB patients in 1990 compared with 45.5% in 1994.¹⁴ If current trends continue, the new century will begin with an epidemic of coinfection that will dwarf that seen in Africa in the late 20th century.

Developed Countries
Throughout the 19th century, developed countries had TB rates and mortality that were similar to those found in developing countries today and in some instances were significantly greater.¹⁵ These rates began to decrease significantly by the beginning of the 20th century, and with the advent of effective chemotherapy, an even greater decline in rates was seen. The apathy brought on by the perceived decreased threat from TB was soon reversed in the mid-1980s when the now well-described resurgence in TB occurred in developed countries.^{15 16} Since that time, renewed efforts have held the resurgence of the white plague in check and, in many instances, resulted in a return to the declining rates seen for much of the 20th century.¹⁷ The TB rates for select developed countries are shown in Table 2 . Overall, rates in those countries are declining and, with the exception of high-risk groups, are expected to continue to decline. However, certain problem areas exist throughout developed countries that stand as formidable barriers to the elimination of TB.

Although HIV infection does not seem to play a significant role in the resurgence of TB in eastern Europe,²⁰ the role of HIV in fueling the epidemic in developed countries in the west has been well established.^{15 21 22 23 24} In the United States, between 26% and 38% of the total number of cases are estimated to be caused by HIV infection.²⁵ Despite the high rate of coinfection with HIV among TB patients in most developed countries, universal testing of all TB patients for HIV is not performed. A study performed in Los Angeles found that nearly 40% of all patients with TB did not have testing for HIV and those that did usually reported risk factors for HIV.²⁶ This same study found that HIV seroprevalence was 2 to 7% in those reporting low risk for HIV infection.²⁶ Universal testing of TB patients for HIV coinfection will be very useful in the coming decades to better define specific high-risk groups in low-incidence countries such as the United States.

In 1986, the Centers for Disease Control and Prevention (CDC) began collecting data on place of birth on patients residing in the United States who were reported to have TB. Since that time, both the number of cases and the percentage of cases occurring in foreign-born individuals in the United States has increased. In 1986, 4,925 cases occurred in the foreign-born compared with 7,702 cases in 1997, and the percentage of foreign-born cases of the total US cases that these numbers represent are 22% and 39%, respectively.²⁷ A similar trend has been documented elsewhere in industrialized countries.^{15 28 29}

Although some studies using restriction fragment length polymorphism (RFLP) analysis have documented transmission while in the United States,^{30 31} most cases (~60%) are the result of reactivation of latent infection.²⁷ A cross-sectional analysis of national surveillance data conducted by the CDC found that the risk of developing TB appears to be greatest in the first 2 years after arrival in the United States, however, the risk may persist for as long as 20 years after arrival in the United States.³² This study confirmed previous data that the risk of developing TB in long-term foreign-born residents correlated with a longer period of time in endemic areas.^{32 33 34}

There is little evidence that substantial transmission of TB occurs from the foreign-born to US-born residents.^{31 35} One exception are children of foreign-born individuals. A recent study from New York showed that a substantial number of US-born infants were actually from immigrant families and had suspected source cases who were foreign-born.³⁶ Despite this fact, the majority of TB cases among children are US-born, but the trend continues to be toward a greater percentage among the foreign-born.³⁶

The foreign-born are only one of multiple well-documented high-risk groups for TB that need specific targeting to not only identify active cases, but also to identify and complete prophylactic therapy.³⁷ Despite the global nature of TB, we must not lose sight of the fact that all TB is local and this requires local identification of high-risk groups that will vary from community to community. Certain ethnic groups,^{38 39} occupations,⁴⁰ and other cultural subgroups, such as veterans,²² homeless persons,⁴¹ and IV drug users,⁴² all provide opportunities for specifically directed control and prevention strategies that will be an integral part of the elimination of TB from low-incidence areas.

A new era in TB control has come with the rise of managed care in the United States. The eventual impact of managed care is unclear. It is likely to be great, however, since approximately 50% of TB patients receive care through the private sector.⁴³ This is in sharp contrast to years past when virtually all TB care was through local health departments.

Managed care may represent an unprecedented opportunity for control inasmuch as it has the potential to create strong incentives for health promotion and disease prevention.⁴⁴ Analysts point out, however, that financial mechanisms, such as capitated reimbursement, may discourage providers from working with public health agencies.⁴⁵ In response to these challenges and opportunities, model contract specifications for TB control have been developed for managed care organizations.⁴³

	Molecular Epidemiology

TOP Abstract Introduction Epidemiology TB Infection TB Disease Molecular Epidemiology Pathogenesis Diagnosis Clinical Aspects Vaccine Development New Therapies Conclusion References

 
Most of what is known about the epidemiology of TB is based on epidemiologic concepts that have changed very little in the century after Koch’s discovery of the tubercle bacillus. In the early 1990s, much began to change in our understanding of TB epidemiology with the advent of DNA-based fingerprinting of M tuberculosis. Although there are limitations regarding the applicability of information gathered by DNA fingerprinting, it is clear that significant changes will be implemented in control programs as a result of this relatively new technology.

