Tuberculosis – Defined
Tuberculosis is a bacterial disease prevalent since ancient times. It existed many centuries before Robert Koch first isolated the tubercle bacillus in 1882. Koch’s discovery initiated a period of intense scientific activity that resulted in a variety of methods for detecting the tubercle bacilli by microscopy, by animal inoculation, and by in vitro culture. The disease remained a scourge for many more decades, however, and it was not until the 1940s that an effective treatment regimen became available (Bass, et. al., 1990).
The principal etiologic agent, Mycobacterium tuberculosis, currently infects one-third of the world’s population, and each year eight million people develop tuberculosis, which results in 1.8 million deaths per year. A variety of types of tuberculosis are known to be caused by this organism such as, bone tuberculosis, disseminated tuberculosis or miliary tuberculosis, lymph node tuberculosis, pleural tuberculosis, genitourinary tuberculosis, central nervous system (tuberculomeningitis) tuberculosis, abdominal tuberculosis, pericardial tuberculosis, and pulmonary tuberculosis. Among all of these, pulmonary tuberculosis is the most common and is responsible for the most deaths per year (Bass, et. al., 1990).
The decline in tuberculosis before the introduction of specific drug treatment was probably brought about by isolating infected individuals. Even today, early diagnosis of patients with infectious tuberculosis can reduce transmission of the disease. Sputum microscopy remains a valuable tool for identifying infectious individuals, although staining for tubercle bacilli requires [10.sup.4] organisms/mL for reliable detection and available techniques are not specific for M. tuberculosis. Sputum culture, which requires 6-8 weeks and [10.sup.3] organisms/mL is the only reliable method of detecting M. tuberculosis and is the ‘gold standard’ for diagnosing the disease (Toman, 1979).
Recent attempts at identifying tubercle bacilli early in disease have used the polymerase chain reaction (PCR) to amplify fragments of mycobacterial DNA to levels that are readily detected in multi-well plates by the development of a color in an enzyme reaction. Targeted DNA sequences have included those for ribosomal RNA, insertion sequences (polymorphic DNA which can be used to identify strains of M. tuberculosis) and specific genes (for example, stress proteins and proteins of phosphate metabolism) containing a sequence unique to M. tuberculosis.
However, false-positive reactions caused by contamination and false-negative reactions arising from inhibitors have caused problems with clinical samples (Toman, 1979). ‘Nesting’ the PCR (the use of a shorter length of DNA, derived from the sequence of nucleotides used for the first round of PCR, for a second amplification) confirms that the correct sequence has been amplified, increases the sensitivity of the test and dilutes any inhibitors of the reaction. Despite these safeguards, variability in sensitivity and specificity with these techniques remains a problem (Bothamley and Rudd, 1994).
Diagnostic tests based on the immune response in tuberculosis have been advanced on the grounds that infection does not always produce disease, while the disease is a result of an abnormal immune response. The skin reaction to tuberculin has been used as a measure of exposure to M. tuberculosis, but is affected by geographical area, BCG-vaccination, exposure to non-tuberculosis mycobacteria, immuno-suppression from whatever cause and may even give a false-negative response in the patient with advanced disease (Bothamley and Rudd, 1994).
As tuberculosis progresses from infection towards overt and then infectious disease, there is a corresponding shift away from a cell-mediated to an antibody response. Blood tests for the diagnosis of tuberculosis have had a long history. However, many of these have failed because most mycobacterial proteins are not very different to other bacterial proteins as far as the immune system is concerned. Crude extracts and even purified proteins or polysaccharides from tubercle bacilli can bind antibodies from uninfected individuals.
One protein (identified by its relative molecular mass of 38kDa) appears remarkably specific in detecting infectious tuberculosis, and as the ‘antigen 5’ preparation has successfully screened army recruits for infectious tuberculosis in Bolivia (Bothamley and Rudd, 1994). Monoclonal antibodies have been invaluable in overcoming the problems of specificity and supply of purified antigens. A competition assay using a monoclonal antibody to the same 38kDa antigen has achieved a good sensitivity and specificity in detecting early disease (Comstock, et. al. 1981).
There are three main drugs for the treatment of tuberculosis that will be proven bactericidal or sterilizing power – rifampicin, isoniazid and pyrazinamide. Three other drugs are important because the are either inexpensive or used widely in developing countries or are recommended for use where drug resistance is anticipated – thiacetazone, streptomycin and ethambutol. The patents on six drugs have expired.
A standard six-month course of treatment using three or four drugs for the first two months followed by two drugs for four months is highly effective in curing tuberculosis (99.6%) However, drug supply is erratic and even in developed countries acute shortages of rifampicin, streptomycin and second-line drugs have occurred. Inadequate treatment regimens and the lack of compliance with such long-term treatment reduce the cure rate to around 60% and relapse or chronic excretors of live tubercle bacilli become a problem. However, close supervision of a recommended treatment regimen has a significant effect on drug resistance and rates of relapse (Comstock, et. al. 1981).
The most effective chemotherapy currently available consists of three specific drugs and must continue for at least 6 months, which often results in low compliance, especially for those in the developing part of the world where most people are illiterate. Lack of compliance further imparts multiple drug resistance (MDR), giving rise to resistant strains of the bacterium, which consequently raises the cost of treatment, making the treatment prohibitive in most developing countries (Watson and Brit, 1993). Despite the enormous numbers of people infected with this organism, it is estimated that less than 10% of affected individuals show evidence of clinical symptoms.
