Tuberculosis and HIV: Background Information
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:
From Exposure to Infection
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:
Two Forms of 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:
This article was provided by Canadian AIDS Treatment Information Exchange. It is a part of the publication CATIE News. Visit CATIE's Web site to find out more about their activities, publications and services.
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