DNA fingerprinting is performed most commonly by using RFLP analysis. RFLP analysis is predicated on the fact that there are repetitive insertion elements throughout the genome of M tuberculosis. IS6110 is the most commonly used insertion sequence that is specific for M tuberculosis and is usually found in 5 to 20 copies throughout the chromosome.⁴⁶ The DNA is isolated and then cleaved with restriction endonucleases, and subsequent electrophoresis and hybridization result in a series of bands of various lengths (Fig 2 ). The variability in band number and position is the basis for distinguishing between different strains.⁴⁷

View larger version (34K): [in this window] [in a new window] [Download PPT slide]   Figure 2. Insertion sequence blotting of two strains of M tuberculosis. 1: The genomic DNA of the organism is shown as a closed circle, shaded ovals indicate IS6110, an insertion sequence whose variability in copy number and genomic location is the basis for genotyping. 2: DNA is cleaved into many fragments with a restriction endonuclease; a subset of fragments contain IS6110. 3: DNA fragments are separated electrophoretically. 4: Hybridization with probe for IS 6110 and chemiluminescent detection reveals those fragments that contain IS6110, and this is the unique strain pattern. Reprinted with permission from Behr and Small.46

RFLP analysis has contributed significantly to our understanding of the dynamics of specific outbreaks of TB. By confirming routes of transmission that many times can only be suspected by conventional epidemiology, RFLP analysis has played the key role in establishing the susceptibility of certain individuals, particularly HIV-infected persons, to nosocomial outbreaks of TB.^{53 54 55 56} Conversely, RFLP analysis is integral to refuting "pseudo-outbreaks" that are caused by laboratory contamination⁵⁷ and contaminated medical equipment.⁵⁸ Diverse outbreaks throughout communities have also been studied using molecular epidemiology, and it has become clear during the last 5 years that epidemiologic investigations would be incomplete without supplementation with newer molecular techniques.^{59 60}

Perhaps even more exciting than the insight gained in the dynamics of outbreaks is the possibility of a better characterization of the phenotypic characteristics of specific strains of M tuberculosis. The increased susceptibility to disease of certain populations has always been a part of the TB epidemic, a fact that has been brought to the fore as RFLP analysis has shown the dramatic rates of disease progression in HIV-infected individuals.⁶¹ Molecular epidemiology is shedding light on the fact that not all strains of TB are created equally. The recent identification of "supertransmitters"⁶² may lead to the identification of genotypic variations that lead to increased virulence and thus be a target for future therapies or vaccines, if proven to exist. Furthermore, it is possible that the clinical and epidemiologic manifestations of TB are the reflection of phenotypic variation among different strains of organisms. Testing this hypothesis will likely yield valuable and essential tools toward the elimination of TB.

Most published studies using RFLP analysis looked at specific outbreaks and were designed to answer short-term questions regarding transmission. In the years to come, evaluation of the population dynamics of TB, particularly in high-incidence areas, will become increasingly important. The studies that are available have yielded very interesting results. A population-based study that looked at the genetic heterogeneity of isolates from Ethiopia, Tunisia, and the Netherlands showed that there was little genetic overlap between the isolates of the three countries, suggesting separate reservoirs of infection.⁶³ This same study postulated that BCG may be more protective against certain genetic variants than others and may lead to selection of BCG-resistant isolates. Another study from South Africa found that an epidemiologic link could not be established in 73% of RFLP analysis–clustered cases, suggesting that a significant proportion of cases in high-incidence areas are the result of casual contact.⁶⁴ Both of these studies and the increasing number of studies looking at genetic diversity of isolates among populations^{65 66 67 68 69} will provide the basis of a better understanding of the global movement of TB.

Despite the promise of molecular epidemiology, this technology certainly has limits that need to be overcome as we begin the new century. As stated previously, RFLP analysis is based on the assumption that persons with identical RFLP patterns are linked epidemiologically. It is also assumed that the particular RFLP pattern will change gradually over time, which is what accounts for the variability among unrelated strains. However, at what rate do the IS6110-based genotypes change? If the rate is rapid, recent transmission would be underestimated. A recent study found that 29% of patients whose cultures spanned 90 days had genotypes that changed.⁷⁰ Other limitations include the questionable applicability of IS6110-based techniques in those populations that have isolates with few or no copies of IS6110⁷¹ and in certain rural populations with low rates of recent transmission.⁷² These studies support the need to follow isolates over time and establish the rates of IS6110 stability among different isolates.

	Pathogenesis

TOP Abstract Introduction Epidemiology TB Infection TB Disease Molecular Epidemiology Pathogenesis Diagnosis Clinical Aspects Vaccine Development New Therapies Conclusion References

 
Robert Koch stated that to effectively design preventive measures against TB, we needed to understand the "living conditions" of the bacillus.⁷³ During the past 100 years since Koch’s proclamation, tremendous advancements have been made that have brought us closer to this understanding. Before Koch’s discovery of M tuberculosis in 1882, the cause of TB was widely debated and varied from hereditary in nature to poor social and living conditions. Even after the seemingly convincing work by Koch, it still took the next 20 years to make believers of the majority of the medical community.

Although M tuberculosis has been proven to be the causative agent of TB, many questions still remain concerning the interactions that exists between the organism and the host’s immune response, which account for the full spectrum of manifestations that are seen with the disease. For example, why do only a small fraction of individuals who are exposed to an infectious patient subsequently become infected, and why do only a small percent of those infected develop disease? The answers to these questions may actually hold important therapeutic clues to improve our weapons against this illness. However, these answers have been slow in coming because of a lack of a reliable animal model in which to study such effects.