Many factors, notably socio-economic, coinfection with HIV, and genetic predisposition of the host, influence susceptibility to this disease. After the appearance of acquired immunodeficiency syndrome (AIDS) in the 1980s, which affects the CD4^sup +^ T cell population, tuberculosis has become a more acute problem as these cells play an important role in protective immune response against the disease. Therefore, it has become necessary to develop an appropriate vaccine for treatment and find a cost-effective method of diagnosing tuberculosis (Bass, et. al., 1990).
Risk of Tuberculosis
As stated, tuberculosis is an infectious, bacterial disease. It is transmitted by droplets coughed into the air by people with pulmonary tuberculosis. A susceptible individual breathing contaminated air may become infected, depending on the concentration of droplet nuclei and the duration of exposure. Once an individual is infected with Mycobacterium tuberculosis, he or she remains infected for many years, perhaps for life (Daniel, et. al. 1986).
Some individuals develop clinically apparent disease within a few weeks of infection, but most are asymptomatic and the only evidence of infection is a positive skin reaction to tuberculin (an extract prepared from tubercle bacilli). Clinical disease can develop at any time, but peaks in incidence occur in young adults and in the elderly, where waning immunity occurs with advancing years.
Estimates suggest that one-third of the world’s population has been infected with tubercle bacilli and 20 million people have the disease at any one time. Tuberculosis is now a leading cause of death in the world (2.6 million deaths per year) and, in developing countries where tuberculosis is common in young adults, significantly affects economic output. During the 17th to 19th centuries, tuberculosis may have accounted for a quarter of all deaths in Europe and the US, but for much of this century there has been a steady decline in incidence to around 10 per 100,000.
Until recently that is, when a slight increase in tuberculosis has been observed in cities where social deprivation is comparable to that found in the developing world. The tuberculosis epidemic probably reached its peak in Asia at the turn of the 20th century, and may soon be at its height in sub-Saharan Africa. Urban development and poverty are responsible for the overcrowding, which encourages transmission by increasing exposure to infected droplet nuclei (Watson and Brit, 1993).
Reducing the Risk of TB Exposure
The persistence of tubercle bacilli in sputum in apparently healthy individuals may reflect the lower virulence of isoniazid-resistant strains. The genetic basis of resistance to isoniazid, rifampicin, streptomycin and ethionamide being elucidated and promises tools for the rapid assessment of drug resistance, using the polymerase chain reaction and probes that detect relevant changes in mycobacterial genes. Where the incidence of drug resistance is high, a four-drug regimen for the first two months is recommended. This treatment, together with the isolation of patients with suspected pulmonary tuberculosis, is likely to be the most effective method of preventing the spread of drug-resistant organisms.
Several new derivatives of rifamycin B have been developed. Their value lies in substantially longer duration of action and greater bactericidal activity on an equimolar basis compared with rifampicin. Thrice- or twice-weekly drug regimens for tuberculosis reduce the cost of supervised treatment, although reactions to rifampicin were a problem. Animal studies suggest that rifabutin and rifapentine could be effective treatments for tuberculosis when given at weekly or even fortnightly intervals.
The quinolones, which are also effective against a number of bacterial pathogens, act by inhibiting DNA gyrase and have in vitro activity against M. tuberculosis. Their potential resides in the treatment of drug-resistant organisms and some success in this field has been reported. Combinations of [Beta]-lactamase inhibitors with penicillins may be effective treatment for drug-resistant tuberculosis.
Vaccines work on the principle that a disease given in a ‘safe’ form can stimulate the immune system in such a way that when the human host meets the virulent organism, an effective immune response is achieved more quickly and is of a greater magnitude than in the non-vaccinated individual (Freiden, 1993).
The idea of immunizing a patient suffering from a chronic viral or bacterial infection with still more of the infecting organism or its antigens seemed foolish, until recently. Jonas Salk of the Immune Response Corporation and Salk Institute for Biological Studies, San Diego, the pioneer of the polio vaccine, first suggested such an approach in 1987. Before this, an experimental model had suggested that, under particular conditions, T-cells responsible for promoting antibody production could change their pattern of response when stimulated by antigen and instead promote a cellular response, delayed hypersensitivity. This approach has already met with moderate success in the treatment of genital herpes (Freiden, 1993).
In general, it should be noted that tuberculosis is a disease that continues to cause the most deaths globally compared with all other diseases. This is the very reason why continuous research is being done to create the best possible vaccine as a remedy or as a prevention method against this disease. Vaccines provide us with immunity (permanent or temporary) against a specific microbe by eliciting an appropriate immune response against the disease (Selwyn, 1989). An individual immunized (vaccinated) either prior to an infection or at the onset of a disease shows an appropriate immune response elicited by the vaccine, which helps the body get rid of the invading pathogen. The properties desired for an ideal tuberculosis vaccine are as follows (Watson and Brit, 1993):
- It should give protection after a single administration.
- It should induce long-lasting (lifelong) immunological memory.
- It should be possible to combine it with other childhood vaccines.
- It should provide protection against disease and infection worldwide.
- It should not compromise the tuberculin skin test.
- It should be safe, stable, and inexpensive.
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