Tuberculosis (TB) is an infectious disease caused by bacteria whose scientific name is Mycobacterium tuberculosis. It was first isolated in 1882 by a German physician named Robert Koch who received the Nobel Prize for this discovery. TB most commonly affects the lungs but also can involve almost any organ of the body. Many years ago, this disease was referred to as “consumption” because without effective treatment, these patients often would waste away. Today, of course, tuberculosis usually can be treated successfully with antibiotics.
There is also a group of organisms referred to as atypical tuberculosis. These involve other types of bacteria that are in the Mycobacterium family. Often, these organisms do not cause disease and are referred to as “colonizers” because they simply live alongside other bacteria in our bodies without causing damage. At times, these bacteria can cause an infection that is sometimes clinically like typical tuberculosis. When these atypical mycobacteria cause infection, they are often very difficult to cure. Often, drug therapy for these organisms must be administered for one and a half to two years and requires multiple medications.
A person can become infected with tuberculosis bacteria when he or she inhales minute particles of infected sputum from the air. The bacteria get into the air when someone who has a tuberculosis lung infection coughs, sneezes, shouts, or spits (which is common in some cultures). People who are nearby can then possibly breathe the bacteria into their lungs. You don’t get TB by just touching the clothes or shaking the hands of someone who is infected. Tuberculosis is spread (transmitted) primarily from person to person by breathing infected air during close contact.
There is a form of atypical tuberculosis, however, that is transmitted by drinking unpasteurized milk. Related bacteria, called Mycobacterium bovis, cause this form of TB. Previously, this type of bacteria was a major cause of TB in children, but it rarely causes TB now since most milk is pasteurized (undergoes a heating process that kills the bacteria).
Over 8 million new cases of TB occur each year worldwide. In the United States, it is estimated that 10-15 million people are infected with the TB bacteria, and 22,000 new cases of TB occur each year.
Anyone can get TB, but certain people are at higher risk, including:
There is no strong evidence for a genetically determined (inherited) susceptibility for TB.
As previously mentioned, TB infection usually occurs initially in the upper part (lobe) of the lungs. The body’s immune system, however, can stop the bacteria from continuing to reproduce. Thus, the immune system can make the lung infection inactive (dormant). On the other hand, if the body’s immune system cannot contain the TB bacteria, the bacteria will reproduce (become active or reactivate) in the lungs and spread elsewhere in the body.
It may take many months from the time the infection initially gets into the lungs until symptoms develop. The usual symptoms that occur with an active TB infection are a generalized tiredness or weakness, weight loss, fever, and night sweats. If the infection in the lung worsens, then further symptoms can include coughing, chest pain, coughing up of sputum (material from the lungs) and/or blood, and shortness of breath. If the infection spreads beyond the lungs, the symptoms will depend upon the organs involved.
Bacille Calmette Guérin, also known as BCG, is a vaccine given throughout many parts of the world. It is derived from an atypical Mycobacterium but offers some protection from developing active tuberculosis, especially in infants and children. This vaccination is believed to be important in parts of the world where TB is quite common. This is not the case in the United States, and the vaccine is not routinely administered in the U.S. When BCG has been administered, future PPD and Tine skin tests remain positive and can cause some confusion when trying to diagnose TB. It is also important to realize that even with a BCG vaccine in childhood, tuberculosis can still occur in an adult exposed to the tuberculosis bacteria, which calls into question the real utility and effectiveness of this vaccination.
A new blood test is now available that can help distinguish between a prior BCG vaccine and a positive PPD due to TB infection (QuantiFERON-TB Gold). This test involves mixing the patient’s blood with substances that produce a TB-like immune response. After a period of time, the immune cells, if infected with TB, produce interferon-gamma, a protein produced by the body to defend against an infection. This test, like most, is not perfect, but with the proper clinical information can help distinguish a real TB infection from a positive reaction on the test due to a prior BCG vaccine.
A person with a positive skin test, a normal chest X-ray, and no symptoms most likely has only a few TB germs in an inactive state and is not contagious. Nevertheless, treatment with an antibiotic may be recommended for this person to prevent the TB from turning into an active infection. The antibiotic used for this purpose is called isoniazid (INH). If taken for six to 12 months, it will prevent the TB from becoming active in the future. In fact, if a person with a positive skin test does not take INH, there is a 5%-10% lifelong risk that the TB will become active.
Taking isoniazid can be inadvisable (contraindicated) during pregnancy or for those suffering from alcoholism or liver disease. Also, isoniazid can have side effects. The side effects occur infrequently, but a rash can develop, and the individual can feel tired or irritable. Liver damage from isoniazid is a rare occurrence and typically reverses once the drug is stopped. Very rarely, however, especially in older people, the liver damage (INH hepatitis) can even be fatal. It is important therefore, for the doctor to monitor a patient’s liver by periodically ordering blood tests called “liver function tests” during the course of INH therapy. Another side effect of INH is a decreased sensation in the extremities referred to as a peripheral neuropathy. This can be avoided by taking vitamin B6 (pyridoxine), and this is often prescribed along with INH.
A person with a positive skin test along with an abnormal chest X-ray and sputum evidencing TB bacteria has active TB and is contagious. As already mentioned, active TB usually is accompanied by symptoms, such as a cough, fever, weight loss, and fatigue.
Active TB is treated with a combination of medications along with isoniazid.Rifampin (Rifadin), ethambutol (Myambutol), and pyrazinamide are the drugs commonly used to treat active TB in conjunction with isoniazid (INH). Four drugs are often taken for the first two months of therapy to help kill any potentially resistant strains of bacteria. Then the number is usually reduced to two drugs for the remainder of the treatment based on drug-sensitivity testing that is usually available by this time in the course. Streptomycin, a drug that is given by injection, may be used as well, particularly when the disease is extensive and/or the patients do not take their oral medications reliably (termed “poor compliance”). Treatment usually lasts for many months and sometimes for years. Successful treatment of TB is dependent largely on the compliance of the patient. Indeed, the failure of a patient to take the medications as prescribed is the most important cause of failure to cure the TB infection. In some locations, the health department demands direct monitoring of patient compliance with therapy.
Surgery on the lungs may be indicated to help cure TB when medication has failed, but in this day and age, surgery for TB is unusual. Treatment with appropriate antibiotics will usually cure the TB. Without treatment, however, tuberculosis can be a lethal infection. Therefore, early diagnosis is important. Those individuals who have been exposed to a person with TB, or suspect that they have been, should be examined by a doctor for signs of TB and screened with a TB skin test.