Tuberculosis (TB) is an ancient human disease caused by Mycobacterium tuberculosis which mainly affects the lungs, making pulmonary disease the most common presentation (K Zaman, 2010). However, TB is a multi-systemic disease with a protean presentation. The organ system most commonly affected include the respiratory system, the gastrointestinal (GI) system, the lymphoreticular system, the skin, the central nervous system, the musculoskeletal system, the reproductive system, and the liver.
Evidence of TB has been reported in human remains dated thousands of years (Hershkovitz et al., 2017, K Zaman 2010). For a human pathogen with no known environmental reservoir, Mycobacterium tuberculosis has honed the art of survival and has persisted in human communities from antiquity through modern time.
In the past few decades, there has been a concerted global effort to eradicate TB. These efforts had yielded some positive dividends especially since 2000 when the World Health Organization (WHO, 2017) estimated that that global incidence rate for tuberculosis has fallen by 1.5% every year. Furthermore, mortality arising from tuberculosis has significantly and steadily declined. The World Health Organization (WHO, 2016) reports a 22% drop in global TB mortality from 2000 through 2015.
Despite the gains in tuberculosis control and the decline in both new cases and mortality, TB still accounts for a huge burden of morbidity and mortality worldwide. The bulk of the global burden of new infection and tuberculosis death is borne by developing countries with 6 countries, India, Indonesia, China, Nigeria, Pakistan, and South Africa, accounting for 60% of TB death in 2015, (WHO, 2017).
Tuberculosis remains a significant cause of both illness and death in developed countries especially among individuals with a suppressed immune system. People with HIV are particularly vulnerable to death due to tuberculosis. Tuberculosis accounted for 35% of global mortality in individuals with HIV/AIDS in 2015. (W.H.O, 2017). Children are also vulnerable, and tuberculosis was responsible for one million illnesses in children in 2015 according to the WHO.
M. tuberculosis causes tuberculosis. M. tuberculosis is an alcohol and acid-fast bacillus. It is part of a group of organisms classified as the M. tuberculosis complex. Other members of this group are, Mycobacterium africanum, Mycobacterium bovis, and Mycobacterium microti. Most other mycobacteria organisms are classified as non-tuberculous or atypical mycobacterial organisms.
M. tuberculosis is a non-spore forming, non-motile, obligate-aerobic, facultative, catalase negative, intracellular bacteria. The organism is neither gram-positive nor gram-negative because of very poor reaction with the Gram stain. Weakly positive cells can sometimes be demonstrated on Gram stain, a phenomenon known as "ghost cells."
The organism has several unique features compared to other bacteria such as the presence of several lipids in the cell wall including mycolic acid, cord factor, and Wax-D. The high lipid content of the cell wall is thought to contribute the following properties of M. tuberculosis infection:
The Ziehl-Neelsen stain is one of the most commonly used stains to diagnose T.B. The sample is initially stained with carbol fuchsin (pink color stain), decolorized with acid -alcohol and then counter-stained with another stain(usually, blue colored methylene blue). A positive sample would retain the pink color of the original carbol fuchsin, hence the designation, alcohol and acid-fast bacillus (AAFB).
Tuberculosis is present globally. However; developing countries account for a disproportionate share of tuberculosis disease burden. In addition to the six countries listed above, several countries in Asia, Africa, Eastern Europe, and Latin and Central America continue to have an unacceptably high burden of tuberculosis.
In more advanced countries, high burden tuberculosis is seen among recent arrivals from tuberculosis-endemic zones, health care workers, and HIV-positive individuals. Use of immunosuppressive agents such as long-term corticosteroid therapy has also been associated with an increased risk.
More recently, the use of a monoclonal antibody targeting the inflammatory cytokine, tumor necrotic factor alpha (TNF-alpha) has been associated with an increased risk. Antagonists of this cytokine include several monoclonal antibodies (biologics) used for the treatment of inflammatory disorders. Drugs in this category include infliximab, adalimumab, etanercept, and golimumab. Patients using any of these medications should be monitored for tuberculosis before and during the period of drug treatment.