Approximately only one third⁷⁴ of all individuals who are in close contact with a person with active TB for a prolonged period become infected. This is a relatively low infection rate as compared with other diseases such as measles or varicella, which may infect 70 to 90% of susceptible, exposed individuals.⁷⁵ The reason for this low infection rate still remains elusive. Some have suggested the low number of organisms in the air of the room of a person with active disease as one cause. Riley et al⁷⁶ showed that there was only one organism per 12,000 cu ft. However, other factors may also be at play, as suggested by recently identified strains of TB that were found to have a higher rate of infection and disease, potentially pointing to a still unknown virulence factor of the organisms.^{62 77}

When a person inhales a droplet nucleus, which may contain between 1 and 400 organisms,⁷⁸ it usually is trapped in the upper respiratory tract and cleared. The smallest droplets, those < 5 µm, make it to the alveolus and are phagocytosed by resident alveolar macrophages. At this primary site of infection, bacilli multiply initially and within 2 weeks are transported through the lymphatics and establish secondary sites. A development of an immune response, heralded by the development of delayed-type hypersensitivity (DTH), during the next 4 weeks leads to granuloma formation with a subsequent decrease in bacillary numbers. Although the initial and secondary sites are rarely completely sterilized by this immune response in most cases, the host appears immune, on the basis of epidemiologic evidence,⁷⁹ to subsequent re-exposures by TB and rapidly eliminates any new, inhaled tubercle bacilli.

The exact mechanism underlying this resistance needs further investigation. If a better understanding of this protection could be further elucidated, a method to confer this acquired resistance may be possible and used to protect previously uninfected hosts. The reasoning behind vaccinating uninfected hosts with BCG was to try to confer this protection. Unfortunately, it has met with varying limited success (0 to 80%).⁸⁰

Once infected with TB, 3 to 5% of immunocompetent individuals develop TB within 1 year, and an additional 3 to 5% develop TB during the remainder of their lifetime.⁷⁹ The majority of individuals who have TB in their body are able to mount an effective immune response that encapsulates these organisms, usually for the rest of the host’s life, thus preventing the progression from infection to disease.

This ability to mount an effective immune response that suppresses this natural progression may hold clues that can be potentially used to develop new therapies. It appears that some individuals are more susceptible to progressing from TB infection to TB disease. This is particularly apparent among individuals with certain medical conditions (HIV, diabetes mellitus, gastrectomy, silicosis, certain cancers, chemotherapy, etc.) that are associated with varying degrees of immunosuppression.⁸¹ Most of these groups have conditions that are believed to impair their cellular immunity. However, it is important to note that a significant number of individuals with apparently normal immune systems still progress from infection to TB disease.

Recently, attention has been focused on cellular immunity as the primary component of the host’s immune system responsible for the containment of TB. Koch recognized that individuals with TB disease who were inoculated with tubercle bacilli had a more toxic reaction as compared with those who were never exposed. This phenomenon was recognized to be an example of DTH. It took > 50 years to begin to understand the responsible mechanisms for this phenomenon, with more insights now being gained daily.

It is now apparent that bacilli are killed in humans and that the killing is a manifestation of acquired resistance.⁷³ However, we still do not know exactly how intracellular mycobacterium are killed. Much work has concentrated on the role of T cells, specifically CD4 cells. Two lines of circumstantial evidence suggest their central role in resistance to TB. First, T cell responsiveness correlates inversely with disease progression in terms of both low blastogenic responses to mycobacterial antigens⁸² in vitro and reduced or absent skin tuberculin hypersensitivity among patients with advanced or uncontrolled disease.⁸³ Second, HIV-infected individuals are exquisitely susceptible to TB.^{61 73} In fact, among this population, disease progression may occur before severe depletion of CD4 cells, suggesting even slight reductions in CD4 may predispose one to progression from infection to TB disease.

Elucidation on exactly how CD4 cells act to control disease continues to be an intense field of investigation. Questions as to whether they exert their influence through secretion of TH1 cytokines, such as interferon-, which induces the secretion of interleukin 2 and subsequent activation of lymphokine-activated killer cells, or through the direct activation of macrophages still need more investigation. These may be only a few of what are probably many processes that are involved in this complex interaction. It is hoped that an enhanced understanding of these mechanisms will lead to new therapeutic alternatives that may allow cure through immunomodulation. This strategy may especially be useful in MDR strains in which available therapeutic options are at times poor.

It has long been postulated that TB prefers areas with high oxygen tension. This has been one of the theories proposed for the apical predisposition of TB in the lungs.⁸⁴ When the host immune response begins to control TB, the microenvironment that TB is exposed to changes. It is thought that the oxygen tension drops in this environment, causing the organism to shift down into a nonreplicating stage.^{85 86} It is believed that this ability of the organism to survive this unfavorable environment may be responsible for the ability of the organism to lie latent in the host for long periods, with the capacity to revive and activate at a later time. The medications we traditionally use to kill TB have a limited capacity to affect the organism when it is in this latent phase. This tendency toward latency has been proposed as the reason for needing to treat disease for prolonged periods (at least 6 months) so that drug is still present in the host when these organisms reactivate. New experimental models have been developed that are successful at inducing a latent stage, enabling the study of drugs that may be effective at killing these latent organisms. If this could be accomplished, these drugs may potentially dramatically reduce the time necessary to treat TB.

