Tuberculosis
Tuberculosis (TB) is caused by one of three species of mycobacteria -- Mycobacterium tuberculosis, M. bovis, and M. africanum. However, M. tuberculosis is generally recognized as the prime cause in humans and is commonly referred to as the tubercle bacillus. This bacterium is non-spore forming, an obligate aerobe, growing very slowly. As with the other Mycobacterium species, it does not Gram-stain well.
The word "tuberculosis" means a tubercle or a small nodule or growth. Historians believe that the disease appeared in the Middle East about 8,000 years ago and crossed over to humans from cattle. A recently discovered Peruvian mummy who is estimated to have died about 1,000 years ago, shows evidence of lung scarring from such a disease. TB did not become a major problem until the Industrial Revolution, which created squalid living conditions that the bacteria needed to thrive. By the late 17th century, TB was killing one out of every five Londoners. Europeans brought it to the New World, adding to the already depleting population as a result of smallpox. René-Théophile-Hyacinthe Laënnec, the inventor of the stethoscope, died of the disease in 1826, as did many famous authors, inventors, etc., over the years. By 1900, TB was killing 200 out of every 100,000 urban Americans, with estimates as high as a billion people over the last two centuries worldwide. It still infects eight to ten million people per year around the world, killing two to three million.
Tuberculosis is generally found only in humans today, but M. bovis, a bovine tuberculosis, was once more of a problem to humans than it is now. This disease is transmitted from an infected cow to humans via contaminated milk. This is the main reason for pasteurization. However, both animals and humans can contract and transmit the disease. Human to human transmission is usually via the airborne route -- coughing, sneezing, etc., yet it usually takes months of close contact to accomplish this. Malnutrition often precludes the disease, while crowded and unsanitary conditions exacerbates it.
TB is a complex disease characteristically seen in different stages. The primary stage of the disease results when a person becomes infected for the first time. Bacilli usually travel to small spaces in the lung (alveoli), where oxygen and carbon dioxide exchange places. Here, they begin to multiply slowly, taking about twenty-four hours to double and three weeks to form colonies. By contrast, it takes the common stomach bacterium, E. coli, only twenty minutes to double and eight hours to go from a single organism to visible lumps of billions of bacteria. Many of the bacilli are phagocytized by macrophages, but they are not able to completely destroy the organism. Consequently, they are carried by the macrophages to lymph nodes. The growth of the bacilli inside the macrophage is slowed if the immune system is able to stimulate the development of enough antibodies. These antibodies allow for the formation of scar tissue around the bacilli called a nodule or tubercle. The tubercle usually prevents any further spread of the bacilli, but the center of it still provides a tiny area where the bacterium is protected from the host's defence mechanisms. It is at this stage that a person can display a positive skin reaction. The primary stage is usually without clinical symptoms, but can progress to other stages of the disease.
The second stage is called the latent-dormant stage. Persons in this stage still remain skin positive, but the tubercles may still not be large enough to be seen on X-ray. This dormancy period can last for weeks, years, or a lifetime. What triggers the activation of the tubercle bacilli is not definitely known, but it appears to have something to do with the host's immune system.
The next stage is called the secondary or active adult-type tuberculosis. It is an "awakening" of a dormant lesion. It is estimated that more than 90% of people with active tuberculosis have harbored the tubercle bacilli in the latent-dormant stage for more than a year. The previously dormant tubercle begins to expand in size, causing an enlarged central area of dead tissue and debris to form. This material is referred to as caseous necrosis. Eventually, the expanding tubercle erodes into a bronchial tube, and the inner contents are expelled into the airways. At this time, the patient can expel tens of thousands of bacilli from the respiratory tract in coughs, sneezes, etc. The fibrous and calcified walls of the tubercle then form an air-filled cavity where it can continue to grow. In this phase, healing or treatment is difficult because the cavity wall forms a barrier against any drug therapies, as well as the body's own defence mechanisms. Surgical removal of such cavities is sometimes necessary before any drug therapy can prove effective. Without treatment, the tubercular lesion will continue to grow, consuming the normal tissue surrounding it, until death results. Because of this progressive destruction or "consumption" of the live tissue cells, an earlier name for tuberculosis was "consumption."
Until the invention of streptomycin, the standard treatment was rest in clean air with good food. This was usually accomplished at rural hospitals where patients were confined for months at a time. The tuberculin test, developed by the German bacteriologist Robert Koch, was the standard method of detection to compliment chest X-rays. Malnutrition and poor hygiene are great contributors to the risk of contracting the disease. Today, treatment is long and expensive, requiring daily drug doses of medication for six to twelve months -- usually -- but can last as long as two years. Most who develop the disease are poor with little education and become frustrated with the extended treatment. Consequently, they give it up long before healing occurs. This not only causes them to continue with the disease, but allows for them to infect others.
There are a number of antibiotics successfully used to treat TB. They are usually given in combinations over long periods of time. However, by shortening the time needed, or taking the drugs sporadically, resistant strains of the bacillus develop which cannot be treated nearly as effectively. Another problem is that, by taking these toxic chemicals for one disease (tuberculosis), secondary illnesses often develop that require even more medications. There are now at least seven major drugs for TB, and some strains are resistant to all seven. Proper diet and hygenic living conditions seem to be the only way of preventing the disease. Scientists reluctantly agree that the spread is caused by the actions of the immune system. Therefore, as with other diseases caused by whatever microbe, the immune system is the first line of defence, and it is in our own best interest to do everything possible to keep the immune system strong and healthy.
There is a vaccine called BCG (Bacille Calmette-Guérin, named after the inventors) developed in France in 1921. It is made from bacteria that are closely related to Mycobacterium tuberculosis but have been bred to be harmless. In theory, this vaccine should work since it stimulates just enough of an infection for the body to develop an immunity to the real thing. However, its effectiveness ranges from dismal to ineffective, and is used only in countries where tuberculosis is prevalent and medical facilities are limited. This live vaccine induces an increased resistance to TB, but not complete immunity. However, it can cause enough hypersensitivity against the tubercle bacillus to render the TB skin test useless as a diagnostic tool. Thus, in countries where incidents of TB are low, only the skin test, followed by chest X-rays, is used for diagnostic purposes.