Other Major Risk Factors
Multi-Drug Resistant Tuberculosis (MDR-TB) and Extremely Multi-Drug Resistant Tuberculosis (XDR-TB)
Although, usually a lung infection, tuberculosis is a multi-system disease with protean manifestation. The principal mode of spread is through inhalation of infected aerosolized droplets.
The body's ability to effectively limit or eliminate the infective inoculum is determined by the immune status of the individual, genetic factors and whether it is a primary or secondary exposure to the organism. Additionally, M. tuberculosis possesses several virulence factors that make it difficult for alveolar macrophages to eliminate the organism from an infected individual. The virulence factors include the high mycolic acid content of the bacteria outer capsule, which makes phagocytosis to be more difficult for alveolar macrophages. Furthermore, some of the other constituents of the cell wall such the cord factor may directly damage alveolar macrophages. Several studies have shown that mycobacteria tuberculosis prevents the formation of an effective phagolysosome, hence, preventing or limiting the elimination of the organisms.
The first contact of the Mycobacterium organism with a host leads to manifestations known as primary tuberculosis. This primary TB is usually localized to the middle portion of the lungs, and this is known as the Ghon focus of primary TB. In most infected individuals, the Ghon focus enters a state of latency. This state is known as latent tuberculosis.
Latent tuberculosis is capable of being reactivated after immunosuppression in the host. A small proportion of people would develop an active disease following first exposure. Such cases are referred to as primary progressive tuberculosis. Primary progressive tuberculosis is seen in children, malnourished people, people with immunosuppression, and individuals on long-term steroid use.
Most people who develop tuberculosis, do so after a long period of latency (usually several years after initial primary infection). This is known as secondary tuberculosis. Secondary tuberculosis usually occurs because of reactivation of latent tuberculosis infection. The lesions of secondary tuberculosis are in the lung apices. A smaller proportion of people who develop secondary tuberculosis does so after getting infected a second time (re-infection).
The lesions of secondary tuberculosis are similar for both reactivation and reinfection in terms of location (at the lung apices), and the presence of cavitation enables a distinction from primary progressive tuberculosis which tends to be in the middle lung zones and lacks marked tissue damage or cavitation.
Type-IV Hypersensitivity and Caseating Granuloma
Tuberculosis is a classic example of a cell-mediated delayed type IV hypersensitivity reaction.
Delayed Hypersensitivity Reaction: By stimulating the immune cells (the helper T-Lymphocyte, CD4+ cells), Mycobacterium tuberculosis induces the recruitment and activation of tissue macrophages. This process is enhanced and sustained by the production of cytokines, especially interferon gamma.
Two main changes involving macrophages occurs during this process namely, the formation of multinucleated giant cells and the formation of epithelioid cells. Giant cells are aggregates of macrophages which are fused together and functions to optimize phagocytosis. The aggregation of giant cells surrounding the Mycobacterium particle and the surrounding lymphocytes and other cells is known as a granuloma.
Epithelioid cells are macrophages which have undergone a change in shape and have developed the ability for cytokine synthesis. Epithelioid cells are modified macrophages and have a flattened (spindle-like shape) as opposed to the globular shape characteristic of normal macrophages. Epithelioid cells often coalesce together to form giant cells in a tuberculoid granuloma.
In addition to interferon-gamma (IFN-gamma), the following cytokines play important roles in the formation of a tuberculosis granuloma, Interleukin-4 (IL-4), Interleukin-6 (IL-6), and tumor necrotic factor-alpha (TNF-alpha).
The appearance of the granuloma in tuberculosis has been described as caseous or cheese-like on gross examination. This is principally explained by the rich mycolic acid content of the mycobacterium cell well. Because of this unique quality, the term caseous or caseating necrosis has been used to described granulomatous necrosis caused by mycobacteria tuberculosis.
Histologically, caseous necrosis would present as a central area of uniform eosinophilia on routine hematoxylin and eosin stain.
The granuloma is the diagnostic histopathological hallmark of tuberculosis.