	Diagnosis

TOP Abstract Introduction Epidemiology TB Infection TB Disease Molecular Epidemiology Pathogenesis Diagnosis Clinical Aspects Vaccine Development New Therapies Conclusion References

 
TB Disease
A test that rapidly and accurately diagnoses TB is essential for the appropriate identification, isolation, and treatment of patients with TB. Unfortunately, no perfect test for the diagnosis of TB exists. Smears, the time-honored test for the diagnosis of TB, are only positive in approximately 50% of cases of active TB.⁸⁷ Even the introduction of fluorescent staining techniques, which facilitate and speed the interpretation of the smear, have not significantly improved the sensitivity of the test. The advantage of the smear is that it is both rapid and cheap. The reason for its low sensitivity is the need for 10,000 to 100,000 organisms/mL of specimen. In addition, the smear is not specific inasmuch as other mycobacteria also may stain with a similar appearance.

Cultures are positive in approximately 80% of cases⁸⁸ but unfortunately take a prolonged time to grow, up to 8 weeks for solid media and 1 to 3 weeks for liquid media. At least 500 organisms/mL are necessary to have a positive culture. Although more sensitive than the smear, the culture is costly and more technically difficult to perform and requires longer times for results. This long period before results are available often leads to patients not being treated or treatment being started on a presumptive basis. In addition, the increased technical skills required preclude using this as a method of diagnosis in much of the developing world.

Susceptibility testing is much more technically demanding and requires an additional 1 to 2 weeks after the cultures are positive. In addition, many of the drugs used in this testing are unstable or need to be performed under specific conditions, further complicating the performance of the test. Thus, new tests that are rapid, sensitive, specific, and technically less sophisticated are needed for the clinician to improve the diagnosis of TB disease.

Underlying some of the reduced sensitivities of the various mycobacteriologic tests is their reliance on sputum as the diagnostic specimen. In situations in which patients are unable to cough or do not produce an adequate sputum specimen (ie, children and the elderly), this lack of a good specimen renders the tests less accurate. For many years, serologic tests have been investigated to overcome this barrier. Most of these tests rely on the detection of various mycobacterial antigens for the diagnosis. Unfortunately, to date, no single marker has proven more useful than the smear.

Recently, nucleic acid amplification (NAA) techniques have gained more attention. These techniques hold the promise of being able to detect even one strand of nucleic acid from TB, to amplify it, and within a matter of hours to identify the presence of TB. Studies using these techniques have shown sensitivities of 60 to 95% and specificities of up to 100%.⁸⁹ The test is > 99% specific when used on smear-positive cases, and for this reason, the Food and Drug Administration approved the test for use on smear-positive, untreated cases. Newer NAA techniques hold the promise of improving the sensitivity and specificity for smear-negative cases. NAA tests have also been used on specimens other than sputum in patients with extrapulmonary disease, including serum in patients with HIV, with good results.^{90 91}

However, a major problem has been the reliance of NAA tests on sputum as a specimen. The tests have proven to be very sensitive at detecting the nucleic acid of the organism if it is present in the specimen and as long as an inhibitor is not present in the specimen. However, if the specimen is not a representative sample of the lower respiratory tract, the test will not be positive. Most authorities recommend using multiple specimens from the patient to improve the sensitivity. However, the clinician must be aware that a negative test does not rule out the possibility of TB and rarely, a positive test might not guarantee a diagnosis of TB. Clinical judgment must be used when using these tests and, of course, realization that the interpretation of acid-fast bacilli smear results also requires clinical correlation by the treating physician. Another problem with these tests is that they are costly, and very technically sophisticated, thereby limiting their usefulness in developing countries. In addition, these tests, if positive, do not tell the susceptibility of the organism, still necessitating the use of cultures. However, overall, these new techniques hold the promise of being more sensitive and specific for the diagnosis of TB disease than the rapid diagnostic test commonly used now, the acid-fast bacilli smear. Issues concerning the increased cost (although in practice they may be more cost-effective) and availability will need to be addressed in the near future.

Progress has been made in understanding the genetic mutations that are associated with resistance to certain drugs. Researchers have identified mutations in the RNA polymerase B gene that are associated with > 90% of all rifampin-resistant strains.⁹² Mutations in the genome associated with isoniazid, streptomycin, ethambutol, ofloxacin, and pyrazinamide have also been identified,⁹³ but no one mutation is as strongly correlated as that found with rifampin. Techniques are now being developed to do genotypic resistance testing that may soon allow NAA tests to diagnose as well as predict resistance. Unfortunately, these tests are costly as well as technically demanding.

With the delineation of the entire genome of M tuberculosis and the rapid development of computer chip technology, it may soon be possible to identify TB and its susceptibility pattern, all in one step, in a rapid and cost-effective way.⁹⁴ It is hoped that this technology will be sensitive and specific as well as potentially available worldwide.

Clinicians have imperfect means of determining the infectiousness of TB patients as well as the rapid assessment of response to effective therapy. Tools to assess these factors would be useful and cost-effective, especially in patients who are hospitalized and need to return to a congregate setting. Currently, most physicians rely on the patient’s clinical response after 2 weeks of therapy and three consecutive negative smears as a guide to when a person is no longer infectious. It is probable that many patients are no longer infectious before these criteria are met and, conversely, may still be contagious even after meeting these criteria. Tests using messenger RNA may be helpful at determining the viability of the remaining organisms,⁹⁵ but further research is needed.

