December 4, 2012
Tuberculosis (TB) is caused by bacteria called Mycobacterium tuberculosis complex and usually affects the lungs. However, among people co-infected with TB and HIV, parts of the body outside the lungs are often affected; this is called extra-pulmonary TB.
The germs that cause TB are spread when a person inhales tiny droplets produced by people who have infectious, or active, TB of the lungs. Infection with TB-causing germs can result in one of two forms of infection:
If left untreated, TB can be fatal. However, timely treatment, consisting of combination therapy with antibiotics, can cure both latent and active TB. Having had TB in the past -- either the latent or active form of the disease -- does not protect a person from future infection with TB-causing bacteria.
TB-causing bacteria are most commonly transmitted when someone inhales tiny droplets released into the air by an infected person during the following activities:
These tiny droplets are invisible and can remain floating in the air for several hours after they have been released.
Transmission of TB-causing bacteria from an infectious person depends on many factors, including the following:
Studies have found that people with TB whose sputum (the thick mixture that people with chest infections produce when they cough) contains TB-causing bacteria are likely to transmit these bacteria. Also, people with cavities in their lungs caused by TB are very infectious.
In high-income countries such as Canada, HIV-positive people exposed to TB-causing germs are prone to have TB-causing bacteria affect other parts of the body, including the following:
The risk of a person developing active disease from TB-causing bacteria after exposure depends on many factors, including the following:
In general, after exposure to TB-causing bacteria, disease does not immediately result. Instead what usually happens once the bacteria enter the lungs is that they are captured by cells of the immune system. Sometimes the immune system is able to destroy these bacteria. At other times the bacteria are able to subvert the immune system's defenses and go on to infect cells of the immune system. Once this happens, the infection can begin to spread slowly. For most people, at least initially, their immune system, if it does not kill the bacteria, is able to put the infection into a latent state. However, people whose immune systems are weakened by the following factors are at risk for ultimately developing active TB:
TB infection can result in two different states -- latent infection (which is symptom free) or active infection.
People with latent TB do not have symptoms; tests are needed to help assess their condition. To help diagnose latent TB, nurses can inject purified protein from TB-causing bacteria just under the skin (this does not cause infection). People with latent TB are very likely to have a reaction -- usually a swelling or bump at the injection site within 48 to 72 hours.
The test is imperfect, and in people with severely weakened immune systems there may not be a reaction. In other cases, people infected with bacteria closely related to those that cause TB might develop a reaction. In some clinics, a blood test may also be used to help identify people with latent TB. These tests check for the presence of a chemical signal -- interferon gamma -- produced by the immune system.
The symptoms of active TB can vary depending on the person and the organ-system affected. However, the following symptoms tend to be relatively common:
In cases where TB has affected the central nervous system (CNS) -- the brain and spinal cord -- additional symptoms might include the following:
In cases where TB-causing bacteria have spread throughout the body, the following symptoms may be present:
In cases where TB has affected the lungs, the following symptoms can occur:
When examining a person with active TB, doctors may find that the following parts of the body are swollen:
Doctors rely on several aids to help them diagnose active TB, including the following:
Usually a long course (between six and nine months) of the antibiotic isoniazid is prescribed. Treatment is prolonged because antibiotics work when the TB-causing bacteria are dividing, and in the case of latent TB, these bacteria grow very slowly.
Due to widespread use of antibiotics and other factors, including improper adherence to therapy, TB-causing bacteria have developed varying degrees of resistance to therapy. In high-income countries like Canada, the U.S., Australia and Western Europe, once the diagnosis of active TB is made, it is often standard procedure to have a fluid sample (from the lungs or other affected tissue) taken and the TB bacteria tested in the lab for their ability to resist antibiotics.
Treatment of TB has two goals:
The most common first-line treatment of active TB is a combination of the following antibiotics:
In HIV-positive people, doctors initiate potent combination therapy for HIV (commonly called ART or HAART) as soon as possible once TB has been diagnosed, as this helps improve the chances of recovery and survival. Doctors may also have to adjust antibiotic regimens or ART to reduce potential drug-drug interactions.
HIV-positive people co-infected with TB are at risk for developing IRIS -- immune reconstitution inflammatory syndrome. This occurs because when ART is initiated, the immune system is strengthened and begins to recognize and fight germs that it previously was unable to fight. As a result, symptoms of an infection (such as TB) may temporarily appear or may temporarily grow worse. Symptoms of IRIS can appear anywhere from two weeks to two years after ART has been initiated and are inflammation related. These can include fever, swollen lymph nodes and other symptoms depending on the type of infection and organ affected.
In some cases, such as in TB-related IRIS, drugs such as corticosteroids may be prescribed to help reduce symptoms caused by inflammation. IRIS tends to occur in people who have very low CD4+ counts (less than 50 cells).
Additional antibiotics for the treatment of TB are available, but these, particularly second-line agents, tend to have more side effects, must be injected and may require extended courses of therapy. This is the case when treating multi-drug resistant (MDR)-TB.
New antibiotics for the treatment of TB are being tested.
Future CATIE publications will feature developments about TB and HIV.