The defining features of the granuloma of tuberculosis are:
A chronic cough, hemoptysis, weight loss, low-grade fever, and night sweats are some of the most common physical findings in pulmonary tuberculosis.
Secondary tuberculosis differs in clinical presentation from the primary progressive disease. In secondary disease, the tissue reaction and hypersensitivity is more severe, and patients usually form cavities in the upper portion of the lungs.
Pulmonary or systemic dissemination of the tubercles may be seen in active disease, and this may manifest as miliary tuberculosis characterized by millet shaped lesions on chest x-ray. Disseminated tuberculosis may also be seen in the spine, the central nervous system, or the bowel.
Tuberculin skin testing: Mantoux test (skin testing with PPD)
The Mantoux reaction following injection of a dose of PPD (purified protein derivative) is the traditional screening test for exposure to Tuberculosis. The result is interpreted taking into consideration the patient's overall risk of exposure. Patients are classified into 3 groups based on the risk of exposure with three corresponding cut-off points. The 3 major groups used are discussed below.
Examples of Patients in the Different Risk Categories
Note that a Mantoux test indicates exposure or latent tuberculosis. However, this test lacks specificity, and patients would require subsequent visits for interpreting the results as well as chest x-ray for confirmation. Although relatively sensitive, the Mantoux reaction is not very specific and may give false positive reactions in individuals who have been exposed to the BCG-vaccine.
Interferon release assays (IGRA, Quantiferon Assays)
This is a tuberculosis screening test that is more specific and equally as sensitive as the Mantoux test. This test assays for the level of the inflammatory cytokine, especially interferon gamma.
The advantages of Quantiferon, especially in those with prior vaccination with BCG vaccine, includes, the test requires a single blood draw, obviating the need for repeat visits to interpret results. Furthermore, additional investigations such as HIV screening could be performed (after patient consent) on the same blood draw.
Quantiferon's disadvantages include cost and the technical expertise required to perform the test.
Screening in Immunocompromised Patients
Immunocompromised patients may show lower levels of reaction to PPD or false negative Mantoux because of cutaneous anergy.
A high level of suspicion should be entertained when reviewing negative screening tests for tuberculosis in HIV-positive individuals.
The Significance of Screening
A positive screening test indicates exposure to tuberculosis and a high chance of developing active tuberculosis in the future. Tuberculosis incidence in patients with positive Mantoux test averages between 2% to 10% without treatment.
Patients with a positive test should have a chest x-ray as a minimum diagnostic test. In some cases, these patients should have additional tests. Patients meeting the criteria for latent tuberculosis should receive prophylaxis with isoniazid.
Screening Questionnaires for Resource-Poor Settings
Several screening questionnaires have been validated to enable healthcare workers working in remote and resource-poor environments screen for tuberculosis.
These questionnaires make use of an algorithm that combines several clinical signs and symptoms of tuberculosis. Some of the commonly used symptoms are:
Several studies have confirmed the utility of using several criteria rather than a focus on only chronic cough or weight loss.
Confirmatory and Diagnostic Tests
The new molecular-based techniques are faster and enable rapid diagnosis with high precision. Confirmation of TB could be made in hours rather than the days or weeks it takes to wait for a standard culture. This is very important, especially among immunocompromised host where there is a high rate of false negative results. Some molecular-based tests such as GeneXpert and DR-MTB also allow for identification of multi-drug resistant tuberculosis.
Drug of choice is isoniazid. It is usually given with vitamin B6, pyridoxine (to prevent nerve damage). Isoniazid is recommended for Mantoux or quantiferon positive individuals and should be continued for 6 or 9 months.
The WHO recommends the following treatment regimen for treating latent tuberculosis:
Treatment of Active Infection
Treatment of confirmed TB requires a combination of drugs. Combination therapy is always indicated, and monotherapy should never be used for tuberculosis. The most common regimen for TB includes the following anti-TB medications:
First-Line Medications, Group 1
Isoniazid and Rifampicin follow a 4-drug regimen (usually including Isoniazid, Rifampicin, Ethambutol, and Pyrazinamide) for 2 months or six months. Vitamin B6 is always given with Isoniazid to prevent neural damage (neuropathies).