TB Infection
It was Arthur Conan Doyle⁹⁶ who first recognized that old tuberculin might be a good tool for the diagnosis of TB infection. It was 30 years later that Mantoux⁹⁷ perfected the use of tuberculin for skin testing. Unfortunately, it was soon realized that up to 10% of patients with a positive reaction to purified protein derivative (PPD) may be falsely diagnosed with infection,⁹⁸ whereas up to 20 to 30% of ill patients with TB may have a negative skin test.⁹⁹ The false-negative rate is higher in patients who are infected with HIV or have other immunosuppressive conditions. Unfortunately, in the years since Koch’s discovery of the tubercle bacillus, we still do not have a more accurate test for diagnosing TB infection.

Recently, tests using interferon- production from sera of infected patients have shown some promise, including in HIV-infected individuals.¹⁰⁰ The problem with these tests is that they require some technically sophisticated procedures. In addition, testing the accuracy of these tests may be difficult because they will be compared with the PPD test, which is itself imperfect.

The problem of the loss of DTH responses (anergy) and a lack of response to PPD in HIV-infected individuals has left clinicians without a way to determine which patients are infected with TB. Given the sometimes ambiguous interpretation associated with anergy testing, especially in those infected with HIV, the CDC recommended that anergy testing not be a routine part of the evaluation of patients suspected of being infected with TB.¹⁰¹ Recently, with the introduction of antiretroviral (ARV) therapy, DTH has been observed to return in up to 80% of previously anergic patients who maintained a satisfactorily suppressed viral load after 12 weeks of ARV therapy (P Alonso, MD; personal communication; June 1999). If this is confirmed, a new protocol for using PPD and anergy testing may be appropriate. It may be applicable to once again perform PPD and anergy tests on all HIV-infected individuals. For those on effective ARV who are found to have intact DTH and negative PPD, no further therapy may be needed. In those who are found to be PPD-positive, preventive therapy would be administered. In those patients who are found to be anergic and not on ARV therapy, effective ARV therapy would be begun and, 3 months later, PPD and anergy tests repeated; if DTH is restored, appropriate intervention depending on the results of the PPD would be initiated. Studies invoking this protocol are now in progress (C Boulanger, MD; personal communication; June 1999).

Most authorities agree that preventing TB disease with the use of preventive therapy in individuals with latent TB infection is a key element in programmatic efforts to control, prevent, and eliminate the disease. This is usually accomplished by Mantoux tuberculin skin testing of individuals at high risk of being infected with TB and administering and completing a course of preventive therapy. Although many trials have demonstrated the efficacy of 6 to 12 months of isoniazid preventive therapy in HIV-seronegative as well as HIV-seropositive individuals, the major obstacle has been compliance with these long regimens. Studies have shown completion rates as low as 30% for such regimens. Identifying latently infected individuals without being able to complete a course of preventive therapy is, in essence, a missed opportunity to prevent disease and further transmission. Two recent trials^{102 103} now show that a 2-month regimen of preventive therapy with rifampin and pyrazinamide is effective at preventing disease. The advantages of these shorter regimens are obvious. Shorter courses of preventive therapy should be easier to comply with and allow TB programs to have the resources to improve completion rates with the use of directly observed preventive therapy (DOT) and hopefully result in lower TB rates. These short-course preventive therapy regimens may be especially effective in groups such as homeless individuals, substance abusers, migratory populations, jail inmates, and HIV-seropositive individuals in whom the completion of preventive therapy has been historically difficult and/or medically imperative.

Drug interactions are an essential consideration when using these preventive regimens with rifampin. Not only does rifampin interact with methadone and other opiates, but it also interacts with protease inhibitors (PIs) and nonnucleoside reverse transcriptase inhibitors (NNRTIs). It is not known whether rifabutin, which can be given more easily to patients taking ARV drugs metabolized by the cytochrome p450 system, is as efficacious in short-course preventive regimens as rifampin, although there is no reason to believe it is not.

	Clinical Aspects

TOP Abstract Introduction Epidemiology TB Infection TB Disease Molecular Epidemiology Pathogenesis Diagnosis Clinical Aspects Vaccine Development New Therapies Conclusion References

 
The resurgence of TB that was witnessed in the United States in the 1980s has been attributed to numerous causes. These include the HIV epidemic, increased immigration of individuals from countries with a high incidence of TB, increased number of individuals living in congregate settings, and a deterioration of the health-care infrastructure.¹⁰⁴

Although TB has been increasing among the foreign-born (see previous sections), these groups at times pose clinical dilemmas and have exposed holes in our immigration policies. Immigrants applying for visas are supposed to be screened before entering the United States. Those found to have a positive PPD or abnormal findings on chest radiographs are supposed to be referred to and followed by the health department where they settle. Unfortunately, frequently this does not occur.^{105 106} This lack of appropriate screening represents potential missed opportunities to prevent disease and further spread of the disease to the community. Significantly, the vast majorities of persons coming from other countries do so as visitors and do not require any sort of screening.

Many individuals coming from other countries have different cultural backgrounds and beliefs. These social differences may pose barriers for the completion of therapy. Programs designed to address and respect these cultural beliefs while assuring the administration of appropriate therapy have been successful. Many health departments in the United States that deal frequently with foreign-born populations employ field workers of the same cultural background or provide training to the workers to enhance sensitivity to the cultural needs of the patient.