Several other antimicrobials are effective against tuberculosis including the following categories:
Second-Line Anti-tuberculosis Drugs, Group 2
Injectables aminoglycosides and injectable polypeptides
Second-Line Anti-Tuberculosis Drugs, Group 3, Oral and Injectable Fluoroquinolones
Second-Line Anti-tuberculosis Drugs, Group 4
Third-Line Anti-Tuberculosis Drugs, Group 5
These are medications with variable but unproven efficacy against TB. They are used for total drug-resistant TB as drugs of last resort.
Multi-drug resistant TB is becoming increasingly common.
The combination of first-line and second-line medications are used at high doses to treat this condition.
On December 28, 2012, the United States Food and Drug Administration Agency (FDA), approved Bedaquiline as a drug for treating MDR-TB. This is the first FDA approval for an anti-TB medication in 40 years. While showing remarkable promise in drug-resistant tuberculosis, cost remains a big obstacle to delivering this drug to the people most affected by MDR-TB.
Clinical and Laboratory Monitoring
Liver function test is required for all patients taking isoniazid. Other monitoring in TB includes monitoring for retinopathies for patients on ethambutol.
Tuberculosis is a great mimic and should be considered in the differential diagnosis of several systemic disorders. The following is a non-exhaustive list of conditions to be strongly considered when evaluating the possibility of pulmonary tuberculosis.
Side Effect associated with most commonly used anti-TB drugs 
1) Isoniazid- Asymptomatic elevation of Aminotransferases (10-20%), Clinical Hepatitis (0.6%), Peripheral neurotoxicity, Hypersensitivity.
2) Rifampin- Pruritis, Nausea & Vomiting, Flulike symptoms, Hepatotoxicity, Orange discoloration of bodily fluid.
3) Rifabutin- Neutropenia, Uveitis (0.01%), Polyarthralgias, Hepatotoxicity (1%))
4) Rifapentine- Similar to Rifampin
5) Pyrazinamide- Hepatotoxicity (1%), Nausea & Vomiting, Polyarthralgias (40%), Acute gouty arthritis, Rash and photosensitive dermatitis
6) Ethambutol- Retrobulbar neuritis (18%)
One of the most important aspects of tuberculosis treatment is close follow up and monitoring for these side effects. Most of these side effects can be managed by either close monitoring or adjusting dose. In some cases, the medication needs to be discontinued and second-line therapy should be considered if other alternatives are not available.
Majority of patients with a diagnosis of TB have a good outcome. This is mainly because of effective treatment. Without treatment mortality rate for tuberculosis is more than 50%.
The following group of patients is more susceptible to worse outcomes or death following TB infection:
Most patients have a relatively benign course. Complications are more frequently seen in patients with the risk factors mentioned above. Some of the complications associated with tuberculosis are:
Tuberculosis is a preventable and treatable infectious disease. Having said that, it's still one of the major contributors of morbidity and mortality in developing countries where we are still struggling to provide adequate access to care. Other challenges include lack of awareness, delayed diagnosis, poor accessibility to medication and vaccination as well as medication adherence. DOTS (Direct Observed Therapy) proposed by WHO has been very effective in recent years to improve adherence to the treatment in tuberculosis patients.  Also, Vaccination drive in developing countries has played a bigger role in decreasing the prevalence of this infection. Preventive effect of BCG vaccination is controversial but many studies have identified vaccination as a very important tool in the fight against tuberculosis and we need to keep our focus on childhood vaccination especially in developing countries. WHO and other health organizations have to continue their investment in developing strategy and research until we eradicate this disease from the world map. New antituberculosis drugs need to be developed to shorten or otherwise simplify treatment of tuberculosis caused by drug-susceptible organisms, to improve treatment of drug-resistant tuberculosis, and to provide more efficient and effective treatment of latent tuberculosis infection.