In some areas of the United States that border Mexico, TB control has been hampered by migrant patients traveling back and forth between the two countries. This can disrupt continuity of care, which may lead to further spread of the disease as well as acquired drug resistance of the organism. Novel programs like the United States-Mexico Bi-National TB Tracking and Referral Project (TBNet) of the Migrant Clinicians Network (1–800-825–8205), in which each patient is given a small medical passport, which chronicles the patient’s TB history, radiographic findings, and medications prescribed and doses administered, have effectively assisted in this effort. This system has recently been transferred to a computer network that can be accessed by health-care providers in both countries to facilitate the exchange of treatment information. The encouragement and support of government and community groups are needed to develop more novel programs and approaches that aim to improve preventive strategies and enhance completion rates among the foreign-born.

One of the most dramatic contributors to the resurgence of TB has been the emergence of HIV. This partnership between the two organisms has augmented each of their deadly potentials. HIV, by destroying the CD4 cells of the host’s immune system, allows the TB that may lie dormant in the patient to activate and rapidly cause disease. In response to the reactivation of TB, CD4 cells become stimulated and begin to replicate. This activation of CD4 cells further renders these cells vulnerable to invasion by HIV and allows for HIV to further replicate within these cells. This leads to a vicious cycle of increasing viral load, which causes a further deterioration of the host’s immune system. Studies have shown that the 1-year mortality rate for treated, HIV-related TB ranges from 20 to 35% and shows little variation between cohorts from industrialized and developing countries.^{107 108}

HIV has changed the classic clinical presentation of TB. The chest radiograph was the tool that clinicians traditionally relied on to aid in the diagnosis of TB. It classically showed upper lobe infiltrates and cavities in 98% of non-AIDS cases. However, it has been shown that clear lung fields may be present in 35% of patients with active TB and AIDS.¹⁰⁹ Approximately 5% of HIV-infected patients with pulmonary TB have positive results on acid-fast staining of sputum, despite normal chest radiographs.¹¹⁰ Atypical radiographic presentations, such as mediastinal or hilar adenopathy alone or tuberculous pleurisy,^{111 112 113} are found more commonly in patients infected with HIV and depleted cellular immunity than those non-HIV-infected patients with TB caused by reactiviation. This lack of classic presentation has been attributed to the induced immunosuppression of the host impairing their ability to have a DTH response and preventing the formation of cavitation in the lungs of severely immunosuppressed AIDS patients.

HIV has also altered the treatment of TB. PIs and NNRTIs, two of the most potent agents available to control HIV, interfere with rifampin, the most important drug available to treat TB. Rifampin is a potent inducer of the hepatic p450 system. This induction enhances the metabolism of the PIs and NNRTIs, causing their serum concentrations to be reduced. Low serum concentrations of the PIs and NNRTIs leads to ineffective viral suppression as well as the development of resistance.^{114 115} Conversely, the PIs and NNRTIs interfere with the metabolism of the rifamycins, causing high serum concentrations of the rifamycins and potential toxicity.¹¹⁶

This interaction posed a quandary for providers. If providers opted to treat the TB with the most potent agent available, rifampin, they would not be able to use the most potent agents for HIV, the PIs or NNRTIs. Conversely, if they treated the HIV using the most effective agents available, then they would have to treat the TB with less potent agents for a longer period of time. This dilemma also potentially stressed the limited resources of the public health systems, which in many cases did not have enough workers to get patients through 6 months of TB therapy by DOT.

More recent experience with rifabutin, a rifamycin derivative, with less effect on the hepatic p450¹¹⁷ system but equivalent efficacy against TB,¹¹⁸ may prove to be an important solution to this problem. Limited data¹¹⁹ have suggested that rifabutin can be used safely and effectively with the PIs, but more experience is necessary to determine the correct dosages and best regimens to use. Clinicians with limited experience in treating patients who are coinfected with TB and HIV should refer to the CDC guidelines on the subject¹²⁰ and should seek expert advice when appropriate.

ARV therapy has proven to be a major clinical advance in the fight against HIV, with preliminary studies showing increased survival with these agents.¹²¹ However, many questions related to TB and ARV therapy still need to be answered. First, the ARVs are still very expensive and are not available for the majority of the world where TB and HIV are causing most of their devastation. Availability of these agents, with the ability to properly use and monitor their use, needs to be achieved worldwide if we are to begin to halt the continued morbidity and mortality from these illnesses.

Second, with the use of ARV therapy and the subsequent improvement in the host’s immune response, patients with TB disease may actually exhibit a worsening of their clinical condition, a paradoxical response or inflammatory response with immune reactivation.¹²² Patients may experience the development of new ascites, lymphadenopathy, fever, pleural effusions, or cerebral lesions. These may be life-threatening depending on the site and size of the lesion. Clinicians treating TB patients who have HIV must be aware of this phenomenon and rule out other causes for this clinical worsening. In selected cases, the use of immunomodulators (ie, steroids) may be indicated to slow the progression of this response. Further studies to elucidate the percentage of patients who develop this reaction and risk factors for the development of this response may help determine which patients may benefit from certain interventions to prevent or lessen this phenomenon.

Third, although morbidity and mortality seem to be improving among individuals infected with HIV who are being treated with ARV therapy, most patients are not experiencing a complete restoration of their immune system. Traditionally, before the advent of ARV therapy, it was found that up to 8% per year of individuals coinfected with HIV and TB developed active TB disease.¹²³ This represents an increased relative risk of 80- to 170-fold when compared with individuals not infected with HIV.

The incidence of progression to TB disease is likely to decrease with improvements in the immune system of HIV-infected individuals. The fact that most ARV-treated individuals are still left with a partially impaired immunity will probably still predispose these coinfected individuals to develop TB disease at a higher rate as compared with immunocompetent individuals. Thus the risk of developing TB disease among these HIV-coinfected individuals will still be high considering their longer expected life spans. This may make the identification of HIV-coinfected individuals with TB even more significant.

The identification of TB-infected patients with HIV is hampered by the immunosuppression that leads to the loss of DTH, including a lack of response to PPD. Studies have shown a significant rate of anergy when the patients’ CD4 counts are < 400 cells/µL.¹²⁴ Studies that examined providing prophylaxis to all HIV anergic patients have not proven to be of benefit.¹²⁵

Because both HIV and TB rely on medications for their control, both face the same two obstacles to their control: adherence and resistance. Both diseases share the need to administer multiple drugs for a long time; which decreases the chances for complete adherence. With poor adherence, not only is continued progression and spread of the disease probable, but also the development of resistance to the drugs being used. With the development of resistance, the prospect of losing the most important drugs we have in our clinical armamentarium becomes a real possibility. The United States faced this dilemma in the early 1990s. In certain areas of the country, rates of resistance were rapidly rising, with some cities reporting as much as 19% of their strains to be MDR.¹²⁶ With the widespread implementation of DOT, rates of resistance are again falling. However, this is not the case worldwide where in countries such as the Dominican Republic, 10% of cases are now reported as MDR.¹⁸ In retrospect, it probably can be argued that it was the deterioration of the health-care infrastructure, with inadequate supervision of TB medication administration, that probably most contributed to the resurgence of TB and the increased rates of resistance. For the last 6 years, TB has been declining in the United States despite the continued presence of HIV, immigration of individuals from countries with a high incidence of TB, and persons in congregate settings. With increased funding from federal, state, and local sources being used to improve TB control efforts (especially in the public health sector), including the wider availability and use of DOT, TB in the United States has now reached its lowest levels ever recorded.¹²⁷ DOT entails representatives of health-care facilities (usually from the public health system) going into the community to observe and assure that patients take their medications. With widespread utilization of DOT, rates of MDR-TB have also dramatically fallen in the United States. With the assurance of completion of therapy and cure of patients with active disease, TB rates have dramatically declined. Triumph must be tempered with caution, as we have been at this juncture before in the 1970s when TB was thought to be on the verge of elimination and support for TB control efforts dramatically diminished. With our guard down, TB had its resurgence. The major battle we will face now is trying to remember our past and keeping up the support for TB control until it is eliminated.

The lessons learned from TB may also prove to benefit the therapeutic future of HIV. Resistance to ARV drugs is rising, and documentation of poor adherence has been noted.¹²⁸ Lessons learned from treating TB may be essential to maintain the therapeutic resources gained in the fight against HIV.

Adherence to TB therapy must be closely monitored and maintained. Wider utilization of DOT must be advocated worldwide to assure the cure of patients with active disease and decreased drug resistance rates. Even with the utilization of DOT, some TB programs have only shown an ability to cure 70% of patients with active TB.¹²⁹ Some have even argued that DOT really does not make a significant difference.¹³⁰

Thus, further efforts to improve adherence are needed. Methods such as the development of longer-acting medications, requiring less dosing, need to be developed. Drugs such as rifapentine, a long-acting rifamycin derivative allowing once a week dosing, may prove to be useful. However, caution with this agent in HIV-infected individuals is required as preliminary studies showed a significant number of HIV-infected patients with active TB disease treated with rifapentine subsequently relapsed with rifampin-resistant disease.¹³¹ Techniques to shorten therapy of active disease need to be explored. As previously stated, further research examining possible medications that may be effective against latent organisms may have the potential to shorten therapy and improve adherence.

Combination pills that include all of the required medications in a bioequivalent form are needed to decrease the chances of acquired drug resistance through the improper administration of medications. Unfortunately, given the disadvantaged populations that TB is currently affecting, there is a lack of financial incentives to warrant the developmental costs incurred by the pharmaceutical companies to develop new formulations or medications for this disease. In addition, many affected countries do not have the financial resources or political infrastructure to maintain an effective TB program; therefore, poor TB control efforts in any part of the globe impacts the rest of world. Financial support from governmental or international agencies to assist in the development of new medications and the maintenance of TB programatic infrastructures may be necessary. The World Bank has ranked an effective TB program using DOT as one of the "most cost effective of all health interventions."¹³²

In the United States, more efficient use of hospitals for TB care must be accomplished. Currently, it has been shown that > 60% of expenditures for TB treatment is spent on hospital costs.¹³³ In our experience in Florida, some of this expense occurs because of a misunderstanding by physicians about when a patient with active disease can be discharged. Most patients with active TB do not have to be admitted to a hospital unless another medical condition requiring hospitalization is present. Studies have shown that patients can be sent home to their prior place of residence on medications (by DOT), as long as the residence is not a congregate setting with young children or immunosuppressed individuals. This is based on the fact that patients are most infectious before the diagnosis is made and are rendered significantly less contagious once they are started on effective medications.¹³⁴ In some cases, in fact, having patients with TB in the hospital may pose a significant risk given the large number of immunosuppressed individuals now commonly treated in hospitals. Efforts need to be undertaken to treat patients in the community or discharge them back home early on DOT when appropriate. In addition, health departments must create alternative housing arrangements when the patient’s original living situation is not suitable to return while infectious (ie, homeless). This has worked well in Florida and other areas.¹³⁵

Health-care providers must also "think TB" early before admitting patients to hospitals and isolate when appropriate to avoid the numerous nosocomial outbreaks that were witnessed earlier in the 1990s. Clinicians must consider TB in patients with chronic respiratory complaints, radiographs suspicious for TB, or immunosuppressed individuals and initiate isolation until the diagnosis has been confirmed or ruled out.

	Vaccine Development

TOP Abstract Introduction Epidemiology TB Infection TB Disease Molecular Epidemiology Pathogenesis Diagnosis Clinical Aspects Vaccine Development New Therapies Conclusion References

 
The currently available tools for TB control are unlikely to be sufficient to achieve the goal of eradicating TB worldwide in the 21st century. No disease has ever been eliminated with medications alone. The use of BCG and the treatment of active disease with unsupervised regimens have had little impact on the overall epidemiology of the disease and in some instances may actually worsen the problem. DOT with short-course regimens, the most effective tool in the armamentarium against TB, has proven difficult to implement universally. These facts have led most to believe that eradicating TB will be impossible without a new vaccine.

The notion that a vaccine against TB would be an important component of control strategies is not new. BCG, an attenuated strain of Mycobacterium bovis, was first used as a vaccine in humans in the early 1920s.¹³⁶ BCG has since become the most widely used vaccine preparation in the world, despite the fact that there are questions regarding its efficacy in preventing pulmonary TB in adults, which ranges from 0 to 80%.^{137 138 139 140 141 142 143} Although the data are less than ideal, numerous studies have consistently shown the effectiveness of BCG in reducing the incidence of TB in infants and young children, particularly potentially fatal disseminated TB.^{143 144 145 146 147} For this reason, BCG remains an important part of vaccine programs because of the effectiveness of BCG in reducing death in children from disseminated TB.

Why is there this great discrepancy between efficacy studies of BCG? Several reasons are possible, but the most important may be the fact that BCG is best thought of as a family of vaccines, each with varying degrees of protective immunity produced in vaccine recipients. Recent breakthroughs in molecular biology may lead to the identification of certain antigens produced by the more effective BCG strains and subsequent development of a more effective BCG. Furthermore, a recent meta-analysis that was designed to identify the cause of the discrepancy between BCG efficacy studies found that the major contributor to the variation was geographic location, particularly geographic latitude.¹⁴⁸ Further study into the question of why geographic latitude is important can also possibly lead to a more effective BCG vaccine. Even if a more effective BCG vaccine can be created, several problems would remain, particularly the fact that since BCG is a live attenuated vaccine, administration to immunosuppressed individuals carries the risk of dissemination. For this and other reasons, other alternatives are being sought.

Recent advances have increased the possibility of a new TB vaccine. Perhaps the most significant of these advances is the complete sequencing of the genome of M tuberculosis.¹⁴⁹ The DNA sequences of the 4,000 genes will help to identify antigens that confer protective immunity and lead to various types of vaccine candidates. Despite these advances, little is known about the immunologic mechanisms that confer resistance to TB; therefore, there is a need to pursue many different vaccine candidates.

Of the different types of vaccines, DNA vaccines hold the most promise. A gene encoding a protective antigen is inserted into an expression plasmid, the plasmid DNA is amplified in transformed bacteria, and the plasmid DNA encoding the antigen is injected into the host. The plasmid directly transfects a living cell, leading to the host becoming immunized against a heterologous protein produced by his own cells.¹⁵⁰ Various studies using a murine model have shown that DNA plasmid vaccination can lead to specific T-cell responses that contain TB infection.^{151 152 153 154} The duration of this effect, however, is unknown. Also unknown are some questions regarding the safety of DNA vaccines. Several of these theoretical concerns regard integration of plasmid DNA into host genome, tolerance induction, and autoimmunity.¹⁵⁰

Despite the optimism fostered by recent advances, a TB vaccine that will have an impact on the epidemiology of this disease is still 20 years away. For this quest for the holy grail of TB control to be successful, several key steps must be taken. The Advisory Council for the Elimination of TB has outlined these steps and focuses attention on the need to develop consensus among public funding agencies, private funding sources, and vaccine manufacturers that a new TB vaccine is an urgent public health priority.¹⁵⁵ In addition to funding needs, basic science research is critical to determine basic requirements such as correlates for immunity and diagnostic tests to determine infection. Finally, ethical issues need to be addressed before vaccine trials begin, especially because these trials will require testing in developing countries with limited resources. These difficulties notwithstanding, it is likely that the momentum toward the development of a new or improved vaccine for TB will continue to gain steam and eventually yield the key thread in the tapestry of interventions to eliminate TB.

	New Therapies

TOP Abstract Introduction Epidemiology TB Infection TB Disease Molecular Epidemiology Pathogenesis Diagnosis Clinical Aspects Vaccine Development New Therapies Conclusion